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Perivascular epithelioid cell neoplasms (PEComas) of the bladder are extremely rare, with ~30 case reports. A subset of PEComas contain TFE3 gene rearrangement, however, the distinct histomorphologic features of these translocation tumors has not been fully explored in bladder PEComas. In our series, 11 cases of bladder PEComas were collected, including 1 internal and 10 consults, with 1 case previously reported. There was a female predominance (9 female, 2 male) with a mean age of 44.2 years (24 to 61 y). In only 1 of the 10 consult cases was PEComa considered in the differential diagnosis. In 10 of 11 cases, prominent epithelioid features were noted, with the final case having focal epithelioid morphology. Mitotic rate was increased in 2 of 11 cases, and 2 of 11 cases had cytological atypia. Two cases were malignant, with invasion into perivesicle tissue in 1 case, and metastases to lungs and brain followed by death in the other case. Immunohistochemically, there was strong, and diffuse staining for cathepsin K in 10/11 cases with the 1 negative case restained on a previously stained slide. HMB-45 was diffusely positive in 8/11 cases, while melan-A was present in only 1/10 cases. Muscle markers were variably expressed with positivity for both smooth muscle actin in 6/10 cases and desmin in 3/10 cases. Keratin AE1/3 was uniformly negative (0/11). In 5/8 cases where TFE3 was rearranged by fluorescence in situ hybridization, the morphology had a predominantly epithelioid, nested architecture. Overall, bladder PEComas are particularly difficult to diagnose given their rarity, are predominantly epithelioid and do not always express melanocytic markers. Diagnosis in the bladder requires a combination of morphologic characterization, exclusion of other diagnostic possibilities, positive Cathepsin K staining, variable melanocytic marker expression, with some cases showing a TFE3 gene rearrangement.

Primary intraosseous rhabdomyosarcomas are extremely rare. Recently two studies reported 4 cases of primary intraosseous rhabdomyosarcoma with EWSR1/FUS-TFCP2 gene fusions, associated with somewhat conflicting histologic features, ranging from spindle to epithelioid. In this study we sought to further investigate the pathologic and molecular abnormalities of a larger group of intraosseous rhabdomyosarcomas by a combined approach using targeted RNA sequencing analysis and fluorescence in-situ hybridization (FISH). We identified 7 cases, 3 males and 4 females, all in young adults, age range 20–39 years (median 27 years). Three cases involved the pelvis, 2 involved the femur and 1 each involved the maxilla and the skull. Molecular studies identified recurrent gene fusions in all 7 cases tested, including: a novel MEIS1-NCOA2 fusion in 2 cases, EWSR1-TFCP2 in 3 cases and FUS-TFCP2 gene fusions in 1 case. One case showed a FUS gene rearrangement, without a TFCP2 gene abnormality by FISH. The MEIS1-NCOA2 positive cases were characterized by a more primitive and fascicular spindle cell appearance, while the EWSR1/FUS rearranged tumors had a hybrid spindle and epithelioid phenotype, with more abundant eosinophilic cytoplasm and mild nuclear pleomorphism. Immunohistochemically, all tumors were positive for desmin and myogenin (focal). In addition, 4 tumors with TFCP2 associated gene fusions also co-expressed ALK and Cytokeratin. In conclusion, our results suggest a high incidence of gene fusions in primary rhabdomyosarcomas of bone, with two molecular subsets emerging, defined by either MEIS1-NCOA2 or EWSR1/FUS-TFCP2 fusions, showing distinct morphology and immunophenotype. Additional studies with larger numbers of cases and longer follow-up data are required to definitively evaluate the biologic behavior of these tumors and to establish their relationship to other spindle cell rhabdomyosarcoma genetic groups.

Perivascular epithelioid cell tumor (PEComa), mesenchymal tumors morphologically characterized by epithelioid cells, coexpress melanocytic and muscle markers. Herein, we describe a heretofore-undescribed tuberous sclerosis complex (TSC)-related neoplasm, morphologically resembling a soft tissue fibroma-like lesion, but showing an immunophenotype resembling PEComa. We identified 3 soft tissue fibroma-like lesions in individuals with TSC. We also evaluated 6 TSC-related periungual fibroma as well as a range of non-TSC fibroma-like lesions (n=19). Immunohistochemistry for HMB-45, desmin, smooth muscle actin, TFE3, and S100 was performed on the TSC-related fibromas. Periungual fibromas and non-TSC fibroma-like lesions were also stained for HMB-45. All 3 TSC patients were female, ranging in age from 4 to 51 years (mean, 26.7 y). Two tumors were located in extremities and 1 on the chest wall. The tumors showed elongated to stellate spindle-shape cells, prominent collagenous background, and lacked mitotic activity and cytologic atypia. Immunohistochemically, all 3 tumors were positive for HMB-45; smooth muscle actin or desmin was positive in both tumors tested. TFE3 was negative. All patients were alive with no evidence of disease with median follow-up of 55 months (range, 6 to 131 mo). Non-TSC fibroma-like lesions and oral and periungual fibromas were negative for HMB-45. Fibroma-like PEComa, a newly recognized soft tissue tumor with a strong association with TSC, mimics soft tissue fibroma but shows reactivity with melanocytic markers.

Cellular myofibroblastic tumors other than desmoid-type fibromatosis are often diagnostically challenging due to their relative rarity, lack of known genetic abnormalities, and expression of muscle markers which may be confused with sarcomas with myogenic differentiation. In this study we investigate the molecular alterations of a group of cellular myofibroblastic lesions with in the myofibroma and myopericytoma spectrum for better subclassification. Two index cases were studied by paired-end RNA sequencing for potential fusion gene discovery. One chest wall soft tissue tumor in a 3-month-old girl case showed a SRF-C3orf62 fusion, while the other, a forearm lesion in an 8-year-old girl, showed a SRF-RELA fusion. Further screening of 42 cellular examples of myofibroma/myopericytoma by fluorescence in situ hybridization identified additional 8 cases with recurrent SRF gene rearrangements, 6 of them showing identical SRF-RELA fusions. The cohort was composed of 7 females and 3 males, with a wide age range of 3 months to 63 years (mean=17). All tumors showed a densely packed growth of oval to spindle cells with fibrillary eosinophilic cytoplasm, arranged either in intersecting fascicles or with a distinct nested pattern around a rich vascular network. Despite the dense cellularity and variable mitotic activity none of the lesions displayed nuclear pleomorphism or necrosis. All tumors showed coexpression for SMA and desmin, in most cases with a strong and diffuse pattern of staining, while myogenin was consistently negative. No distant metastases were seen in the few cases with follow-up information. A control group of 34 well-characterized myofibroblastic and perivascular tumors, including 10 typical myofibromas and 3 myopericytomas, were also investigated for SRF gene abnormalities by fluorescence in situ hybridization and were negative. In summary, we report a subset of cellular variants of myofibroma and myopericytoma showing a smooth muscle-like immunophenotype and harboring recurrent SRF-RELA gene fusions, which mimic sarcomas with myogenic differentiation.

Alveolar soft part sarcoma (ASPS) is a morphologically distinctive neoplasm of unknown differentiation that bears a characteristic gene fusion involving ASPSCR1 and TFE3. ASPS can occur in the female genital tract, but is rare. Eleven cases with an initial diagnosis of ASPS at female genital tract sites were evaluated for their morphologic features and immunoprofile using a panel of antibodies (TFE3, HMB45, melan-A, smooth muscle actin, desmin, and h-Caldesmon). In addition, the presence of TFE3 rearrangement and subsequent ASPSCR1-TFE3 fusion were determined by fluorescence in situ hybridization. Ten tumors retained their classification as ASPS based on their morphologic appearance, immunohistochemical profile, and demonstration of ASPSCR1-TFE3 fusion. The remaining case was reclassified as conventional-type PEComa due to its pattern of HMB45, melan-A, and desmin positivity as well as absence of TFE3 rearrangement. Sites of the 10 ASPS were uterine corpus (3), cervix (2), uterus not further specified (2), vagina (2), and vulva (1). The age of the patients ranged from 15 to 68 years (mean 34 y, median 32 y). The tumors demonstrated a spectrum of morphologic features, but all had a consistent immunophenotype of strong TFE3 nuclear expression and lack of muscle (smooth muscle actin, desmin, h-Caldesmon) and melanocytic (melan-A, HMB45) markers, except focal positivity for HMB45 in 1. Follow-up was available for 4 patients ranging from 1 to 35 months (mean 15 mo, median 25 mo) and they were alive and had no evidence of recurrence or metastasis at last follow-up. Distinguishing ASPS from its morphologic mimics, particularly PEComa, is important due to increasingly efficacious targeted agents such as MET-selective and VEGF signaling inhibitors in the former and mTOR inhibition therapy in the latter.

Recurrent gene fusions involving EWSR1 with members of the cAMP response element binding protein (CREB) family (ATF1 and CREB1) have been reported in a diverse group of tumors including angiomatoid fibrous histiocytoma (AFH), soft tissue and gastrointestinal clear cell sarcoma, primary pulmonary myxoid sarcoma, and hyalinizing clear cell carcinoma of salivary gland. We have recently encountered a group of 5 myxoid mesenchymal tumors positive for EWSR1 fusions with one of the CREB family member (ATF1, CREB1, and CREM), with histologic features distinct from any of the previously described pathologic entities. Tumors occurred in children or young adults (12 to 23 y; mean, 18 y), with equal sex distribution. All except 1 were intracranial (intra-axial, 2; meningeal, 2), whereas 1 was perirectal. Histologically, the tumors were well circumscribed, often lobulated, composed of uniform ovoid to round cells, and arranged in cord-like or reticular structures in a myxoid background. All except 1 displayed unique sunburst amianthoid fibers. Immunohistochemically, tumors were positive for epithelial membrane antigen (5/5; 4 focal, 1 diffuse) and desmin (3/5). A novel EWSR1-CREM fusion was identified by RNA sequencing in the perirectal tumor, which was further confirmed by fluorescence in situ hybridization (FISH) and reverse transcription-polymerase chain reaction (RT-PCR). A second case with similar EWSR1-CREM fusion was identified by RT-PCR and FISH in a meningeal tumor. The remaining cases studied by FISH showed the presence of EWSR1-CREB1 fusion in 2 cases and EWSR1-ATF1 in 1. In conclusion, we report a distinct group of myxoid mesenchymal neoplasms occurring in children or young adults with a predilection for intracranial locations. Although the immunoprofile [epithelial membrane antigen (EMA), desmin] and the fusion type raise the possibility of a myxoid AFH, none of the typical histologic findings of AFH were present, suggesting a novel entity.

Myolipoma of soft tissue, which was first described by Meis and Enzinger (1991), is a rare benign neoplasm characterized by the admixture of mature adipocytes and well-differentiated smooth muscle cells. Recently, cytogenetic alteration of the HMGA2 gene has been reported in 2 myolipomas. We present the clinicopathologic features of 34 cases of myolipoma of soft tissue, study immunoreactivity for HMGA2, and review the previous literature. In our series, there were 32 women and 2 men, with age at presentation ranging from 35 to 94 years (median, 55 y). The most frequently affected site was retroperitoneum (47%), followed by pelvis (15%), abdominal wall (12%), and intra-abdominal sites (9%). Follow-up information was available for 17 patients (50%), ranging from 1 to 202 months (mean, 41 mo). None has developed local recurrence or metastasis. Grossly, tumors were well circumscribed, and the cut surface showed an admixture of yellowish adipose tissue and tan-whitish nodules. The size ranged from 2.4 to 60 cm (median 10.5 cm). Histologically, the tumors were composed of an intimate admixture of mature fat cells and bland spindle-shaped cells with brightly eosinophilic cytoplasm, arranged in fascicles. Some cases showed the following unusual features focally: hypercellular fascicular pattern (N=2), degenerative nuclear atypia (N=1), round cell morphology (N=1), hemosiderin deposition (N=1), metaplastic cartilage (N=1), metaplastic bone (N=1), and eosinophil infiltrates (N=1). Immunohistochemically, spindle cells showed strong and diffuse positivity for desmin (26/26 cases), SMA (20/21), and ER (13/15). Nuclear positivity for HMGA2 was identified in 15 of 25 cases (60%). MDM2 and CDK4 were usually negative (14/15, 8/9, respectively). In summary, myolipoma of soft tissue is a distinctive benign tumor composed of mature fat cells and smooth muscle cells and arises most commonly in deep-seated locations of middle-aged women. In our study, 60% of cases showed nuclear staining for HMGA2 by immunohistochemistry, which supports the possibility that these tumors harbor aberration of the HMGA2 gene, as seen in lipomas and leiomyomas elsewhere.

Sugita et al. recently reported a Ewing-like sarcoma within the right posterior neck of a 63 year old man that was found to have a novel CIC-FOXO4 gene fusion caused by translocation t(X;19)(q13;q13.3).1 The tumor was composed of sheets of undifferentiated small round blue cells within abundant desmoplastic stroma containing coarse chromatin and conspicuous nucleoli which expressed CD99, CD56, and WT1 but not SMA, desmin, CD34, or EMA. This patient is reportedly alive and disease-free at six months after gross total resection followed by adjuvant radiation and chemotherapy with adriamycin and ifosfamide.1 The prevalence, pathologic characteristics, and clinical behavior of this Ewing-like sarcoma with novel CIC-FOXO4 fusion are undefined.

Mesenchymal proliferations presenting as mucosal polyps are relatively uncommon and are represented by gastrointestinal stromal tumors, smooth muscle and neural tumors, and inflammatory fibroid polyps. In this report, we describe the clinicopathologic features of a distinctive type of mucosal polyp composed of cytologically bland spindled cells with fibroblastic features. Fourteen cases with histologic features of"fibroblastic polyps" were identified from our case files from January 2000 to December 2003. The clinical and endoscopic findings were reviewed. Immunohistochemistry using a panel of antibodies (vimentin, smooth muscle actin, desmin, CD31, CD34, Bcl-2, c-Kit, S-100, and epithelial membrane antigen) was performed in all cases, and electron microscopy was performed in two cases. The lesions were solitary in all cases and not associated with an identifiable polyposis syndrome. Associated adenomata and/or hyperplastic polyps at different sites were present in 10 cases and hyperplastic polyps were seen in close association in 3 cases. These polyps were characterized by a monomorphic spindle cell proliferation in the lamina propria, without necrosis or mitotic activity. The lesions were intimately associated with the muscularis mucosae and resulted in wide separation and disorganization of the colonic crypts. Immunohistochemical analysis revealed strong and diffuse positivity for vimentin only. Weak and focal reactivity was noted in 2 cases for CD34 and smooth muscle actin, while staining for other antibodies was negative. Electron microscopy revealed sparse cytoplasmic organelles and many intermediate filaments. The histology and ultrastructural and immunohistochemical findings are suggestive of fibroblastic differentiation of these spindle cells. In summary, these lesions represent a distinctive type of colonic mucosal polyp that should be distinguished from other stromal polyps of the gastrointestinal tract.

Clear cell myomelanocytic tumors are a recently described neoplastic growth considered to be a member of the family of perivascular epithelioid cell tumor. These tumors have a predilection for falciform ligament/ligamentum teres. We report an additional case arising from the muscularis propria of the urinary bladder in a 33-year-old woman. The tumor consisted of clear to eosinophilic, epithelioid, and spindled cells arranged in fascicles or packets. A delicate vascular stroma was found among the nests. Immunohistochemically, the tumor cells were typically positive for HMB-45 and smooth muscle actin but negative for S-100 protein, Melan-A, desmin, and pan-cytokeratin. The patient has been free of the disease since the excision of the tumor 6 years ago. This case, in association with the expanding list of perivascular epithelioid cell tumor reported in different sites, suggests that this type of neoplasm may be ubiquitous.

Desmoplastic infantile ganglioglioma is a rare intracranial tumor of early childhood with a usually excellent prognosis despite malignant features both radiologically and histologically. We present the case of a desmoplastic infantile ganglioglioma with histologically highly anaplastic features and both intracerebral and pial metastases. After partial resection the tumor was rapidly progressive and new metastases appeared. A combination of vincristine and carboplatinum was used according to the Low Grade Glioma Protocol of the International Society of Pediatric Oncology, with a temporary good response. When histologically characterized by highly anaplastic features, it seems the biologic behavior of this tumor remains uncertain. The aggressive behavior and the responsiveness to chemotherapy in this case may challenge the belief in the benign nature of these rare tumors.

Diagnosing monophasic fibrous and poorly differentiated synovial sarcoma (SS) on morphology alone is often a source of problems for pathologists. SS bear the t(X;18)(p11.2,q11.2) translocation, which proved to be specific for this tumor type and is currently considered one of the most reliable diagnostic criteria. To evaluate the sensitivity of immunohistochemical techniques in diagnosing monophasic fibrous SS (MFSS) and poorly differentiated SS (PDSS), we examined 60 t(X;18)(SYT-SSX)-positive cases (47 MFSS and 13 PDSS) for cytokeratin AE1/AE3, cytokeratin KL1, epithelial membrane antigen, E-cadherin, CD34, S-100 protein, alpha-smooth muscle actin, desmin, h-caldesmon, CD99, bcl2, and C-kit (CD117) antibodies. Of the four epithelial markers tested, epithelial membrane antigen proved to be the most sensitive, reacting with 100% of MFSS and 92% of PDSS, followed by cytokeratin AE1/AE3 (70% of MFSS, 46% of PDSS), cytokeratin KL1 (49% of MFSS, 38% of PDSS), and E-cadherin (47% of MFSS, 54% of PDSS). A staining for cytokeratin AE1/AE3 and/or E-cadherin was observed in 79% of MFSS and 69% of PDSS, and a staining for cytokeratin KL1 and/or E-cadherin was observed in 74% of MFSS and 62% of PDSS. S-100 protein was positive in 38% of MFSS and 23% of PDSS, and alpha-smooth muscle actin in 21% of MFSS and 8% of PDSS. Tumor cells were rarely positive for CD34 (6% of MFSS, 0% of PDSS) and desmin (2% of MFSS, 0% of PDSS). Most SS were strongly positive for bcl-2 (91% of MFSS, 92% of PDSS) and CD99 (91% of MFSS, 100% of PDSS). A weak and focal cytoplasmic reactivity for CD117 was observed in 11% of MFSS (only one case had a strong immunoreactivity) and 8% of PDSS. Staining with h-caldesmon was consistently negative. In conclusion, in keeping with literature data, our results show that reactivity for epithelial membrane antigen, cytokeratin AE1/AE3, and E-cadherin, in combination with CD34 negativity, are the most useful and sensitive markers for diagnosing monophasic fibrous and poorly differentiated t(X;18)-positive SS. They also support the fact that about one third of MFSS and one fourth of PDSS are positive for S-100 protein, a finding of diagnostic relevance when considering their distinction from other spindle to round cell sarcomas, especially malignant peripheral nerve sheath tumors.

A group of lesions show morphologic and immunophenotypic evidence of differentiation toward a putative perivascular epithelioid cell. These so-called PEComas include angiomyolipoma, lymphangiomyoma, lymphangioleiomyomatosis, renal capsuloma, clear cell myomelanocytic tumor of the falciform ligament/ligamentum teres, and clear cell "sugar" tumor. PEComas are characterized by strong immunoreactivity with the HMB45 antibody and variable expression of muscle markers. This family of lesions may be composed of a spectrum of cells from epithelioid to spindle cells with clear to granular eosinophilic cytoplasm. One member of this family, composed of epithelioid cells with glycogen-rich clear cytoplasm, is descriptively called a clear cell "sugar" tumor. This tumor, originally described in the lung, is being recognized increasingly in extrapulmonary sites. We report a case of a primary extrapulmonary clear cell "sugar" tumor occurring in the right breast of a 16-year-old girl. The tumor was composed of clear epithelioid cells with abundant glycogen and distinct cell borders. The tumor showed strong immunoreactivity with HMB45 antibody and Melan-A. There was focal vimentin staining. In addition, there was diffuse and strong nuclear staining for progesterone receptor. Antibodies to actins, S-100 protein, cytokeratins (AE1/AE3 and CAM5.2), desmin, and estrogen receptor were negative. The tumor was completely excised, and the patient is well without evidence of disease 9 months postexcision.

The expression of desmin, h-caldesmon, calponin, CD10, CD34, CD99, inhibin, and keratin (AE1/3-Cam 5.2) was studied in 10 conventional leiomyomas, 9 highly cellular leiomyomas, 9 epithelioid smooth muscle tumors, 9 leiomyosarcomas, 10 endometrial stromal tumors (4 with smooth muscle metaplasia), and 7 uterine tumors resembling ovarian sex cord tumors (UTROSCTs). c-kit expression was tested in 10 endometrial stromal tumors, 7 UTROSCTs, and 9 leiomyosarcomas. Desmin was positive in almost all smooth muscle tumors except those of epithelioid type, which were positive in only about half of the cases. It also stained areas of smooth muscle differentiation in endometrial stromal tumors and five of seven UTROSCTs. h-caldesmon was positive in almost all nonepithelioid smooth muscle tumors and in areas of smooth muscle differentiation in endometrial stromal tumors; it was positive in only about half of the epithelioid smooth muscle tumors and negative in all UTROSCTs. Calponin was positive in most tumor types. CD10 was positive in nine of 10 endometrial stromal tumors and five of seven UTROSCTs, although very focally in the latter group. It was also expressed, however, in almost all leiomyosarcomas, almost 50% of highly cellular leiomyomas, and rarely in the other smooth muscle tumors. CD34 was negative in the tested tumors with rare exceptions. CD99 and inhibin were positive in four of seven and one of seven UTROSCTs. Keratin positivity was found in most (five of seven) UTROSCTs and occasionally in smooth muscle tumors (seven of 37). c-kit was negative in all endometrial stromal tumors, UTROSCTs, and leiomyosarcomas. The major conclusions of this study are as follows: 1) Pure endometrial stromal tumors are usually desmin negative. 2) In contrast to some previous studies, CD10 expression was often seen in smooth muscle tumors, including most leiomyosarcomas and almost half of highly cellular leiomyomas. As a result, a panel of CD10, h-caldesmon, and desmin should be used and will distinguish endometrial stromal tumors from highly cellular leiomyomas in most cases. 3) In contrast to a previous study, no significant differences in immunoreactivity were seen between h-caldesmon and desmin in tumors with smooth muscle differentiation. 4) The absence of h-caldesmon in UTROSCTs helps separate them from epithelioid smooth muscle tumors. 5) UTROSCTs may express epithelial, stromal, and smooth muscle markers, suggesting divergent differentiation. 6) Our study shows less frequent inhibin expression in the sex cord-like elements of the UTROSCTs than in other studies. 7) c-kit may help distinguish metastatic endometrial stromal tumors of the uterus (c-kit negative) from gastrointestinal stromal tumors (c-kit positive). 8) CD34, CD99, and keratin have no or minimal role in this area, but keratin positivity in smooth muscle tumors should not lead to their confusion with epithelial tumors.

Glomus tumors usually occur in the peripheral soft tissues, but similar tumors have also been reported in the stomach and occasionally in the intestines. However, the relationship of these tumors to peripheral glomus tumors and gastrointestinal stromal tumors has not been fully clarified because previous series of gastrointestinal glomus tumors predate availability of immunohistochemistry. This clinicopathologic study examined 32 gastrointestinal glomus tumors. All but one of the tumors were located in the stomach and the remaining tumor was from the cecum. The tumors occurred with a strong female predominance (23 females and 9 males) and a median age of 55 years (range 19-90 years). The gastric tumors typically presented with gastrointestinal bleeding or ulcer-like symptoms, and 14 tumors had mucosal ulceration. Five tumors were incidental findings. The tumor sizes varied from 1.1 to 7 cm (median 2 cm), and most were located in the antrum. Histologically, the tumors typically had a solid pattern of sharply demarcated, round glomus cells with prominent, mildly dilated pericytoma-like vessels. Vascular invasion and focal atypia were relatively common (seen in 11 and 13 cases, respectively), and low mitotic activity (1-4 per 50 high power fields), was seen in 10 cases. Immunohistochemically, all tumors were positive for alpha-smooth muscle actin and calponin, and nearly all had a net-like pericellular laminin and collagen type IV positivity. All tumors were negative for desmin and S-100 protein. Three tumors had focal synaptophysin positivity, but none was positive for chromogranin. All tumors lacked KIT expression and the GIST-specific mutations in the c-kit gene. Follow-up revealed one patient death of metastatic disease to liver at 50 months; this tumor had 1 mitosis per 50 high power fields, but had spindle cell foci, mild atypia, and vascular invasion. Thirteen patients were well and alive after long-term follow-up. Gastrointestinal glomus tumors occur almost exclusively in the stomach, and they have a good overall prognosis, but a small, unpredictable potential for malignant behavior exists. These tumors are phenotypically similar to peripheral glomus tumors and differ from epithelioid GISTs.

Ewing's sarcoma/primitive neuroectodermal tumor (ES/PNET) is an extraordinarily rare primary tumor in the kidney and can be mistaken for a variety of other round cell tumors, including blastema-predominant Wilms' tumor (WT). Approximately 90% of ES/PNET have a specific t(11;22), which results in a chimeric EWS-FLI-1 protein. Immunohistochemistry for the carboxy-terminus of FLI-1 is sensitive and highly specific for the diagnosis of ES/PNET. WT-1, the WT-associated tumor suppressor gene, is overexpressed in WT but not in ES/PNET. No study has examined FLI-1 or WT-1 expression in renal ES/PNET. The clinicopathologic features of 11 renal ES/PNET were studied along with immunohistochemistry for cytokeratin, desmin, CD99, FLI-1, and WT-1. WT were also immunostained for CD99 (5 cases), FLI-1 (10 cases), and WT-1 (9 cases). The patients (6 men, 5 women) ranged from 18 to 49 years of age (mean, 34 yr). The mean tumor size was 11.8 +/- 3.8 cm (mean +/- standard deviation). Presenting symptoms included abdominal/flank pain and/or hematuria. Grossly, all tumors showed necrosis and hemorrhage, and 4 had cystic change. Microscopically, all tumors showed vaguely lobular growth, primitive round cells, and variable rosette formation. Epithelial, myogenous, or cartilaginous differentiation was not seen. Immunohistochemical results on the renal ES/PNET were cytokeratin (2/8 focal), desmin (0/9), CD99 (8/8), FLI-1 (5/8), and WT-1 (0/8). In comparison, the WT only rarely expressed CD99 (1/5) and did not express FLI-1 (0/10), but were usually WT-1-positive (7/9). Follow-up on 8 cases (mean, 28 mo; range, 6-64 mo) showed 4 lung and pleural metastases, 1 bone metastasis, liver metastasis, 2 local recurrences, and 5 deaths from disease (median time to death, 16.8 mo). No case had distant metastatic disease at presentation. Adjuvant therapy included chemotherapy (8 cases), radiation (3 cases), and bone marrow transplantation (1 case). Our study affirms a unique proclivity of renal ES/PNET for young adults and that it is a highly aggressive neoplasm, with rapid death in many cases, usually after the development of treatment-resistant lung metastases. These tumors must be distinguished from blastema-predominant WT and other primitive renal tumors that require different therapy. FLI-1 and WT-1 immunohistochemistry may be valuable in this differential diagnosis, given the known immunophenotypic overlap between ES/PNET and blastema-predominant WT with regard to CD99, cytokeratin, and desmin. The accurate distinction between these two entities has clear prognostic and therapeutic implications.

Desmoplastic small round cell tumor (DSRCT) and blastemal-predominant Wilms tumor (WT) share overlapping histologic features, yet accurate distinction is critical because of differing prognosis and treatment. We encountered a neck mass in a young adult with a concomitant abdominal mass. The neck mass was initially concerning for DSRCT on the basis of the poorly differentiated morphology, desmoplastic stroma, and immunoreactivity for desmin and cytokeratin. However, focal triphasic elements and glomeruloid bodies were identified on deeper sections, WT1 immunoreactivity was seen with antibodies to both the amino-terminus and carboxy-terminus, and EWSR1-WT1 rearrangement studies were negative, supporting the ultimate diagnosis of WT. On the basis of the overlapping histologic features of DSRCT and blastemal-predominant WT, we undertook a comparison study of desmin and cytokeratin reactivity patterns. We found that, whereas desmin reactivity was more frequent in DSRCT (11 of 12) than in WT blastema (11 of 22, P=0.024), dot-like perinuclear reactivity was seen with equal frequency in desmin-reactive DSRCT (10 of 11) and WT blastema (10 of 11), illustrating an important diagnostic pitfall. Moreover, we demonstrate coexpression of desmin and cytokeratin in both DSRCT (9 of 12) and WT blastema (11 of 22). Importantly, although dot-like desmin reactivity and coexpression of desmin and cytokeratin are historically associated with DSRCT, we demonstrate that these features can be seen in either DSRCT or blastemal-predominant WT. In these challenging cases, detection of an EWSR1-WT1 rearrangement and selective WT1 carboxy-terminus immunoreactivity (characteristic of DSRCT) or dual immunoreactivity for the WT1 amino-terminus and carboxy-terminus (characteristic of WT) remain the most discriminating diagnostic tools.

Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma and is mostly represented by the embryonal (ERMS) and alveolar (ARMS) histotypes. Whereas ERMS shows variable genetic alterations including TP53, RB1, and RAS mutations, ARMS carries a gene fusion between PAX3 or PAX7 and FOXO1. Epithelioid RMS is a morphologic variant of RMS recently described in adults. Five cases of epithelioid RMS were identified after histologic review of 85 cases of ARMS enrolled in Italian therapeutic protocols. Immunostaining analyses (muscle-specific actin, desmin, myogenin, AP-2β, EMA, cytokeratins, INI-1) and reverse transcription polymerase chain reaction assays to detect MyoD1, myogenin, and PAX3/7-FOXO1 transcripts were performed. In 4 cases DNA sequencing of TP53 was performed; and RB1 allelic imbalance and homozygous deletion were analyzed by quantitative real-time polymerase chain reaction. Histologically, epithelioid RMS displayed sheets of large cells without rhabdomyoblastic differentiation or anaplasia in 3 and prominent rhabdoid cells in 2; necrosis was evident in 4, often with a geographic pattern. Immunostainings for INI, desmin, myogenin (scattered cells in 4, diffuse in 1) were positive in all; EMA and MNF116 were positive in 2; AP-2β was negative. PAX3/7-FOXO1 transcripts were absent. In all cases RB1 was wild type, and a TP53 mutation at R273H codon was found in 1. All patients are in complete remission, with a median follow-up of 6 years. Epithelioid RMS may occur in children and is probably related to ERMS, as suggested by lack of fusion transcripts, weak staining for myogenin, negative AP-2β, evidence of TP53 mutation (although only in 1 case), and a favorable clinical course.

Myofibroblastic tumors are fairly recently established soft tissue neoplasms. Although most of them appear to be benign, myofibrosarcoma of the soft tissue, seemingly their malignant counterpart, have been reported. We describe the clinicopathologic and radiologic features of four cases of myofibrosarcoma arising from the bone. All but one of the patients were women ranging in age from 60 to 71 years. Two tumors occurred in the metaphyses of distal femurs and the others arose in the iliac bones. On radiologic examination all tumors exhibited well-demarcated lytic destructive lesions without periosteal reaction. Two tumors were localized in the bone, whereas the other two extended into surrounding soft tissues. Histologically, all tumors were composed principally of a mixture of a cell-rich fascicular area and a hypocellular fibrous area. In the former area tumor cells had rather eosinophilic spindle-shaped wavy cytoplasm and were arranged in interlacing fascicles and small storiform patterns with variable numbers of inflammatory cells. Tumors occasionally showed prominent pleomorphism, and large cells with hyperchromatic nuclei were seen. In contrast, hypocellular areas had various features, including collagenous, hyalinous scar-like and rarely keloid-like areas. Focal coagulation necroses were present in all but one tumor. Immunohistochemically, the tumors were positive for vimentin, muscle actin (HHF35), alpha-smooth muscle actin, calponin, and desmin, whereas all of them were negative for high molecular weight caldesmon. On follow-up there was one fatal case with distant metastases, whereas the clinical courses of other cases after wide resection were excellent. Myofibrosarcoma of the bone has distinctive histopathologic features, which should be distinguished from those of other bone tumors with myoid differentiation.

The objective of the present study was to analyze the role of the mesothelial markers desmin and N-cadherin in the diagnostic panel of serous effusions. A total of 181 pleural and peritoneal effusions consisted of 101 cases cytologically diagnosed as malignant (89 carcinomas, 12 mesotheliomas), 78 benign, and 2 inconclusive specimens. All specimens were immunostained using 11 antibodies, against epithelial membrane antigen, Ber-EP4, carcinoembryonic antigen, E-cadherin, CA 125, N-cadherin, desmin, calretinin, p53, vimentin, and CD45. After evaluation of immunocytochemistry results, 110 specimens were diagnosed as malignant (98 carcinomas, 12 mesotheliomas) and 71 as benign (56 cellular, 15 paucicellular). The presence of desmin was detected in benign mesothelial cells in 47 of 56 (84%) reactive cellular specimens compared with 1 of 12 (8%) malignant mesotheliomas and 2 of 98 (2%) carcinomas. N-cadherin was expressed in 48 of 56 (86%) reactive cases, 12 of 12 (100%) mesotheliomas, and 47 of 98 (48%) carcinomas. In carcinomas, N-cadherin expression was most often seen in ovarian carcinoma but was also found in other carcinomas. Calretinin, an established marker of mesothelial cells, was detected in 52 of 56 (93%) reactive specimens, 11 of 12 (93%) mesotheliomas, and 3 of 98 (3%) carcinomas. Evaluation of staining results led to reclassification of six malignant specimens as benign, whereas 17 cases diagnosed as benign and the two diagnosed as inconclusive were classified as malignant. In conclusion, desmin appears to be a promising marker for the distinction between reactive mesothelium and malignant epithelial cells in terms of both specificity and sensitivity, and its complementary use with calretinin is recommended. Unlike calretinin, it may also prove valuable for the distinction between benign and malignant mesothelial cells. N-cadherin does not have a role in the distinction between mesothelial and epithelial cells. However, it may prove useful in the characterization of carcinomas of unknown origin. As has previously been shown, a significant number of diagnoses that are based on morphologic examination alone are modified after the use of a broad antibody panel.

Metanephric adenoma (MA) is a renal tumor that is generally detected in adults and occasionally in children. These tumors usually behave in a benign fashion. Although the histogenesis of MA is unclear, a morphologic similarity to Wilms' tumor (WT) complex exists. Six cases of MA, five cases of childhood WT (CWT), two cases of adult WT (AWT), and four cases of treated MWT and/or nephrogenic rests (MWT/NR), with paraffin blocks available for use, were retrieved from the surgical pathology files of the Mayo Clinic. Clinical information was extracted from the medical record. Immunoperoxidase stains for WT1, AE1, CK7, CD57, CD56, and desmin were performed on paraffin sections from all cases. All six cases of MA were strongly and diffusely positive with antibodies to WT1 and CD57 and focally positive with antibodies to CK7. Three cases showed focal faint staining in

Rhabdomyosarcoma (RMS), the most common soft tissue sarcoma of childhood, displays a variety of histologic patterns. Immunohistochemistry is used extensively to distinguish RMS from its mimics. Myogenin and MyoD1, myogenic transcriptional regulatory proteins expressed early in skeletal muscle differentiation, are considered sensitive and specific markers for RMS and are more specific than desmin and muscle-specific actin and more sensitive than myoglobin. Previous studies have focused on expression of myogenin and MyoD1 in small round cell tumors. This study assesses myogenin and MyoD1 in rhabdomyosarcoma subtypes and spindle cell tumors considered in the differential diagnosis of RMS. Formalin-fixed, paraffin-embedded archival tissue from 32 RMS, 107 non-RMS, and 11 benign skeletal muscle samples was stained for myogenin and MyoD1 with standard immunohistochemical techniques. Nuclear positivity was scored on a three-tiered scale. All RMSs expressed myogenin. Alveolar RMS (ARMS) showed strong nuclear staining, especially in tumor cells lining fibrous septae and perivascular regions. In cases with a subtle alveolar architecture on routinely stained sections, myogenin highlighted and enhanced visualization of the alveolar morphologic pattern. Embryonal RMSs (ERMSs) were more variable in myogenin staining pattern and intensity. No cases of nodular fasciitis, malignant fibrous histiocytoma, malignant peripheral nerve sheath tumor, inflammatory myofibroblastic tumor, myofibrosarcoma, leiomyoma, leiomyosarcoma, or alveolar soft part sarcoma stained for myogenin. Focal nuclear reactivity was seen in desmoid (2 of 10), infantile myofibromatosis (2 of 10), synovial sarcoma (1 of 10), and infantile fibrosarcoma (2 of 10). Non-neoplastic skeletal muscle fiber nuclei stained positively for myogenin in both tumor-associated samples (25 of 40) and benign skeletal muscle samples (5 of 11). Although all RMSs were immunoreactive for MyoD1, cytoplasmic and nonspecific background staining and reactivity of nonmyoid tissues hindered its practical utility in paraffin-embedded samples in this study. Although myogenin is a highly sensitive and specific marker for RMS, it is rarely seen in other spindle cell soft tissue tumors. As previously reported, ARMS stained more strongly than ERMS. In contrast to previous studies, rare non-RMS (7 of 107) displayed focal nuclear reactivity, and entrapped atrophic or regenerative skeletal muscle fibers also stained positively. Although these are potential pitfalls in the interpretation of myogenin, careful attention to morphology and other features, to the relative paucity of myogenin-positive nuclei in non-RMS. and to the presence of entrapped muscle fibers should prevent incorrect interpretation. Because the extent of myogenin expression in RMS is much greater than in non-RMS, it is a very useful marker when interpreted in the context of other clinicopathologic data.

Pleomorphic leiomyosarcoma (PLMS) was recently described as a morphologic variant of leiomyosarcoma; however, its diagnostic criteria, as shown by morphologic features and biologic behavior, remain controversial. We describe 28 cases of pleomorphic sarcoma with pleomorphic areas in more than two thirds of the tumor and an ordinary leiomyosarcomatous fascicular area covering less than one third as PLMS. PLMS comprised 8.6% of all the leiomyosarcomas (322 cases) registered in our institute. Patients ranged in age from 31 to 89 years (average, 57.9 years). Seventeen patients (60.7%) were male and 11 were female. Tumor location was as follows: the extremities in 17 cases, the retroperitoneum or abdominal cavity in 7 cases, the chest/abdominal wall in 3 cases, and the scalp in 1 case. Histologically, all cases showed at least small foci of fascicles consisting of smooth muscle tumor cells, in addition to pleomorphic areas mimicking storiform-pleomorphic malignant fibrous histiocytoma. The border between pleomorphic and leiomyosarcomatous fascicular areas was sharp in 3 cases, gradual in 2 cases, and blending in 23 cases. Sixteen cases (57.1%) showed a typical storiform pattern, 6 cases revealed extensive stromal hyalinization, 6 cases showed a chronic inflammatory infiltrate, 2 cases had the foci of foamy xanthomatous cells, and 7 cases contained myxoid malignant fibrous histiocytoma-like areas covering less than 50% of the tumor. The tumors had a tendency to be of a morphologically higher grade (10 tumors were French Federation of Cancer Centers grade 2, 18 were grade 3). Five of 28 cases (18%) showed rhabdoid features. Immunohistochemically, all of the 28 tumors examined showed a positive reactivity for at least one smooth muscle marker (desmin, muscle-specific actin, and alpha-smooth muscle actin) in the leiomyosarcomatous fascicular areas. In the pleomorphic areas the expression of smooth muscle markers (desmin 10 of 28, muscle-specific actin 13 of 28, and alpha-smooth muscle actin 14 of 28) was significantly reduced, compared with that in leiomyosarcomatous fascicular area (desmin 18 of 28, muscle-specific actin 26 of 28, and alpha-smooth muscle actin 24 of 28). No significant difference was observed between the MIB-1 labeling index in the leiomyosarcomatous fascicular areas (26.10 on average) and that in the pleomorphic areas (26.17 on average). However, the MIB-1 labeling index in PLMS was significantly higher than that in ordinary leiomyosarcoma (n = 20, 12.86 on average) or storiform-pleomorphic malignant fibrous histiocytoma (n = 16, 16.63 on average). In 23 patients follow-up data were available with a duration of 1-239 months. Eleven patients developed metastases, and lung accounted for the most common site of metastasis (9 cases). Fifteen of 23 patients (65.2%) died of disease. Our results indicate that PLMS should be differentiated from ordinary leiomyosarcoma because of its high proliferative activities and rather aggressive biologic behavior.

The authors present 18 cases of a hitherto unrecognized variant of cutaneous neurofibroma. The tumors presented in adults (10 occurred in men and eight occurred in women) as a solitary, well-circumscribed, superficial lesion located in the dermis measuring 3 to 17 mm (mean size, 6.2 mm). The tumors formed oval-shaped masses that ran perpendicular to the epidermis. In the deep part of the tumor there was multinodular arrangement with two types of cells: Type I cells were small, dark, lymphocyte-like cells with a slightly irregular nucleus and inconspicuous cytoplasm. Type II cells were larger, with pale-staining vesicular nuclei, with frequent invaginations and intranuclear inclusions, and had copious clear eosinophilic cytoplasm that formed a stellate growth pattern, which was poorly visible on hematoxylin and eosin staining. Type I cells were grouped concentrically around type II cells and formed pseudorosettes. Most of the type I and type II cells were S-100 protein and CD57 positive, and various proportions of both cell types were CD56 and PGP9.5 positive. All cells were chromogranin A, synaptophysin, glial fibrillary acidic protein, cytokeratins, CD1a, CD21, CD31, alpha-smooth muscle actin, muscle-specific actin, desmin, and HMB-45 negative. CD34 stained intralesional fibroblasts. Antibody to epithelial membrane antigen stained only the perineurium around the tumor masses, suggesting that the tumors arose inside the nerve sheath. No signs of neurosecretory granules were present at ultrastructural level. None of the lesions recurred and none metastasized over a mean follow-up of 8.1 years.

The difficulty in distinguishing between smooth muscle and endometrial stromal-derived neoplasms of the uterine corpus is a notorious and clinically relevant problem in pathology of the female genital tract. Immunohistochemistry offers some aid in resolving this difficulty, because the expression of smooth muscle markers is reputed to indicate smooth muscle derivation. This expression, however, is not entirely specific, and difficult cases may still present in which immunohistochemistry is of little help. To explore this problem, the authors evaluated the expression of traditional muscle markers and high-molecular-weight caldesmon (h-cal), an actin and tropomyosin binding protein that has recently been described as a useful muscle marker, in uterine leiomyosarcoma (LMS), cellular leiomyomata (CL), and endometrial stromal sarcoma (ESS). Formalin-fixed and paraffin-embedded tissue sections from nine LMSs, 11 CLs, and 12 ESSs were evaluated with commercially available monoclonal antibodies against smooth muscle actin (SMA), desmin, and h-cal. Established morphologic criteria were used to classify the neoplasms. We found that there was, as expected, a significant difference in the expression of traditional smooth muscle markers (SMA and desmin) between tumors derived from smooth muscle and those derived from endometrial stroma (p = 0.005 for LMS and 0.013 for CL). We further found that h-cal was most useful in distinguishing between CL and ESS (p = 0.01). A significant difference between h-cal expression in LMS versus ESS was not found. Of note, one ESS expressed both SMA and desmin but lacked h-cal expression. Our findings confirm the most useful immunohistochemical data to date; smooth muscle neoplasms are generally distinguishable from endometrial stromal tumors by the expression of conventional muscle markers. We also report here that h-cal is useful more specifically in the differentiation of CL from ESS.

Microphthalmia transcription factor (Mitf) is a nuclear protein involved in the development of melanocytes and the regulation of melanin synthesis. Recent studies have suggested that Mitf may be a more sensitive and specific melanocyte marker than S-100 protein and gp100. However, there is insufficient knowledge on the specificity of Mitf, and a systematic examination of its use for the recognition of desmoplastic melanoma has not yet been performed. In this study, we compared the expression of Mitf with S-100 protein, gp100, and tyrosinase in 20 desmoplastic melanomas by using the antibodies D5 (anti-Mitf), anti-S100P, HMB-45 (anti-gp100), and T311 (anti-tyrosinase). All 20 melanomas were positive for S-100 protein, 7 were positive for Mitf, 6 for gp100, and 11 for tyrosinase. To examine the specificity of Mitf, a panel of normal tissue and 386 samples of miscellaneous tumors, including dermal and subcutaneous spindle cell lesions relevant for the differential diagnosis of desmoplastic melanoma, were examined by immunohistochemistry. Furthermore, normal tissue samples were tested for Mitf mRNA by reverse transcriptase polymerase chain reaction (rt-PCR). Immunoreactivity for Mitf was seen not only in melanocytes of normal skin, but also in macrophages, lymphocytes, fibroblasts, Schwann cells, and smooth muscle cells at various sites, and tumors derived thereof. Our results indicate that the antibody D5 lacks sufficient sensitivity and specificity for widespread diagnostic use. Especially in re-excisions, when immunohistochemistry is often needed to distinguish an inflamed scar tissue from tumor, the presence of immunopositive inflammatory cells and fibroblasts limits the diagnostic use of D5.

Leiomyomas (LMs) of the gastrointestinal tract arise within the muscularis mucosae (superficial) and muscularis propria (deep). There are isolated reports of KIT-positive cells, presumed interstitial cells of Cajal (ICCs), within gastrointestinal LMs. We have encountered esophageal LMs with a high proportion of KIT-positive and DOG1-positive spindle-shaped cells, an appearance that mimicked gastrointestinal stromal tumor. Our aim was to explore the prevalence of ICCs in LMs of the gastrointestinal tract and the etiopathogenic significance of these cells in this benign neoplasm. We identified 34 esophageal LMs (28 deep, 6 superficial), 8 gastric LMs, and 5 small-bowel LMs (all lesions in muscularis propria). We performed immunohistochemical staining studies for desmin, DOG1, and KIT on these neoplasms. We also evaluated 12 superficial colonic LMs. ICCs were distinguished from mast cells on the basis of morphology (elongated and occasionally branching spindle-shaped cells) and the presence of DOG1 reactivity. Four cases were screened for mutations in PDGFRA exons 12, 14, and 18 and KIT exons 9, 11, 13, and 17. ICCs were identified in all deep esophageal LMs and constituted an average of 20% of the lesional cells; focally, these cells comprised >50% of cells. The density of these cells was significantly higher than the background muscularis propria, and hyperplasia of ICCs was not identified in the adjacent muscle. ICCs were identified in 6 of 8 gastric LMs and 1 of 5 small-bowel LMs and were entirely absent in all superficial esophageal and colonic/rectal LMs. There were no mutations in KIT or PDGFRA. ICCs are universally present in deep esophageal LMs, and thus these neoplasms could be mistaken for gastrointestinal stromal tumors, particularly on biopsy samples, an error associated with adverse clinical consequences. ICCs are also identified in gastric and intestinal LMs, albeit in a smaller proportion of cases. Colonization and hyperplasia by non-neoplastic ICCs likely account for this phenomenon.

Two cases of primary meningeal sarcoma with leiomyoblastic differentiation are presented. In case no. 1, the tumor showed anaplastic spindle cell tumor components intermingled with anaplastic meningothelial components. Meningothelial tumor cells gradually became transformed into spindle tumor cells. Spindle tumor cells reacted with antisera to muscle actin (HHF-35) and alpha-smooth muscle actin. However, unchanged meningothelial tumor cells did not react with the antisera to HHF-35 and alpha-smooth muscle actin. Electron microscopy showed condensations of cytoplasmic fibers and pinocytotic vesicles in spindle tumor cells similar to those seen in smooth muscle cells. In case no. 2, the tumor cells consisted predominantly of sheets of round or polygonal cells as seen in an epithelioid leiomyosarcoma. The neoplastic cells had frequent nuclear inclusions, such as those seen in meningiomas. Immunohistochemically, the tumor cells reacted with antisera to desmin and to HHF-35. Electron microscopy showed a basal lamina around the cytoplasm of tumor cells. Intranuclear inclusions with various cytoplasmic organelles were frequently observed in the tumor cells, as in meningiomas. Interdigitating cytoplasmic processes and intercellular junctional complexes, however, were not found in the tumor cells. Two possible hypotheses explain the occurrence of leiomyoblastic characteristics of these cases. In case no. 1, leiomyoblastic cells originated from meningothelial cells with the advancement of meningothelial anaplasia. In case no. 2, pluripotential mesenchymal cells in the meninges differentiated into meningothelial and smooth-muscle cell lines at the time of tumor growth. With consideration of previous publications on primary meningeal sarcoma, these cases are the first reported primary meningeal sarcoma with leiomyoblastic and meningothelial differentiation.

HMB-45-positive clear-cell epithelioid tumor arising in the ligamentum teres hepatis of a 13-year-old Japanese girl is described. The well-defined tumor was completely removed and measured 9 x 7 x 6 cm. Cut sections showed a tan-white, homogeneous appearances with no hemorrhage or necrosis. The tumor was composed of nests or sheets of polygonal or oval-shaped cells rich in clear or finely granular cytoplasm. Capillary network was well developed, and sinusoid vessels were often seen with occasional perivascular hyalinization. There was moderate nuclear atypia but mitotic figures were absent. Periodic acid-Schiff stain showed a large amount of glycogen digested by diastase. Immunohistochemical stains for smooth muscle actin, Melan-A, and HMB-45 were positive in most of the tumor cells. Stains for vimentin, muscle actin, and HAM56 were focally positive, whereas stains for desmin, cytokeratin, epithelial membrane antigen, S-100, CD34, CD68, CD99, neurofilament proteins, and estrogen/progesterone receptors were negative. Ultrastructurally, the cytoplasm contained a considerable number of mitochondria, monoparticipate or membrane-bound glycogen, and longitudinally oriented thin filaments with focal condensations and subplasmalemmal densities. The histopathology of the present case, originally interpreted as epithelioid leiomyoma, was consistent with clear cell "sugar" tumors. The present case may indicate ubiquitous distribution of clear cell "sugar tumors" of which histogenesis remains unknown but is presumed to be of perivascular epithelioid cell origin.

Three cases of neuroendocrine carcinoma showing skeletal muscle differentiation are presented. The tumors were located in the skin and subcutaneous tissue, the urinary bladder, and the nasal cavity respectively, and were composed by two cell types admixed intimately with each other. One cell type had features identical to those seen in conventional small cell neuroendocrine carcinoma, including scanty cytoplasm, round nuclei with fine granular chromatin, immunohistochemical reactivity for neuron-specific enolase, chromogranin and cytokeratins, and electron-dense granules on ultrastructural examination. The second cell type was either plasmacytoid or elongated and straplike, with abundant eosinophilic cytoplasm and irregular nuclei with prominent nucleoli. These cells showed immunohistochemical positivity for desmin, sarcomeric actin, myoglobin, and myogenin. They also exhibited ultrastructural evidence of rhabdomyoblastic differentiation in the form of contractile filaments with abortive Z-band formation. An origin from a cell capable of dual differentiation toward neuroendocrine and rhabdomyoblastic elements is postulated for these tumors.

The clinical and histological features of 39 cases of desmoplastic small round cell tumor (DSRCT) diagnosed at M.D. Anderson Cancer Center are presented. Thirty-two of the patients were men and seven were women ranging in age from 10 to 41 years (mean, 25 years). Twenty-five of the 35 patients for whom follow-up information was available died of widespread metastases 8 to 50 months (mean, 25.2 months) from the time of their diagnosis and the remaining 10 were alive with disease. With the exception of two cases that occurred in the liver and in the scrotum, respectively, all of the cases originated within the abdominal and/or pelvic peritoneum. Eight tumors also involved the retroperitoneum. Although the characteristic histologic pattern of "small, blue cells" embedded in a dense fibrous stroma was seen in most cases, about one third of the tumors exhibited a wide range of morphologic features. The recognition of these uncommon morphologic variants of DSRCT is of paramount importance to avoid a misdiagnosis because these tumors could potentially be confused with other neoplastic conditions.

We reviewed six cases of rhabdomyosarcoma as a rare second primary malignancy in children with bilateral retinoblastoma after irradiation treatment. The patients comprised four females and two males (age range I year 4 months-7 years 1 1 months). Second tumors arose in the temporal muscle inside or close to the previously irradiated fields. All the children were alive and well 24-72 months after diagnosis. Microscopic examination showed proliferation of closely packed, small round cells with scanty cytoplasm, coarse nuclear chromatin, and increased mitotic activity without a myxoid background nor obvious alveolar architecture. The most characteristic feature was the presence of rosette-like structures in four tumors. Immunoreactivity for many skeletal muscle markers was evident, including desmin (six of six), muscle-specific actin (HHF35) (six of six), sarcomeric actin (six of six), myogenin (six of six), vimentin (six of six), and myoglobin (three of six). On reverse transcriptase-polymerase chain reaction examination, three second tumors lacked specific chimeric transcripts for alveolar rhabdomyosarcoma and Ewing's sarcoma. Unexpectedly, variable reactivity for neurofilament (150 kd) was identified in six of six second tumors as well as 15 of 20 sporadic primary rhabdomyosarcomas (75%) examined as controls, the result being confirmed by Western blot analysis. In addition, staining for retinoblastoma-susceptibility gene protein was negative in all second tumors, in contrast to positivity in 14 of 17 sporadic primary tumors (82%), This finding suggests that retinoblastoma-susceptibility gene abnormalities could be associated with the development of second primary rhabdomyosarcoma. We consider that knowledge of the occurrence of rhabdomyosarcoma and appropriate immunohistochemical study are helpful for avoiding a misdiagnosis of recurrent retinoblastoma or Ewing's sarcoma when encountering patients with a history of bilateral retinoblastoma who developed second small round cell neoplasms.

The clinicopathologic, immunohistochemical, and ultrastructural features of a seemingly distinctive low-grade spindle cell sarcoma showing myofibroblastic differentiation have been analyzed in a series of 18 patients. The age range of the patients (7 women and 11 men) was 19-72 years (median: 42 years). A painless, enlarging mass was the most common clinical presentation. Five tumors arose in the oral cavity (including four lesions in the tongue), four in the lower extremities and three in the upper extremities, four cases in the abdominal/pelvic cavity, and two on the trunk. Eight soft-tissue cases involved skeletal muscle, three cases were located in perifascial tissues, and two arose in subcutaneous tissue. Tumor size ranged from 1.4 to 17 cm (median: 4 cm); in six cases (of which four were abdominal/pelvic) the lesion was larger than 5 cm. All patients were treated surgically, and four received additional adjunctive therapy. Histologically, most cases were cellular lesions showing a diffusely infiltrative pattern, and were composed of spindle-shaped tumor cells arranged mainly in fascicles. Tumor cells had poorly defined, palely eosinophilic cytoplasm and fusiform nuclei, which were either tapering and wavy or plumper and vesicular with indentations and small inconspicuous nucleoli. Tumor cells were set in a collagenous matrix often with prominent hyalinization. Mild nuclear atypia was noted in 16 cases; in the other 2 cases, and in the metastases of one other lesion, a greater degree of nuclear atypia was seen. In all but one case, the mitotic rate ranged from 1 to 6 mitoses in 10 HPFs (mean: 2/10 HPFs); in a single case, there were more than 20 mitoses in 10 HPFs. Immunohistochemically, all cases stained positively for at least one myogenic marker; 12 cases were positive for desmin, 11 for alpha-smooth muscle actin, and 6 for muscle actin (HHF35). Seven neoplasms were desmin positive/ alpha-smooth-muscle actin negative, and five cases were desmin negative/alpha-smooth-muscle actin positive emphasizing the variable immunophenotype of myofibroblastic lesions. In addition, 7 of 10 tumors stained at least focally positive for fibronectin. Ultrastructural examination in five cases showed characteristic features of myofibroblasts. Follow-up in 11 patients (median: 29 months) revealed local recurrence in 2 cases, and multiple distant soft-tissue, intraosseous, and pulmonary metastases in one other patient. Low-grade myofibroblastic sarcoma seems to represent a distinct entity in the spectrum of low-grade myofibroblastic neoplasms and is distinguishable from fibromatosis, myofibromatosis, solitary fibrous tumor, fibrosarcoma, and leiomyosarcoma.

Seven examples of Clement and Scully's type II uterine tumors resembling ovarian sex cord tumors were studied immunohistochemically with markers of sex cord, steroid cell differentiation (inhibin, O13, A103), or both, and were found to be immunoreactive for one or more of these markers in all cases. There was also constant immunoreactivity for vimentin and for hormone receptors and variable positivity for keratin, actin, and desmin. These findings provide strong evidence for the presence of true sex cord elements in these tumors, the derivation of which remains speculative.

We present five cases of a hitherto unreported cutaneous neoplasm. The tumors appeared as solitary slow-growing flesh-colored nodules arising in young and middle-aged adults. They were located on the trunk, the upper extremities, and the face, and did not recur after complete excision. Clinically, they were diagnosed as dermal nevus, Spitz's nevus, fibroma, or neurofibroma. Histology revealed polypoid flat-dome-shaped lesions with a sharply demarcated matrix consisting of coarse hyalinized collagen bundles arranged in a prominent storiform pattern and separated by mucin-containing clefts. Despite a low overall cellularity, the tumors contained numerous, occasionally bizarre-shaped, multinucleate giant cells with crowded vesicular nuclei and a pale staining foamy cytoplasm, as well as plump fibroblastlike cells with analogous nuclear morphology. Atypical nuclei or mitotic figures were not observed. The cells were strongly positive for vimentin but negative for cytokeratin, smooth muscle actin, desmin, S-100 protein, CD34, factor XIIIa, and the macrophage markers KP1, Mac 387, and Ki-M1p, suggesting a fibroblastic origin. Based on the overall architecture, we conclude that these tumors probably represent a distinctive variant of solitary circumscribed storiform collagenoma (sclerotic fibroma) and propose the designation of giant cell collagenoma.

Leiomyosarcoma of bone is a rare tumor in an unusual location. Previous analysis of this entity mostly involved small numbers of cases with limited follow-up. Thirty-three patients with leiomyosarcoma of bone between 1977 and 1996 were studied, and the histologic appearance and grade were correlated with subsequent treatment and clinical behavior. To be included in this study the tumor had to be intraosseous, with other primary sites of origin clinically excluded. Also, most of the sarcomatous tissue (> or =70%) had to be of intramedullary location with only limited extraosseous extension. The patient's age at diagnosis ranged from 13 to 77 years (average 44.4). The gender distribution was equal. The long bones were preferentially affected (64%), with the lower extremity, around the knee joint, predominantly involved. Five patients (15%) developed postradiation leiomyosarcomas. The histologic analysis showed that the osseous leiomyosarcomas are most commonly of the classic type, followed by the epithelioid, myxoid, and pleomorphic variants. Immunoreactivity for smooth muscle markers (smooth muscle actin, common muscle actin, desmin) was positive in all tumors, and ultrastructural confirmation was obtained in 21% of cases. All sarcomas were histologically graded, which accurately reflected the subsequent prognosis. Seventy-five percent of the lesions were high-grade and the rest low-grade. The histologic grade of the tumors correlated with both the recurrence as well as the metastatic rates and together with the clinicopathologic stage of disease represented the cornerstone on which prudent therapy should be based.

We report on a locally recurrent vulvar tumor in an 80-year-old woman that we believe represents the first example of malignant transformation of an angiomyofibroblastoma. The tumor was predominantly a typical angiomyofibroblastoma, composed of epithelioid or oval cells with eosinophilic cytoplasm that tended to cluster in small groups and around blood vessels. These areas merged imperceptibly with a high-grade sarcoma that resembled a myxoid malignant fibrous histiocytoma. The tumor cells in the benign areas were diffusely immunoreactive for vimentin; many cells were positive for smooth muscle actin, and focal positivity for muscle actin and desmin was observed. The tumor cells in the sarcomatous areas were diffusely positive for vimentin, but negative for smooth muscle actin, muscle actin, and desmin. No staining for keratin, S-100 protein, or CD34 was noted. Ultrastructural examination of the sarcomatous area showed that the cells had the features of fibroblasts. All previously reported cases of angiomyofibroblastoma have exhibited banal histologic features and have behaved in a benign fashion. This case shows that these tumors may rarely be associated with a malignant component, and the designation "angiomyofibrosarcoma" may be appropriate in such cases.

We report a case of pleuropulmonary blastoma occurring in the right upper lobe and pleura of a 29-month-old boy. Histologically, the tumor was composed of undifferentiated mesenchymal cells with occasional rhabdomyoblastic and chondroid differentiation. Immunohistochemical analysis showed vimentin immunoreactivity in most of the tumor cells, myoglobin, desmin, and actin in the rhabdoid cells, and S-100 protein in the chondroid cells. Fluorescence in situ hybridization (FISH) analysis showed trisomy 2 and normal chromosomal copy numbers for chromosomes 7, 12, 17, 18, and X in the majority of the tumor cell nuclei. The identification of trisomy 2 in the current pleuropulmonary blastoma confirms a previous cytogenetic finding of chromosome 2 abnormality with cultured metaphase cells of a similar case, indicating that the acquisition of an additional chromosome 2 copy in pleuropulmonary blastoma is a nonrandom chromosomal alteration and that pleuropulmonary blastoma is cytogenetically distinct from pulmonary blastoma in which chromosome 2 is bisomic. It is also suggested that pleuropulmonary blastoma may have an intimate tumorigenetic relationship with embryonal rhabdomyosarcoma.

A case of myofibrosarcoma of the breast is described. A 55-year-old woman presented with a small mammary nodule. A large recurrent lesions appeared a month later, and the patient died 11 months following initial presentation from diffuse pleuropulmonary metastases. Histologically, the primary tumor consisted mainly of spindled cells, arranged in fascicles and surrounded by varying quantities of dense hyaline collagen. The recurrent lesion had a more pleomorphic organization. In both lesions, there was positive immunostaining for vimentin, smooth-muscle actin, and fibronectin, and negative results for desmin, laminin, and type IV collagen. Electron microscopy revealed abundant rough endoplasmic reticulum, myofilaments with focal densities, and the fibronexus junctions and fibronectin fibrils characteristic of myofibroblasts. Given these cellular features and behavior, the tumor was interpreted as a malignant neoplasm showing myofibroblastic differentiation, i.e., a myofibrosarcoma. This case enlarges the group of myofibrosarcoma of breast, also with the demonstration of fibronexus and fibronectin fibrils. The paper emphasizes the criteria required for myofibroblastic differentiation and reviews lesions of the breast reported in the literature as myofibroblastic.

In its classical form, embryonal rhabdomyosarcoma (ERMS, botryoid type) is a vaginal neoplasm occurring in infants and young girls and is often not considered in the differential diagnosis of uterine corpus and cervical spindle cell tumors in adult women. Clinicopathologic and immunohistochemical features of 25 cases of ERMS in women 20 years of age or older were analyzed. Patient age ranged from 20 to 89 years (mean, 44.4 y; median, 46 y), with 8 patients aged 20 to 39 years, 14 patients aged 40 to 59 years, and 3 patients older than 60 years of age. Tumors originated in the cervix in 20 cases and in the uterine corpus in 5. They were characterized by an edematous hypocellular spindle cell proliferation, typically with cellular condensation beneath epithelial surfaces (cambium layer), in which tightly packed hypercellular foci were scattered. Neoplastic cells had hyperchromatic nuclei and minimal cytoplasm, usually with delicate cytoplasmic processes. Occasionally, elongated or globular cells with eosinophilic cytoplasm (rhabdomyoblasts) were evident, but cytoplasmic cross-striations were only rarely identified. Apoptotic bodies and mitotic figures were usually identified in the hypercellular foci. Hemorrhage was common, often making recognition of the hypercellular foci difficult. Desmin and myogenin were coexpressed in 22 of 23 (95.6%) tumors evaluated. Proliferative activity, as assessed by Ki-67 expression, was notably elevated in all tumors evaluated, typically concentrated in the hypercellular foci. Estrogen and progesterone receptors were expressed focally in only 3 of 12 (25%) and 1 of 8 (12.5%) tumors evaluated, respectively. Follow-up was available in 7 cases. Five patients were alive without evidence of disease with follow-up of 3 to 8 years, and 1 patient was alive with disease at 5 months. One patient died at 5 months with pulmonary nodules, but it was not determined whether this was due to metastatic ERMS or the patient's known ductal breast carcinoma. ERMS has a broader clinical profile than classically expected and should be considered in the differential diagnosis of a uterine corpus or cervical spindle cell tumor, regardless of patient age. Recognition can be rendered difficult by the hypocellular background, which can suggest a benign polyp or low-grade tumor, and hemorrhage, which can obscure the characteristic hypercellular foci. Identification of hypercellular foci in which mitotic activity and apoptotic bodies are found, desmin and myogenin are coexpressed, proliferative activity is notably elevated, and hormone receptor expression is usually absent is very useful for establishing the diagnosis.

The clinical, histologic, and immunohistologic features of 22 desmoplastic melanomas (DMM), 10 mixed desmoplastic and spindle-cell melanomas (DMM/SMM), and two cellular spindle-cell melanomas (SMM) were studied. Patients ranged in age from 35 to 91 years (mean, 67) and included 23 men and 11 women. Seventeen cases occurred in sun-damaged skin of the head and neck. 11 were on the extremities, and six on the trunk. Except for two cases, all were Clark's level IV or V. Twenty-two (65%) cases were associated with a recognizable overlying pigmented lesion. Thirty of 32 (94%) DMM and DMM/SMM were clearly positive for S100. S100 staining was limited to < 5% of the spindle cells in two DMM/SMM. All DMM were negative when stained with HMB45. Three DMM/ SMM were immunoreactive with HMB45, as were both SMM. CD68 staining was limited to < 5% of the spindle cells in two of 32 DMM and DMM/SMM and 20% of the cells in one of two SMM. Nine (32%) DMM and DMM/SMM contained significant numbers of spindle cells immunoreactive for SMA but not desmin. In five cases, the number of actin-positive spindle cells. Two color stains for SMA and S100 demonstrated that these smooth-muscle actin positive cells constituted a separate spindle-cell population, consistent with reactive myofibroblasts. This study indicates that the immunohistologic features of desmoplastic melanoma differ from those of conventional melanoma. If a problematic spindle-cell skin lesion is a suspected melanocytic process, HMB45 is unlikely to provide confirmatory (or exclusionary) evidence for the diagnosis of DMM. Similarly, because of the variability in S100 expression in this neoplasm, the absence of S100 staining should not be relied on too heavily to exclude DMM if the clinical and histologic features favor that diagnosis. Caution should be exercised in the interpretation of numerous actin-positive spindle cells in isolation of additional confirmatory or exclusionary data as desmoplastic melanomas may contain significant numbers of these cells.

The study documents the immunohistochemical features of a case of infantile hemangioendothelioma (IHE) of the liver, which was found incidentally at autopsy in a 44-day-old girl. A precardial apical systolic murmur and hepatomegaly were found on day 4 of life. The tumor was multifocal and histologically composed of vascular channels lined by endothelial cells that were positive for von Willebrand factor, CD31, vimentin, and Ulex europaeus agglutinin 1, and that were invested in a continuous basement membrane (BM) on the antiluminal border. The endothelial cells, especially in the region of intravascular buds, showed intracytoplasmic synthesis of BM components (laminin and collagen IV). Underlying the endothelial cells were cells with cytoplasm that was positive for alpha-smooth muscle actin and antimuscle actin and negative for desmin, and that were enveloped with BM. The immunophenotype, appearance, and location of these cells are characteristic of pericytes. We found neither signs of endocrine secretion nor hepatitis B virus in the tumor tissue. The appearance of this tumor in the neonatal period supports a fetal origin of IHE.

We report a case of desmoplastic small-cell tumor occurring in the CNS in relation to the tentorium in a 24-year-old man. Morphologically, the neoplasm had the typical appearance of small, round tumor cells of primitive appearance growing as well-defined nests separated by abundant desmoplastic stroma. The diagnosis was confirmed through the demonstration of immunoreactivity for keratin, desmin, and neuron-specific enolase and the detection by Southern blot analysis of a unique gene resulting from the fusion of the WT1 gene in chromosome 11 and the EWS gene in chromosome 22. This is the first documented instance of the occurrence of this tumor type at a distance from a mesothelial-lined surface.

Pseudosarcomatous myofibroblastic tumor (PMT) is the result of reactive proliferation of myofibroblasts. In children, PMT of the urinary bladder can be mistaken for embryonal rhabdomyosarcoma clinically, radiologically, and by light microscopy. We are reporting the clinical, histological, and immunohistological features of 11 patients with childhood PMT of urinary bladder that were diagnosed initially as a sarcoma, usually rhabdomyosarcoma. The morphologic spectrum of PMT is broad, with mixtures of myxoid, leiomyomatous, and sclerosing matrix patterns, the myxoid type being the most common. The proliferating cells consist of three forms of myofibroblastic cells: long spindle cells (type I), intermediate spindle cells (type II), and ganglion-like cells (type III), together with various types of inflammatory cells. The immunohistologic profile of the proliferating cells was characterized by positive reactions to vimentin, muscle-specific actin, alpha-smooth-muscle actin, polyclonal desmin, and keratin. Ultrastructural studies showed myofibroblastic differentiation of the tumor cells. No patients have had metastases or local recurrence. Histologic, immunohistochemical, and clinical data from 71 cases of PMT, including the 11 cases in this report, confirm the benign behavior of these lesions. The etiology of these lesions is unclear, including the absence of surgical or other trauma in all of the children.

We report three cases of a unique, previously undescribed soft tissue tumor composed of mature adipocytes and hemangiopericytomatous areas, for which we propose the term lipomatous hemangiopericytoma. The tumors occurred in adults and were located in the sinonasal area, the soft tissue of the shoulder, and the retroperitoneum. The tumors ranged in size from 4 to 10 cm in greatest diameter and grossly were solid and ranged from tan to yellow. Histologically, they were composed of a variable admixture of benign lipomatous and hemangiopericytomatous components. Immunohistochemically, they stained with antibodies to vimentin and not to alpha-smooth-muscle actin, muscle-specific actin, desmin, S-100 protein, glial fibrillary acidic protein, epithelial membrane antigen, or keratin. Ultrastructurally, the cells constituting the hemangiopericytomatous areas had the features of pericytes, and no lipoblasts or transitional forms between lipocytes and pericytes were found. The histologic differential diagnosis of this neoplasm includes spindle-cell lipoma, angiolipoma, liposarcomas, tumors showing smooth muscle and adipocytic differentiation, and hemangiopericytoma infiltrating fat. Because of the small number of cases and the limited follow-up, we cannot be certain of their biologic behavior, although we expect that they are benign. Lipomatous hemangiopericytoma represents a distinctive pathologic entity that should be recognized and studied further.

Sneddon's syndrome is a rare, but potentially severe, arterioocclusive disorder characterized by generalized livedo racemosa of the skin and various central nervous symptoms due to occlusion of medium-sized arteries of unknown cause. We have recently shown that, in skin, small to medium-sized arteries of the dermis-subcutis boundary are affected in a stage-specific sequence. An initial phase (stage I), characterized by the attachment of lymphohistiocytic cells and detachment of endothelial cells (endothelitis), is followed by an early phase (stage II), which displays partial or complete occlusion of the lumen by a plug of lymphohistiocytic cells and fibrin. In an intermediate phase (stage III), the occluding plug is replaced by proliferating subendothelial cells accompanied by the occurrence of dilated capillaries in the adventitia of the occluded vessel. The late phase (stage IV) shows fibrosis and shrinkage of the affected vessels. We investigated sections of paraffin-embedded specimens of 18 patients by immunohistochemistry using a panel of antibodies to detect endothelial cells, macrophages, T cells, smooth muscle-specific actin, and intermediate filaments (vimentin, desmin). We found that the cells involved in subendothelial proliferation were vimentin and actin positive (smooth-muscle-specific), but desmin negative and thus displayed the phenotype characteristic of smooth muscle cells, which was confirmed by ultrastructural studies. CD3+, UCHL-1+, and HLA-DR+ cells constituted a significant proportion of the inflammatory infiltrate in the early stages. The endothelial cells in the dilated capillaries of the adventitia were strongly HLA-DR positive. In later stages, endothelial cells and leukocytes were scarce. The data confirm the hypothesis that Sneddon's syndrome starts as an inflammatory and possibly immunologically mediated disorder, leading to a migration and proliferation of smooth cells of small arteries, resulting in a partial or complete narrowing of the vessel lumen.

Adult pleomorphic rhabdomyosarcomas are difficult to distinguish from other pleomorphic soft tissue sarcomas. Immunostaining with the monoclonal antibody MyoD1 5.8A has recently shown promise as a sensitive and specific marker of rhabdomyosarcomas of childhood. To determine the usefulness of MyoD1 in distinguishing adult pleomorphic rhabdomyosarcoma from other pleomorphic sarcomas, we studied 21 cases of pleomorphic sarcoma, including six cases diagnosed as rhabdomyosarcoma. Immunostaining was accomplished using monoclonal antibodies against MyoD1, desmin, alpha-smooth-muscle actin, and muscle-specific actin on cryostat sections of frozen tumor. All cases of pleomorphic rhabdomyosarcoma stained positively with both desmin and MyoD1. Five cases diagnosed as nonrhabdomyosarcomatous pleomorphic sarcomas stained positively with desmin, but only one showed weak MyoD1 positivity. Three of these five cases were also positive with alpha-smooth-muscle actin. The results of this study suggest that the sensitivity and specificity of the MyoD1 antibody in the differential diagnosis of adult pleomorphic soft tissue sarcomas approaches that seen in pediatric rhabdomysarcomas.

Angiomatoid fibrous histiocytoma (AFH) is a rare soft tissue neoplasm of intermediate biological potential, predominantly occurring in the extremities of children and young adults. It has only recently been reported as a primary lung tumor. We describe 2 cases arising endobronchially harboring EWSR1 gene rearrangements by fluorescence in situ hybridization and, respectively, EWSR1-CREB1 and EWSR1-ATF1 gene fusions by reverse transcription polymerase chain reaction. Histologically, both tumors showed classical features of AFH, comprising multiple nodules of bland spindle to epithelioid cells surrounded by lymphoplasmacytic inflammation and at least a partial fibrous capsule. Both tumors showed focal but strong desmin immunoreactivity, with focal pancytokeratin and epithelial membrane antigen in 1 case. The lung is now a recognized site of AFH occurrence, but tumors arising here can be associated with different gene fusions. It is important to recognize AFH in the pulmonary region, as its behavior at other sites is generally relatively indolent; however, it may be mistaken for metastatic or more aggressive primary lung tumors. It is likely that cases of AFH in the lung may have been previously missed because of their morphologic and genetic overlap with other pulmonary lesions.

Myofibroblastoma of the breast is a rare, benign neoplasm, seen predominantly in men. The gross appearance is that of a well-circumscribed nodule, characteristically small, seldom exceeding 3 cm. We report a case of giant myofibroblastoma measuring 10 cm and weighing 169 g in the breast of an 83-year-old man. Light microscopic, immunohistochemistry, and electron microscopic features are described. Histologically, these neoplasms may exhibit a varied cellularity that can be misinterpreted as sarcoma. However, they lack marked cellular pleomorphism, tumor necrosis, and mitosis and are characteristically composed of plump and long bipolar, spindly cells arranged in swirling fascicles with intervening broad collagen bands. As we report, immunostaining is strongly positive for vimentin, desmin, and muscle common antigen and negative for cytokeratins and S-100-associated protein. Electron microscopy shows predominantly cells suggestive of myofibroblastic differentiation. The patient has remained free of disease 2 years after mastectomy.