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Lipoblastomas can occasionally require further molecular confirmation when occurring outside of the usual age groups or demonstrating unusual morphology. We reviewed 28 lipoblastomas with 16 controls. Lipoblastomas were subdivided into myxoid (n = 7), classic (n = 9), or lipoma-like (n = 12) subtypes. PLAG1 immunohistochemistry, PLAG1 fluorescence in situ hybridization (FISH), and targeted RNA sequencing were performed on formalin-fixed paraffin-embedded tissue. Karyotypes were available in a subset of lipoblastomas (n = 9). Gene rearrangements were identified in 17/25 (68%) lipoblastomas, including PLAG1 (15/25, 60%) and HMGA2 (2/25, 8%). Five novel fusion partners (DDX6, KLF10, and KANSL1L with PLAG1 and EP400 and FGD6 with HMGA2) were found. PLAG1 immunohistochemistry was positive (nuclear, moderate/strong) in myxoid and classic subtypes lipoblastomas with preferential expression in mesenchymal cells within myxoid stroma and fibrous septa and negative in all controls. When comparing PLAG1 immunohistochemistry with molecular testing (FISH and/or RNA sequencing and/or karyotype), concordant results were noted in 13/25 (52%) cases, increasing to 15/25 (60%) after slight adjustment of the PLAG1 FISH positive threshold. In myxoid and classic lipoblastomas, PLAG1 immunohistochemistry seems to be a better surrogate marker for PLAG1 rearrangement, as compared with lipoma-like subtypes. In lipoma-like subtypes, targeted RNA sequencing appears to detect PLAG1 fusions better than FISH and immunohistochemistry. The preferential expression of PLAG1 in the mesenchymal and fibroblast-like cells deserves further investigation as the putative cell of origin in lipoblastoma., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Fibrous histiocytoma (FH) or dermatofibroma is a common cutaneous lesion mostly seen in adults and rare in the first two years of life. Two hundred sixty-seven patients younger than 18 years with a diagnosis of FH or dermatomyofibroma, a lesion with morphologic overlap with FH, were identified from the files of a single institution, with only 13 (4.8%) occurring in patients younger than 5 years. Ten patients had either underlying neurologic, autoimmune, or metabolic disorders or a family history of autoimmune conditions. Histologic review of hematoxylin and eosin staining and immunostaining on 75 FHs and dermatomyofibroma in 70 patients showed the following results: 33 classic FHs, 8 classic FHs characterized by a peculiar retiform morphology with thin fascicles of elongated cells forming a network reminiscent of the eruptive variant of FH, 19 deep/cellular variants, 5 aneurysmal variants, 3 lipidized variants (including two lesions in a patient affected by mucopolysaccharidosis IV), 3 dermatomyofibromas, and 4 isolated cases of hemosiderotic, granular cell atypical, and epithelioid FH. Immunostaining for factor XIIIa highlighted a dense network of dendritic cells in FH, which was significantly reduced in the FH with retiform morphology. Smooth muscle actin staining was positive in a high percentage of FHs (85.3%). The current series demonstrates that FH in children may show unique clinical and morphologic features. The retiform pattern with decreased dendritic cells found in congenital lesions and in two older patients with lesions in two locations might have a different pathogenesis, probably related to an altered immune response in very young patients., (Copyright © 2020 Elsevier Inc. All rights reserved.)

Although littoral cell angiomas (LCAs) are phenotypically well characterized, the antibodies used to support the diagnosis identify many other cells in the normal spleen, and some may be found in other angiomatous lesions. Based on a langerin/CD207+ LCA index case, langerin and other selected immunohistochemical staining was performed on 10 LCAs, 20 other splenic angiomatous lesions, and 7 reactive lymph nodes to further investigate the role of langerin as a diagnostic tool. Ninety percent (9/10) of LCAs were langerin positive, whereas only 1 (5%) of 20 other splenic vascular lesions was partially positive (P < .00001). All LCAs were CD1a-, CD68+, CD34-, and CD8-; 20% were S100+, 70% CD21+, and 90% cyclin D1+. Ultrastructural studies of one LCA did not show Birbeck-type granules in definite lining cells. Sinus lining cells in 7 of 7 reactive lymph nodes showed partial langerin positivity, and 4 of 4 showed partial cyclin D1 positivity. In conclusion, langerin staining is an easily interpreted and highly sensitive and specific (sensitivity [0.90], specificity [0.95]) ancillary study to help distinguish LCA from other vascular tumors of the spleen. Whether this represents cross-reactivity or true CD207 expression is uncertain, as other immunohistochemical and ultrastructural studies do not support a Langerhans cell origin. The cyclin D1 staining seen in most LCA would be consistent with their expression of other selected vascular and histiocytic markers. The similar staining pattern in some lymph node sinus lining cells suggests a possible similar cell of origin, although LCA of lymph nodes is not described., (Copyright © 2018 Elsevier Inc. All rights reserved.)