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Introduction: True thymic hyperplasia following chemotherapy has been described mostly in children. There are a few cases of thymus hyperplasia that have been reported in breast cancer patients. Diagnosis of this unusual entity is very crucial to pretend unnecessary surgery or interventional diagnostic procedures. Case Presentation: We report a case of thymus hyperplasia in a patient who was operated and treated with adjuvant chemotherapy for stage 2 breast cancer two years ago. In the follow-up CT scans, an anterior mediastinal mass was noted. Radiologic evaluation and follow-up revealed thymus enlargement. Discussion: Thymic hyperplasia following chemotherapy has been described in both children and adults, but occurs mostly in children and adolescents treated for lymphoma and several other types of tumors. Few cases are reported in literature describing thymus hyperplasia following chemotherapy in a breast cancer patient. The imaging findings of thymic hyperplasia on CT, MRI and PET CT are discussed. Conclusion: Radiologists must be aware of this unusual finding in breast cancer patients treated with chemotherapy to guide the clinicians appropriately in order to avoid unnecessary surgical intervention, additional invasive diagnostic procedures, or chemotherapy.

Thymoma is a rare neoplasm of the anterior mediastinum, which originates from the epithelium of the thymic gland; it occurs mainly in middle-aged adults and is much less common in children. The tumor has slow growth and is asymptomatic in most pediatric cases, thus resulting in an accidental discovery; one-third of the young patient presents symptoms related to the compression of the tumor mass on the surrounding anatomic structures and/or related to paraneoplastic syndromes. Surgery is the treatment of choice and complete resection of the thymoma achieves excellent long-term results in terms of disease-free survival. In this article, we report the clinical case of a 21-month-old girl who came to our observation for persistent cough for over a month investigated with a chest X-ray, performed in another hospital. The X-ray showed an extensive opacification of the left hemithorax with contralateral dislocation of the mediastinum. The instrumental investigations carried out in our hospital (ultrasound, computed tomography, and magnetic resonance of the chest) showed a voluminous expansive mass of the left antero-superior mediastinum, which occupied the entire ipsilateral hemithorax and not dissociable from the thymus. At the histologic examination, the mass resulted to be a B1 thymoma with a low degree of malignancy according to the histologic classification of thymic tumors of the World Health Organization.

Background True thymic hyperplasia is a rare condition characterized by enlargement of the thymus while its normal structure is retained. True thymic hyperplasia is known to accompany Graves’ disease, but no association between true thymic hyperplasia and thyroid follicular tumor has been reported so far. We report a case of true thymic hyperplasia in a patient with a thyroid follicular tumor. Case presentation A 52-year-old Japanese man was referred to our hospital for evaluation of a thyroid mass and a mediastinal mass. His serum thyroglobulin level was high, and hemithyroidectomy was performed to remove the thyroid mass. The resected mass was diagnosed as a follicular tumor of uncertain malignant potential. After resection of the thyroid lesion, the patient’s serum thyroglobulin levels were markedly decreased. Seven months later, the patient underwent resection of the mediastinal mass. On pathological examination, the mass was found to consist of lobules, which formed a corticomedullary structure with Hassall’s bodies, indicating a normal thymic mass with hyperplastic thymic tissue, less organized cellular cords, and intermingled adipose tissue. Immunostaining for cytokeratin 19 and cytokeratin 7 indicated that the lesion was consistent with thymic tissue. The lesion was diagnosed as true thymic hyperplasia, and the histological findings suggested that secondary atrophy had occurred. No evidence of recurrence was observed at 24 months after surgery. Conclusions We present a case of a combination of true thymic hyperplasia and thyroidal follicular tumors that, to our knowledge, has not been reported previously. High serum thyroglobulin levels might play a role in hyperplasia of the thymus. Although true thymic hyperplasia is a rare disorder, it should be included in the differential diagnosis of a mediastinal mass in patients with thyroid disease.

Pure red cell aplasia caused by true thymic hyperplasia is extremely rare. We report the case of a 25-year-old female diagnosed with pure red cell aplasia. Following a thymectomy confirming true thymic hyperplasia and corticosteroid therapy, complete response was achieved. Patients diagnosed with pure red cell aplasia should be investigated with a computerized tomographic scan to assess for thymic pathology and if present, this should be resected. Follow-up is essential to monitor for recurrence.

Summary True thymic hyperplasia has been reported sporadically in the human literature as an immunological rebound phenomenon following immunosuppressive treatment or disease. There are limited accounts in domestic species, mainly following vaccination, and thymic hyperplasia has not been a recognised condition in the horse to date. This report details a case of true thymic hyperplasia in a 10-week-old Arabian colt diagnosed by histopathology of core biopsy samples. The colt developed pulmonary stenosis caused by compression due to a space occupying lesion in the cranial mediastinum following a 3-month history of hospitalisation for treatment of traumatic oesophageal rupture with perioesophageal abscess formation. Diagnostic imaging of the cranial mediastinum was indicative of a thymic mass, and histopathology confirmed the mass was normal (hyperplastic) thymic tissue. The colt was treated with a tapering dose of corticosteroids, which led to involution of the hyperplastic tissue and resolution of pulmonary artery compression. Thymic hyperplasia may be an unrecognised sequela to chronic inflammation in horses and was only identified in this case when the size was sufficient to compress right cardiac outflow.

Key Clinical Message A surgical approach is the choice in young infants with MTH, who are furthest from the time of physiological involution of the thymus, and when the thymus achieves the largest relative size, a surgical approach is the choice. Steroid therapy has been shown to be ineffective (4, 9, 16, 18‐20). No surgical complications have been reported, and the outcome is excellent. Recurrence has been seen in only one case.

BACKGROUND True thymic hyperplasia (TTH) is characterized as a distinct increase in both size and weight of thymus, which retains normal microscopic and immunohistochemical appearances. Massive true thymic hyperplasia (MTTH) is an extremely rare but significant subtype of TTH in pediatric ages due to its potentially serious consequences. It was reported that the age of cases with MTTH was predominantly between 1 and 15 years, while those before 1 year rarely occurred. By presenting the diagnosis and treatment process of our case as well as reviewing the related literature, we aimed to analyze the clinical characteristics of MTTH for patients younger than 1 year. CASE A 3-month-old male infant was admitted to our department with a chief complaint of gradually increasing polypnea over 9 days, whose preoperative imaging examination showed a large intrathoracic soft tissue shadow predominantly on the right side. The percutaneous fine-needle biopsy guided by ultrasonography was performed to identify its diagnosis. However, proliferating lymphocytes and Hassall`s corpuscles were seen microscopically in the biopsy tissues, which were immunohistochemically positive for CD3, CD19, CD20, CD99, TdT, PCK and Ki67 ( > 90%). Due to the aggravating symptoms, a second operation with total thymectomy was carried out successfully for this infant, which confirmed the diagnosis of TTH again by both morphological study and immunohistochemical staining from the surgical specimen. CONCLUSIONS By reviewing the literature, there were only 10 cases with MTTH reported between 1975 and 2020 for children aged < 1 year of life, together with our present one. In MTTH patient`s sex had an obviously male predominance (70%). Nine out of 10 presented initial symptoms or signs related to respiratory system and 6 patients showed respiratory distress. All patients were successfully treated by surgical thymectomy without any postoperative complications. The prognosis of MTTH was very successful.

We report a new case of a mediastinal mass in a 19-year-old patient corresponding microscopically to an association of unilocular thymic cyst and true thymic hyperplasia. Our aim is to highlight the absence of specificity of clinical and radiological findings and the necessity of a thorough sampling of the tumor in order to establish the diagnosis.

Introduction: A causal relationship between the thymus gland and myasthenia gravis (MG) has been assumed for decades. However, this relationship is still a matter of conjecture and speculations and awaits definitive proof. Recently, myoid cells have been considered as effectors in the pathogenesis of MG as well as thymic enlargement (thymolipoma, true thymic hyperplasia). Material and methods: we report a retrospective study about 6 cases of thymic enlargement with 4 cases of true thymic hyperplasia (TTH) and 2 cases of thymolipoma (TL). Aim: Our objective is to discuss the role of myoid cells in the pathogenesis of MG associated with thymic enlargement. Results: The majority of our patients were men with a sex ratio of 4/2. Only 2 patients had a past medical history consistent for MG. Myoid cells were found in all cases. Conclusion: Through our small study, the implication of myoid cells in the pathogenesis of the association MG-TL or TTH remains unproved.

The association between Graves' disease (GD) and thymic hyperplasia (TH) was first described in 1912 and has been reported numerous times thereafter. TH associated with GD presents as an incidental mediastinal mass on chest X-ray or computed tomography (CT). The pathogenesis of TH in the setting of GD is unclear but seems to involve a complex interplay of hormonal and immunological mechanisms.Here, the effect that thyroid hormones and autoimmunity have on thymic growth and size is reviewed. The authors' experience, along with a review of published case reports, reveals that general physicians may be unfamiliar with this association. This lack of familiarity may result in an aggressive management course, including surgical intervention, along with its associated risks and costs. The differential diagnosis and diagnostic workup of thymic enlargement associated with GD is discussed in light of the available clinical evidence.Recent literature confirms the generally benign nature of TH associated with GD, and supports a conservative approach for the diagnostic workup and initial management. Practical management recommendations for thymic enlargement associated with GD have been formulated and are presented here.

The initial clinical management of mediastinal masses essentially depends on the assumption whether surgery is indicated or not. Thymic masses that are thought to be operable based on imaging studies may not need to be biopsied, if surgery is deemed to be the treatment of choice. However, if nonsurgical therapies (as is the case in lymphomas) or neoadjuvant strategies are options, core needle biopsies may be an invaluable tool to guide therapeutic decisions. Like small biopsies elsewhere, sampling error is also a key problem in the prevascular and visceral mediastinum where it is, however, aggravated by (i) the high degree of microscopic similarity between normal thymus, thymic hyperplasias, T-cell-rich thymomas, and T-lymphoblastic lymphoma; (ii) the common histological heterogeneity across individual type B thymomas; (iii) the striking morphological overlap between spindle cell lesions with thymic epithelial, neuroendocrine, mesenchymal, and dendritic differentiation; (iv) the occurrence of acquired thymic cysts, the development of which is driven by different inflammatory processes and a broad spectrum of mediastinal tumors that may be dwarfed by the cystic process and escape sampling in small biopsies; and (v) reactive epithelial proliferations that can mimic neoplasias. These challenges encountered in small biopsies imply inevitable diagnostic limitations, suggest cautious reporting of diagnoses and argue not only for the thoughtful use of ancillary immunohistological and molecular studies but also for thorough clinicopathological correlation. The below sections on thymic epithelial tumors, various types of thymic hyperplasias and thymic cysts pinpoint diagnostic pitfalls and suggest strategies to avoid them.

中年女性に認められた胸腺過形成の2例を報告する．症例1は46歳，女性．CTで胸腺腫を疑い，胸腔鏡下に腫大した胸腺の生検を施行し，迅速組織診では，反応性リンパ節の診断であった．しかし，永久組織標本では胸腺腫を否定出来ず，胸骨正中切開下に胸腺胸腺腫瘍摘出術を施行した．最終的にリンパ濾胞性胸腺過形成と診断した．症例2は48歳，女性．CTおよびMRIで縦隔腫瘤を指摘された．MRIで腫大した胸腺内に16mm大の結節を認め，こちらは胸腺嚢胞を疑ったが，悪性腫瘍も否定できず，胸骨正中切開下に胸腺胸腺腫瘍摘出術を施行し，真性胸腺過形成および胸腺嚢胞の合併と診断した．自己免疫性疾患の合併を伴わないリンパ濾胞性胸腺過形成や，外傷や化学療法などの治療歴のない非若年者における真性胸腺過形成の報告は極めて稀であるので報告した．

The thymus gland possesses the ability to regrow in children leading to a newly developed anterior mediastinal mass. This condition may represent a rebound phenomenon during recovery from a stressful event such as post-chemotherapy and hence was described as RTH. RTH after LT has not been well documented. We are reporting an infant with BA who underwent LT and presented with a symptomless anterior mediastinal mass, detected on follow-up imaging 6 months thereafter. Surgical partial excision was performed to rule out other differential diagnoses of a solid mass in the anterior mediastinum of an infant particularly lymphoma-that may arise as post-transplant lymphoproliferative disorder-and teratoma, as well as the other aggressive lesions such as thymoma and thymic carcinoma. The final pathological analysis revealed true thymic hyperplasia, consistent with RTH. The diagnosis of RTH should be considered for a child presenting by anterior mediastinal mass after LT.

We present the case of a 24-year-old woman with thymic enlargement accompanied by hyperthyroidism. An x-ray incidentally detected her thymic enlargement and the size was estimated to be 226 cm(3) by computed tomography (CT) using three-dimensional analysis. Physical examination revealed a soft diffuse goiter, increased sweating and restlessness; thyroid function tests showed hyperthyroidism. Because the possibility of a thymoma could not be completely excluded, a mediastinal biopsy via a supracervical approach was done that determined the specimen was composed of normal thymic tissue. Together with these findings, the patient's thymic enlargement was likely to be consistent with true thymic hyperplasia. Moreover, we have demonstrated the presence of the thyrotropin (TSH) receptor in her thymus using reverse transcription-polymerase chain reaction (RT-PCR). She was treated with methimazole, resulting in the resolution of not only the thyroid dysfunction but also thymic enlargement. Thymic enlargement has often been recognized as a complication of hyperthyroidism. However, the pathophysiologic mechanisms underlying these conditions remain obscure. Our result raises the speculation that the thymus is also a target organ of autoimmune attack as are the orbital and fibroblasts, which lead to Graves' ophthalmopathy and pretibial dermopathy, respectively.

True thymic hyperplasia (TH) is an age-dependent increase in size and weight of the thymus gland, which by definition maintains a normal histologic architecture. TH can mimic other important diseases, including lymphofollicular hyperplasia, thymoma, lymphoma, and germ-cell tumors. Traditionally, separating these entities has required a formal surgical biopsy. Given that many of these conditions occur in children, this can be a traumatic experience for both the patient and family members. Fine-needle aspiration biopsy has the distinct advantage of being able to obtain diagnostic material without requiring general anesthesia. We are aware of only one previously reported case of an enlarged thymus being subjected to aspiration cytology. We therefore present a case of thymic hyperplasia in a 5-mo-old child diagnosed by combined radiologic and cytologic parameters, including flow cytometric analysis.

Thymic hyperplasia is a common feature in Graves' disease. However, in most cases, thymic enlargement is minimal, and radiologically detectable massive enlargement of the thymus is infrequently reported. Half of them undergo thymectomy due to the concern about a thymoma. We report a 28-year-old woman with untreated Graves' disease. She had an anterior mediastinal mass that was diagnosed as true thymic hyperplasia by biopsy and disappeared after treatment of the hyperthyroid state. Recognizing the association between thymic hyperplasia and Graves'disease, and existence of the benign course after treatment of the hyperthyroidism may be useful for avoiding unnecessary surgical procedure.

Purpose To report five cases of patients diagnosed with differentiated thyroid carcinoma (DTC) with uptake in the thymic area after high-dose treatment with I-131 and to evaluate the potential causes and therapeutic management. Methods Five cases of young female patients with a mean age of 36.6 years (24–43) who had been treated with a mean dose of 106 mCi of I-131 (100–150 mCi) showing tracer uptake in the thymic area are reported. An I-131 whole-body scan (131I-WBS) was performed 7 days after therapeutic dose administration to each patient. Anterior and posterior planar images, followed by SPECT/CT of the head, neck and superior mediastinum were acquired in all patients. Thyroglobulin levels were measured with and without hormone replacement therapy in all cases. Samples taken from the superior mediastinum were sent to pathology for analysis, which confirmed the presence of thymic tissue. Results Two patients underwent elective total thymectomy due to the gross characteristics of the gland, local 131-I uptake, and high thyroglobulin levels. The remaining three patients had already undergone thymectomy as part of neck dissection during initial surgery, and no further invasive interventions were therefore performed. Pathological examination revealed no metastases in these five patients. Conclusions Thymus visualization in young patients after administration of therapeutic doses of I-131 seems to be a more common finding than usually thought. Absence of metastasis in the thymus despite high thyroglobulin levels was confirmed in all cases. Based on these results, we suggest that a more expectant and less aggressive therapeutic approach could be used. We also suggest that I-131 therapy for DTC should be considered in classification of the potential causes of true thymic hyperplasia in the subgroup of patients recovering from a stressor.

Thymic hyperplasia is a rare cause of an anterior mediastinal mass in children. True thymic hyperplasia is characterized by massive thymic hypertrophy with retention of normal thymic architecture, and must be distinguished from more commonly seen tumors of the anterior mediastinum. Previous reports of thymic hyperplasia primarily have been descriptive, with minimal analysis of the cellular characteristics of the tumor. To better describe the cellular characteristics of thymic hyperplasia, the authors report on a 10-year-old boy found to have an asymptomatic left paracardiac mass during cardiac evaluation. The mass enlarged rapidly during the following 2 weeks and filled the entire left side of the chest. An open biopsy specimen showed normal thymic architecture. Because of the size and rapid growth of the mass, the authors decided to resect it. Cellular analysis of the mass did not show differences from normal thymic elements. There was conserved thymic architecture. The expression of cell surface markers and the proliferative response of thymocytes to cytokines appeared to be normal. Similarly, a long-term cell culture of the thymocytes did not result in clonal proliferation. Immunohistologic staining of stromal cells showed no striking differences from normal thymic elements. Moreover, immunologic analysis of the patient by standard hematological parameters, lymphocyte subsets, quantitative immunoglobins, and immunoelectrophoresis showed no abnormalities except for lymphocytosis, which resolved after tumor resection. The authors conclude that thymic hyperplasia occurs in immunologically normal children and may exhibit (1) rapid growth with minimal associated symptoms, (2) normal thymic architecture, (3) normal thymocyte and stromal growth characteristics in vitro, and (4) a normal thymocyte response to cytokine stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

True thymic hyperplasia is a very rare entity. We present an instance of idiopathic true massive thymic hyperplasia in a 9-month-old girl with a very large left-sided mediastinal mass noted on diagnostic imaging. Percutaneous biopsy revealed normal thymic tissue. Steroids were administered with no response. Surgery may be required in patients with respiratory distress unresponsive to steroids.

A case of true thymic hyperplasia (TTH) associated with thymic hemorrhage (TH) was observed in a 22-year-old male patient who presented with persistent cough and thoracic pain due to an anterior mediastinal mass. The diagnosis of TTH was supported by the observation that the mediastinal mass was essentially composed of histologically normal thymic lobules with preserved cortico-medullary differentiation. The TTH tissue presented multiple areas of hemorrhage associated with the presence of large, tortuous, abnormal vessels in the thymic stroma. Foci of spindle cell proliferations resembling an epitheliod hemangioma were also seen. This finding raises the possibility that vascular malformations, perhaps due to an abnormal growth of the thymus, may be responsible for some cases of TH associated with TTH.

症例は2歳11ヵ月の男児.出生時特に異常所見はなかったが, 生後3ヵ月の胸部単純X線写真で右上縦隔の拡大を指摘された.その後右上葉無気肺, 肺炎等の呼吸器症状が出現し入退院を繰り返した.胸腺肥大の疑いでステ獄イド負荷療法を2回施行したが, いずれも無反応であった.2歳9ヵ月時, 胸腺腫の疑いで手術を施行した.び漫性に肥大した胸腺を認め胸腺全摘除術を施行した.摘出した胸腺は大きさ10×7×6cm, 重さ210gであった.病理組織は正常胸腺構造を呈し, 観察した範囲内に胚中心の形成はみられなかった.最終診断はTrue thymic hyperplasiaとした.

Tumor necrosis factor (TNF) alpha inhibitors are effective in the treatment of inflammatory rheumatic diseases. Despite their effectiveness, anti-TNF drugs have some drawbacks such as severe adverse effects including infections and possibly lymphoproliferative disorders. In this report we described a case of juvenile idiopathic arthritis patient who developed thymic enlargement (true thymic hyperplasia), mediastinal lymphadenopathy and pleurisy associated with systemic symptoms under Etanercept treatment. The clinical presentation was highly suggestive of malignancy and the patient underwent diagnostic mediastinoscopy with biopsy.

A case of massive true thymic hyperplasia in a child of eleven months is described. This rare disorder must be included in the differential diagnosis of a mediastinal mass in children. Diagnosis and management are discussed and the relevant literature is reviewed.

Thymic hyperplasia describes a nonneoplastic condition of the thymus, characterized by an increased number of the constituent cells of the organ. This concept was initially introduced by Castleman in the late 1940s to define the thymic changes seen in association with myasthenia gravis [1]. Years later, Rosai and Levine [2] in the A.F.I.P. fascicle on “Tumors of the thymus” identified two distinct types of thymic hyperplasia on the basis of histopathologic criteria: true thymic hyperplasia, defined by an increase in the size and weight of the organ that retains a normal microscopic morphology, and lymphoid (or follicular) hyperplasia, characterized by the presence of lymphoid follicles with active germinal centers in the thymic medulla. In the last case, the size and weight of the thymus can be increased, but in most instances they are within normal limits. The term thymitis (or autoimmune thymitis), originally coined by Goldstein in 1966 [3], has been used interchangeably by some authors to denote the lymphoid hyperplasia of the thymus, since its morphological features, both at the light and electron microscopic levels, are markedly similar to those observed in any chronic inflammatory process of other organs [4, 5], being characterized by the presence of peripheral B lymphocytes, lymphoid follicles, and diffuse plasmacytosis.

We report on a 5-week-old male infant with recurrent respiratory distress since birth and congenital thymic hyperplasia. Acute life-threatening thymic bleeding apparently from ruptured thymic cysts into the pleural spaces complicated the clinical situation. Thoracotomy and complete thymectomy were performed. Histologic examination revealed normal thymic architecture with cysts of different sizes and an increased thymic weight of 30 g. The combination of true thymic hyperplasia and cyst bleeding in a newborn has not been previously reported and will be discussed in relation to the available literature on respiratory distress due to thymic pathology in childhood.

CD3+ T-cell large granular lymphocyte (LGL) leukaemia is a mature T-cell neoplasm of cytotoxic T-lymphocytes. Neutropenia represents the most frequent peripheral blood cytopenia associated with CD3+ T-cell LGL leukaemia. A wide variety of diseases associated with LGL leukaemia have been reported, both autoimmune and neoplastic. We describe for the first time the association of true thymic hyperplasia with CD3+ T-cell line LGL leukaemia. The patient presented with severe symptomatic neutropenia. Complete and persistent haematological and molecular remission was induced with an association of low-dose methotrexate and cyclosporin A, followed by thymectomy.

A mediastinal tumor mass in a 53-year-old woman with the sole complaint of persistent cough was studied and found to be a true thymic hyperplasia associated with a unilocular thymic cyst, felt to be of the congenital type. The bilobed thymus obtained was macroscopically and microscopically normal except for the size (12 x 12 x 3.5 cm) and weight (97 g). It showed normal thymic tissue segregated in lobules separated by adipose septa and with good corticomedullary separation. There were numerous Hassall corpuscles in the medullary portion. The thymus-to-adipose ratio was approximately 80:20. In the anterior aspect of the thymus, there was a 5-cm unilocular cyst, filled with a clear watery fluid, and with smooth and gray white inner surfaces. Microscopically, the cyst was lined by a cubic and low cylindrical nonciliated epithelium with normal-appearing thymic tissue in the cyst wall. This very unusual combination of true thymic hyperplasia and unilocular thymic cyst has not been previously reported in the literature.

A 24-year-old woman with a history of papillary thyroid carcinoma presented with increased thyroglobulin. A whole-body scan performed after 1-131 treatment showed uptake in the anterior mediastinum and the neck. A computed tomographic scan showed a prominent thymus with slightly lobulated borders. This computed tomographic appearance is atypical for thymic hyperplasia, so the possibility of metastatic disease was raised. A right neck dissection, mediastinal exploration, and thymectomy were performed. Pathologic analysis revealed metastatic carcinoma of the right cervical nodes, minimal residual thyroid, and true thymic hyperplasia with no evidence of metastatic carcinoma to the thymus. False-positive anterior mediastinal uptake of radioiodine can be caused by thymic hyperplasia, and thus should be considered even if the thymus has lobulated borders.

Thymic neoplasms are a common cause of an anterior mediastinal mass and may be benign or malignant. Thymic cysts are congenital or acquired and may be associated with a thymic malignancy. True thymic hyperplasia and thymic lymphoid hyperplasia may enlarge the thymus and simulate a neoplasm. Thymoma and thymic carcinoma are epithelial malignancies with distinct clinicopathologic features. Thymic carcinoid is a rare aggressive neuroendocrine malignancy associated with multiple endocrine neoplasia 1. Thymolipoma is a benign neoplasm. Hodgkin and non-Hodgkin lymphoma may primarily or secondarily involve the thymus. Primary mediastinal germ cell tumors may arise primarily within the thymus and include mature teratoma, seminoma, and non-seminomatous malignant germ cell tumors.

Different benign and malignant processes should be included in the differential diagnosis of a patient presenting with a mediastinal mass, depending on the age and clinical features. These include thymoma, T-cell lymphoblastic lymphoma/leukemia, thymic follicular hyperplasia, true thymic hyperplasia, and others. Thymomas are mediastinal neoplasms arising from thymic epithelial cells that exhibit heterogeneous morphology and, in most cases, are admixed with various proportions of non-neoplastic cortical thymocytes1–5. The histological diagnosis of thymoma is usually based on the recognition of neoplastic epithelial cells, but some diagnostic problem can be encountered in cases with abundant lymphoid component, especially when only small tissue fragments (e.g. by mediastinoscopy) or cytological material are available. Immunohistochemistry can provide some help in difficult cases by demonstrating more precisely the presence of epithelial cells using cytokeratin-specific antibodies6. An extremely precise characterization of cortical thymocytes can be obtained by immunophenotypic demonstration of several membrane, cytoplasmic or nuclear antigens such as CD1, coexpressed CD4 and CD8 antigens, cytoplasmic CD3 and nuclear Terminal deoxynucleotydil transferase (TdT)7.