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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.

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 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.

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.

Pure red cell aplasia was diagnosed in a 35-year-old otherwise healthy woman. Chest computed tomography (CT)-imaging investigation, detected an upper mediastinal mass corresponding to the thymus gland. A thoracotomy was performed and an enlarged thymus mass was removed, rapidly followed by a full hematologic recovery. Thymic histology confirmed a significant degree of hyperplasia. We conclude that not only thymomas but also other types of thymic pathology may be associated with this type of hematologic dyscrasia.

Massive thymic hyperplasia is an extremely rare form of true thymic hyperplasia most often described in infants and children. Hyperplasia of this order is not known to occur in any other organ, and its etiology and prognostic significance remain unknown. As there is no accurate way of preoperatively differentiating massive thymic hyperplasia from other tumors of the thymus and anterior mediastinum, we advise excision in all cases for histological analysis and relief of mediastinal compression. This description of 4 cases updates the 30 previously reported cases, and includes a literature review.

Surgery of the thymus in childhood is uncommon. Thymectomy may be indicated for true thymic hyperplasia, myasthenia gravis, thymic tumors (benign or malignant), thymic cysts, and other rare conditions. Thymic lesions may involve a normally located gland or ectopic thymic tissue. The principal difference between adult and childhood thymic surgery concerns the pathology and the surgical indications. Once thymectomy or thymic surgery has been considered, the surgical technique is similar to that in adults.

True thymic hyperplasia and ectopic thymus are very rare in children. In embryologic aspect, thymus is distributed around cervical area and ends up in mediastinum. This case is simultaneous thymic hyperplasia of neck and mediastinum. Ectopic thymus in the neck and thymic hyperplasia in the mediastinum in children were reported 2 and 7 cases respectively in Korea. In Clinical aspects, these thymic hyperplasia were presented by mass. So we should suspect these benign condition to avoid unnecessary operation or biopsy. We report a case of true thymic hyperplasia in the mediastinum with ectopic thymus in the neck in a 4-month-old male infant and review the relevant literature. We believe this is the first reported case in the world of true thymus hyperplasia in the mediastinum with cervical ectopic thymus in the neck

A case of massive true thymic hyperplasia with non-Hodgkin's lymphoma of the mediastinum is reported in a 14-year-old boy. Computed tomographic scan of the chest showed a mass of the anterior mediastinum and conspicuous swelling of the lymph nodes in the upper and lower mediastinum. They were grossly resected. The tumor of the anterior mediastinum was histologically diagnosed as true thymic hyperplasia. All of the mediastinal lymph nodes were diagnosed as non-Hodgkin's lymphoma, diffuse, mixed small and large cell type.

Diffuse thymic hyperplasia following chemotherapy for nodular sclerosing Hodgkin's disease is reported in an 18-year-old woman. The patient's course was complicated by drug-induced pulmonary fibrosis and disseminated herpes zoster. Subsequently she developed genuine diffuse thymic hyperplasia with disappearance of herpes zoster and apparently complete remission of Hodgkin's disease. We stress that restoration of the host immune system, particularly the cell-mediated immunity, is essential for successful control of Hodgkin's disease and the thymic hyperplasia is most likely a favorable sign for it, which should not be confused with recurrence of Hodgkin's disease.

A large thoracic mass and a mediastinal lymph node were excised from an infant with a peripheral blood and bone marrow lymphocytosis. The 224-mass was composed of histologically normal thymus, and the lymph node architecture was partially effaced. Hypogammaglobulinemia was detected two years after thymectomy. The enormous thumus in this case fits the classic gross pathologic definition of hyperplasia. The possibility of associated thymic hyperfunction in this case is discussed.

Two cases of massive thymic hyperplasia (MTH) are reported in boys 11 and 14 years of age. Each was detected on routine chest roentgenogram as a large anterior mediastinal mass. Total thymectomy was successfully performed and the two patients were alive and well 9 and 17 years later. The thymectomy specimens weighed 324 and 490 gm, respectively, and had uniform histologic anatomy without lymphoid nodules. Morphometric study documented the massive increase in thymic parenchyma (i.e., true hyperplasia) with retention of a normal proportion of connective tissue expected for age. Since thymic size and structure normally become modified with age, it is essential to consult standard reference data when evaluating hypertrophy and hyperplasia. Gross and histologic features distinguishing MTH from childhood thymoma are discussed.

Massive thymic hyperplasia (weight, greater than 100 gm) is a rare lesion in adolescents, and most of the anterior mediastinal masses are true neoplasms. We present a case of massive thymic hyperplasia, in which a 680-gm thymus was resected from a 15-year-old boy. We also present the computed tomographic scan appearance of this lesion, which differed from that previously described, and correlate the histological and radiological findings.

Although the term thymic hyperplasia is used most commonly to indicate the occurrence of germinal centers in the thymus, cognizance must be taken of the fact that such centers may occur in apparently normal thymuses in both children and adults. A concept of thymic compartmentalization is proposed with origin of germinal centers in the perivascular space (extraparenchymal compartment) of the thymus. These germinal centers contain a high percentage of B lymphocytes in contrast to the true thymic parenchyma. Although the significance of germinal centers in the thymus parenchyma. Although the significance of germinal centers in the thymus in myasthenia gravis remains controversial, removal of nonneoplastic thymus in this condition is of proven therapeutic value. A variety of neoplasms originating in the thymus have previously been lumped together under the single term "thymoma." It is apparent, however, that thymoma, thymic carcinoid, various lymphomas, and germ cell tumors that arise in the thymus differ not only pathologically but also in their clinical behavior. Thymoma is regarded as an epithelial neoplasm and ultrastucturally is characterized by many desmosomes and tonofilaments. The lymphocytes do not behave in a malignant manner, and lymphomas of the thymus should be sharply separated from true thymoma. Poorly differentiated thymic carcinoma and histiocytic lymphoma may be distinguishable only by the electron microscopic demonstration of desmosomes and filaments in the thymic carcinoma. The evidence that Hodgkin's disease of the thymus ("granulomatous thymoma") is not a variant of thymoma appears overwhelming. Lymphoblastic lymphoma of the thymus is a distinctive neoplasm that is especially prevalent in teenage males. High levels of terminal transferase characterize the lymphoblasts and there is a striking tendency for leukemia to occur. Thymic carcinoid is usually nonfunctional, although one-third of the reported cases are associated with Cushing's syndrome. On light microscopy a ribbon pattern and punctate necroses are characteristic of thymic carcinoids. Electron microscopic demonstration of many dense core granules is invaluable in establishing this diagnosis. An important clue to the diagnosis of thymic seminoma (a neoplasm that shows the same radiosensitivity as its testicular counterpart) is the frequent presence of epithelioid and giant cell granulomas and germinal centers. Separation of the various thymic neoplasms described not only is justifiable on pathologic grounds but is often essential for appropriate patient investigation and treatment.

In this report, we describe the clinicopathological features of 4 patients with true thymic hyperplasia. This controversial thymic lesion has only recently been defined as a variable, often massive enlargement of the thymus characterized by a nearly normal microscopic structure. Our study of 4 patients and review of the literature indicate that true thymic hyperplasia has a well-defined clinicopathological profile: prevalence in children or young male patients, absence of associated autoimmune diseases, and often presence of respiratory distress or peripheral blood lymphocytosis, or both. True thymic hyperplasia should be considered in the differential diagnosis of anterior mediastinal masses in children and young adolescents.

A 12-year-old boy with massive true thymic hyperplasia presented with respiratory distress and dysphagia. The thymus weighed 245 g and demonstrated normal cortical and medullary components histologically. The findings in this case were compared with the clinical and pathologic features of seven previously published cases of massive hyperplasia and with cases of mild or "borderline" hyperplasia. By electron microscopy and immunoperoxidase techniques, myoid cell differentiation was demonstrated, the first documented example of myoid cells in thymic hyperplasia. These findings support the hypothesis that myoid cells are a normal component of thymic parenchyma.

The condition of true thymic hyperplasia is defined as an increase of both size and weight of the gland while it maintains normal microscopic architecture. It occurs in three different clinicopathologic forms: True thymic hyperplasia without any other disease is extremely rare and only seven well documented examples could be found in a review of the literature. Enlargement of the thymus gland is reported as a form of rebound phenomenon in a number of conditions like recovery from severe stress situations, after administration of steroids, and after treatment of malignant tumors. Finally, thymic hyperplasia has been described in association with endocrine abnormalities, sarcoidosis, and Beckwith-Wiedeman syndrome.

Background Thymic enlargement is a common and physiological finding in children and neonates’ X-rays, but it is usually asymptomatic. Occasionally it can cause respiratory distress. In most cases the aetiology of this expansion remains unclear and it is diagnosed as a thymic hyperplasia. True thymic hyperplasia is defined as a gland expansion, both in size and weight, while maintaining normal microscopic architecture. Often it is a diagnosis of exclusion and prognosis is good. Thymic haemorrhage is an unusual condition related to high foetal and neonatal mortality. Case Presentation We report a case of spontaneous massive thymic haemorrhage in a newborn developing at birth acute respiratory distress associated with severe bilateral haemothorax. Thymic enlargement was evident after pleural evacuation and confirmed by radiographic, Computed Tomography (CT) images and Magnetic Resonance Imaging (MRI) sequences. The spontaneous resolution of this enlargement seen with CT scan and MRI sequences suggested a thymic haemorrhage; surgery was not necessary. Conclusion Thymic haemorrhage should be considered in newborn infants with pleural effusion, mediastinal space enlargement and Respiratory Distress.