Sarcomatoid carcinoma of the thymus arising in metaplastic thymoma

A Rare Case of Metaplastic Thymoma Presenting With Myasthenia Gravis

Thymomas are tumors of the mediastinum often associated with autoimmune conditions, in particular myasthenia gravis. In contrast, among the fewer than 40 reports of metaplastic thymoma, myasthenia gravis is rarely found. We describe the fourth patient, and first man, with metaplastic thymoma and myasthenia gravis. A 34-year-old had acute onset of double vision with associated dysphagia and was found to have an elevation of serum acetylcholine receptor antibodies. He underwent a transsternal thymectomy. Tissue sections showed a biphasic proliferation of keratin-positive epithelial cells with a complement of spindle cells confirming the diagnosis of metaplastic thymoma. Terminal deoxynucleotidyl transferase (TDT)-positive T lymphocytes were rare and only found in the periphery of the tumor, consistent with thymic remnant. A YAP1::MAML2 gene fusion, with an in-frame fusion between genes YAP1 Exon5 (NM_001130145) and MAML2 Exon2 (NM_032427) was found, supporting further the diagnosis of metaplastic thymoma (Anchored multiplex RNA sequencing [Archer Dx, Boulder, CO] assay). The patient's gender and relatively young age, the presence of an autoimmune condition, and the lack of lymphocytic infiltrate all contribute unusual features to this case and suggest avenues for further exploration.

Loss of YAP1 C-terminus expression as an ancillary marker for metaplastic thymoma: a potential pitfall in detecting YAP1::MAML2 gene rearrangement

AIMS

Metaplastic thymoma is a rare thymic tumour characterized by Yes Associated Protein 1 (YAP1) and Mastermind Like Transcriptional Coactivator 2 (MAML2) gene fusions resulting from an intrachromosomal inversion of chromosome 11. Immunohistochemistry with an antibody directed against the C-terminus of YAP1 has shown loss of expression in YAP1-rearranged vascular neoplasms, poromas, and porocarcinomas. This study aimed to validate an anti-YAP1 C-terminal antibody as an ancillary immunohistochemical marker for the diagnosis of metaplastic thymoma.

MATERIALS AND METHODS

Ten metaplastic thymomas were selected for the current study. Fluorescence in situ hybridization (FISH), next-generation sequencing (NGS), and reverse transcription-polymerase chain reaction (RT-PCR) analyses were performed to detect YAP1::MAML2 fusions. We then performed immunohistochemistry to detect YAP1 C-terminus expression in 10 metaplastic thymomas, 50 conventional thymomas (10 each of type A thymoma, type AB thymoma, type B1 thymoma, type B2 thymoma, and type B3 thymoma) and seven thymic carcinomas.

RESULTS

All 10 cases showed narrow split signals with a distance of nearly two signal diameters and sometimes had false-negative results in YAP1 and MAML2 break-apart FISH (BA-FISH). Abnormal colocalized signals of the YAP1::MAML2 fusion were observed in all 10 cases using fusion FISH (F-FISH) assays. Eight of 10 cases with adequate nucleic acids were successfully sequenced and all showed YAP1::MAML2 fusions; in two cases the fusions were detected by both DNA and RNA sequencing and in six cases by RNA sequencing only. YAP1::MAML2 fusion transcripts were identified in four cases by RT-PCR. Metaplastic thymoma showed loss of YAP1 C-terminus expression in all 10 (100%) cases. All other thymic neoplasms showed retained YAP1 C-terminus expression.

CONCLUSION

YAP1 C-terminus immunohistochemistry is a highly sensitive and specific ancillary marker that distinguishes metaplastic thymoma from its mimics. BA-FISH assays could not effectively detect YAP1::MAML2 fusions due to the proximity of the two genes. Loss of YAP1 C-terminus expression is a reliable surrogate for the detection of YAP1::MAML2 fusions in metaplastic thymoma.

Combined Thymic Epithelial Neoplasms - a Review

Thymic epithelial neoplasms are the most common tumors of thymic origin but are overall rare in the general population. Their morphologic diversity, ranging from low grade to overtly malignant lesions, along with various histologic growth patterns make them a diagnostically challenging group of tumors. Very occasionally, thymomas and thymic carcinomas may develop in combination with other benign or malignant lesions of thymic origin, further complicating the diagnostic process. The focus of this review lies on the spectrum of thymic epithelial tumors that present with other thymic lesions in the same tumor mass, such as multilocular thymic cysts, neuroendocrine neoplasms, lymphomas, and germ cell tumors among others. Awareness of the existence of such unusual tumors may not only aid in their diagnosis but may also have implications for prognostic and therapeutic purposes.

Malignant Transformation of Metaplastic Thymoma into High-Grade Sarcomatoid Carcinoma: A Case Report

The correlation of histogenesis between metaplastic thymoma and thymic sarcomatoid carcinoma is unclear. We report a case of metaplastic thymoma transformed into high-grade sarcomatoid carcinoma. A 64  ×  54  ×  32 mm anterior mediastinal mass in a 61-year-old woman microscopically consisted mainly of classic metaplastic thymoma, with the center dominated by high-grade sarcomatoid carcinoma. In some areas, both epithelial and spindle cell components of the metaplastic thymoma showed increased cellular atypia, mitotic activity, and focal necrosis and gradually transformed into the polygonal/pleomorphic and spindle cell components of sarcomatoid carcinoma. Immunohistochemically, the characteristics of the polygonal/pleomorphic sarcomatoid cells were similar to those of the epithelial component of metaplastic thymoma, while the spindle sarcomatoid cells were more similar to the spindle cells component of metaplastic thymoma. The Ki-67 index was less than 5% in the metaplastic thymoma areas but up to 70% in the sarcomatoid carcinoma area. Radical operation and postoperative radiotherapy were performed. Multifocal relapses at the pleura occurred 13 months after surgery.

YAP1-MAML2 Fusion as a Diagnostic Biomarker for Metaplastic Thymoma

BACKGROUND

Metaplastic thymoma is a very rare tumor with only a few case reports documented in literature. Hence, its molecular features have not been well explored.

MATERIAL AND METHODS

Seventeen specimens of metaplastic thymoma were sequenced and retrospectively analyzed by fluorescence in situ hybridization (FISH) and immunohistochemistry in the study. In addition, seven cases of micronodular thymoma with lymphoid stroma and nine cases of type A thymoma were also investigated.

RESULTS

Among these metaplastic thymomas, fifteen cases showed classical histological features, and two cases displayed characteristic micronodular-like growth patterns. DNA and RNA based next-generation sequencing identified and confirmed highly recurrent Yes Associated Protein 1 (YAP1) - Mastermind Like Transcriptional Coactivator 2 (MAML2) translocation (13/17, 76.5%) in metaplastic thymoma but not in micronodular thymoma with lymphoid stroma (0/7, 0%) and type A thymoma (0/9, 0%). In addition, six nonsense mutations were also detected in the metaplastic thymoma. FISH in microdissection specimens indicated that both epithelioid and spindle cell components harbored YAP1-MAML2 gene rearrangements.

CONCLUSIONS

Our study explored the genetic alterations in epithelioid and spindle cell components in metaplastic thymoma. Furthermore, YAP1-MAML2 gene rearrangements emerged as a potential diagnostic biomarker helpful for distinguishing metaplastic thymoma from type A and micronodular thymoma with lymphoid stroma.

Metaplastic thymoma: a distinctive thymic neoplasm characterized by YAP1-MAML2 gene fusions

Metaplastic thymomas are rare biphasic thymic tumors that are characteristically well-circumscribed, confined to the thymus, and follow a benign to indolent clinical course. Their relationship to other thymic neoplasms remains unclear, and their molecular characteristics have not been defined. We report for the first time recurrent translocation events in metaplastic thymomas involving the Yes Associated Protein 1 (YAP1) and Mastermind Like Transcriptional Coactivator 2 (MAML2) genes. Eight metaplastic thymomas were retrieved from two institutions' archives over a 21-year period. Paraffin-embedded material from all cases underwent targeted DNA-based hybrid capture next-generation sequencing. Cases showing no somatic alterations subsequently underwent targeted RNA sequencing. Allele-specific real-time polymerase chain reaction was performed to detect GTF2I c.74146970T>A (p.L424H) mutations. All cases showed characteristic histologic features of metaplastic thymoma and demonstrated no local recurrence or distant metastatic disease at 1-22 years of follow-up. Six of eight cases were successfully sequenced, all showing YAP1-MAML2 fusions; in four cases the fusions were detected by DNA sequencing and in two cases by RNA sequencing. Two distinct products were identified: 5' YAP1 exon 1 fused to 3' MAML2 exons 2-5 or 5' YAP1 exons 1-5 fused to 3' MAML2 exons 2-5. All cases underwent allele-specific real-time polymerase chain reaction and demonstrated no GTF2I L424H mutations. Metaplastic thymoma is a distinct, clinically indolent thymic epithelial neoplasm characterized by YAP1-MAML2 fusion and lacking the GTF2I mutations found in Type A and AB thymomas.

Sarcomatoid Adenocarcinoma of the Ampulla of Vater

Sarcomatoid adenocarcinoma of ampulla of Vater is an extremely rare malignant neoplasm that displays both carcinomatous and sarcomatous component. A 58-year-old woman was admitted to our hospital under the suspicion of an ampulla of Vater cancer. Abdominal computed tomography and endoscopy demonstrated a bulging of ampulla and the biopsy specimen revealed an adenocarcinoma, well differentiated in the background of tubulovillous adenoma. So we performed the pylorus preserving pancreaticoduodenectomy. At postoperative biopsy, the tumor was composed of adenocarcinoma component and sarcomatoid component. Thus, a diagnosis of sarcomatoid adenocarcinoma of ampulla of Vater could be made. Here, we present a case of sarcomatoid adenocarcinoma of ampulla of Vater.

[Thymic carcinomas]

Thymic carcinomas (TC) are approximately 10 times less prevalent than thymomas but of high clinical relevance because they are more aggressive, less frequently resectable than thymomas and usually refractory to classical and targeted long-term treatment approaches. Furthermore, in children and adolescents TC are more frequent than thymomas and particularly in this age group, germ cell tumors need to be a differential diagnostic consideration. In diagnostic terms pathologists face two challenges: a), the distinction between thymic carcinomas and thymomas with a similar appearance and b), the distinction between TC and histologically similar metastases and tumor extensions from other primary tumors. Overcoming these diagnostic challenges is the focus of the new WHO classification of thymic epithelial tumors. The objectives of this review are to highlight novel aspects of the WHO classification of thymic carcinomas and to address therapeutically relevant diagnostic pitfalls.

Immunohistochemistry in the diagnosis of thymic epithelial neoplasms

The diagnosis of thymic epithelial neoplasms is normally rendered after careful evaluation of their histologic features. In some cases, however, the morphological heterogeneity or overlap with neoplasms of other organ systems can lead to diagnostic uncertainty. On the basis of this, the use of immunohistochemical stains as a diagnostic adjunct has become a popular tool. Although undoubtedly, immunohistochemistry has its role in the diagnosis of difficult cases, to date there are no specific markers that would distinguish thymic epithelial neoplasms from other tumors and interpretation of immunohistochemical results should only ever be made in conjunction with accurate morphologic analysis and careful clinical evaluation. This article will review the current knowledge of the immunohistochemical phenotype of thymic epithelial neoplasms with particular emphasis on its use for diagnostic purposes and the latest advances in this field.

Sarcomatoid carcinoma of the remnant stomach: report of a case

We herein report a case of sarcomatoid carcinoma that developed in a remnant stomach. A 76-year-old male with a history of distal gastrectomy for a duodenal ulcer 28 years earlier underwent investigation for a tumor in the remnant stomach. An endoscopic survey showed a round elevated tumor measuring 6 cm in diameter, and a biopsy specimen suggested carcinosarcoma. A total gastrectomy of the remnant stomach was performed, and the excised tumor was identified to be a malignant neoplasm consisting of both carcinomatous and sarcomatous components. A diagnosis of sarcomatoid carcinoma was made since the epithelial markers were positive even in the mesenchymal elements of the tumor. To our knowledge, only 4 cases of sarcomatoid carcinoma of the stomach have been reported in the English literature so far.

Metaplastic thymoma: report of 4 cases

Metaplastic thymoma (MT), accepted in the World Health Organization 2004 scheme, is a circumscribed tumor of the thymus exhibiting biphasic morphology. We herein describe the clinicopathologic features of four MTs and the differential diagnoses of this unusual tumor. There were three women and one man with mean age of 49.5 years. The patients were found to have mediastinal masses, and underwent surgical excision. One exhibited symptoms of myasthenia gravis, and the serum titer for anti-acetylcholine receptor antibody was positive. Grossly, the tumors were encapsulated, and showed vaguely multinodular, solid, tan-white to yellow cut surfaces. Histologically, they comprised epithelial islands intertwining with bundles of delicate spindle cells. The patients remained well after surgical excision at 5-55 months. Because of the distinctive histological appearance and benign clinical course, MT should be distinguished from other more aggressive mediastinal neoplasms displaying biphasic feature.

Metaplastic thymoma of the mediastinum. A clinicopathologic, immunohistochemical, and genetic analysis

Metaplastic thymoma is a rare primary thymic epithelial tumor, and its biologic behavior is uncertain. We report 7 cases of metaplastic thymoma. The spindle components of the tumor seemed to be undergoing epithelial-mesenchymal transition, which is characterized by the loss of E-cadherin expression. Epstein-Barr virus seemed not to have an etiologic role in metaplastic thymoma. No gross genetic alterations were found by using comparative genomic hybridization. The patients were alive and well in follow-up ranging from 11 to 172 months (mean, 81.7 months). The findings suggest that metaplastic thymoma has a benign clinical course. Unless the histomorphologic findings show malignant features, surgical excision alone is recommended.

Sarcomatoid thymic carcinoma arising in metaplastic thymoma: a case report

Metaplastic thymoma is an extremely rare tumor. To date, only 17 cases of metaplastic thymoma have been reported. To the best of the authors' knowledge, this is the second reported case of a sarcomatoid carcinoma arising in metaplastic thymoma; the carcinoma in this case is larger than that in the previous case. A 63-year-old woman with cough and asthenia for 2 weeks was admitted to the hospital. Computed tomography (CT) revealed a giant mass on the right side of the front mediastinum medium. The mediastinal tumor was excised, and additional pathological examinations, immunohistochemical tests, and electron-microscopic tests were performed. The tumor was diagnosed as a sarcomatoid carcinoma arising in metaplastic thymoma. Here, the authors discuss the clinical pathology of the sarcomatoid carcinoma arising in metaplastic thymoma and describe the biological behaviors with respect to the pathological features.