A rare case of primary thymic adenocarcinoma mimicking small cell lung cancer

Thymic enteric type adenocarcinoma: A case report with cytological features

Primary thymic adenocarcinoma is an extremely rare tumor, and thymic enteric type adenocarcinoma has recently been proposed as a distinct pathological entity. Herein, we report the first cytological description of thymic enteric type adenocarcinoma. A 29-year-old Japanese female without a significant past medical history was found to have an abnormal chest shadow. Chest computed tomography demonstrated a well-circumscribed tumor in the anterior mediastinum, and thymectomy was performed. The Papanicolaou staining of the touch smear of the resected tumor demonstrated tightly cohesive epithelial cell clusters in a necrotic background. These cells were cuboidal to columnar in shape and had large round to oval nuclei with conspicuous nucleoli. Some of these neoplastic cells had intracytoplasmic mucin. Immunocytochemically, the neoplastic cells were positive for cytokeratin 20 and CDX-2. Histopathological study revealed tubular and papillotubular neoplastic growth composed of cuboidal to columnar neoplastic cells that contained large round to oval nuclei. Some of the neoplastic cells had intracytoplasmic mucin. Immunohistochemical study confirmed the expression of cytokeratin 20 and CDX-2. The final diagnosis of thymic enteric type adenocarcinoma was made. The cytological and immunocytochemical features of this case led to a diagnosis of enteric type adenocarcinoma. However, these features alone cannot be differentiated from a metastatic adenocarcinoma arising from the gastrointestinal tract. Cytological examination of a fine-needle aspiration of the mediastinal tumor has been reported to be useful in making a diagnosis. Therefore, an awareness of this new pathological entity is important for differentiating a thymic tumor from a metastatic carcinoma in the thymus.

Cytologic Characteristics of Thymic Adenocarcinoma with Enteric Differentiation: A Study of Four Fine-Needle Aspiration Specimens

Thymic adenocarcinoma is extremely rare. Although its histologic features have been occasionally reported, a lack of description of the cytologic features has hampered the prompt and accurate diagnosis of this condition. Herein, we describe the cytologic findings and histology of four aspiration cytology specimens of thymic adenocarcinoma. The specimens were obtained from primary tumors, metastatic lymph nodes, and pericardial effusions. All four specimens showed three-dimensional glandular clusters with a loss of polarity and nuclear overlapping. One specimen had extensive extracellular mucinous material. Three specimens contained tumor cells with intracytoplasmic vacuoles. While the specimen with extracellular mucin showed relatively mild cytologic atypia, other specimens exhibited more atypical cytologic changes: irregular nuclear membranes, a coarse chromatin pattern, and prominent nucleoli. The cytologic features were correlated with the histologic features in each case of enteric type thymic adenocarcinoma. The differential diagnosis included other thymic carcinomas, yolk sac tumors, and metastatic adenocarcinoma from the lung or colorectum.

Characterization of genetic aberrations in a single case of metastatic thymic adenocarcinoma

BACKGROUND

Thymic adenocarcinoma is an extremely rare subtype of thymic epithelial tumors. Due to its rarity, there is currently no sequencing approach for thymic adenocarcinoma.

METHODS

We performed whole exome and transcriptome sequencing on a case of thymic adenocarcinoma and performed subsequent validation using Sanger sequencing.

RESULTS

The case of thymic adenocarcinoma showed aggressive behaviors with systemic bone metastases. We identified a high incidence of genetic aberrations, which included somatic mutations in RNASEL, PEG10, TNFSF15, TP53, TGFB2, and FAT1. Copy number analysis revealed a complex chromosomal rearrangement of chromosome 8, which resulted in gene fusion between MCM4 and SNTB1 and dramatic amplification of MYC and NDRG1. Focal deletion was detected at human leukocyte antigen (HLA) class II alleles, which was previously observed in thymic epithelial tumors. We further investigated fusion transcripts using RNA-seq data and found an intergenic splicing event between the CTBS and GNG5 transcript. Finally, enrichment analysis using all the variants represented the immune system dysfunction in thymic adenocarcinoma.

CONCLUSION

Thymic adenocarcinoma shows highly malignant characteristics with alterations in several cancer-related genes.