# Case report: Primary pulmonary rhabdomyosarcoma exhibiting epithelial morphology and unusual immunophenotype – A significant diagnostic pitfall

## TL;DR

A rare case of lung rhabdomyosarcoma was misdiagnosed due to its unusual appearance and protein markers, highlighting the challenges in identifying this cancer.

## Contribution

This is the first reported case of epithelioid rhabdomyosarcoma with strong TTF-1 expression using two antibody clones.

## Key findings

- The tumor showed epithelioid morphology without typical rhabdomyoblastic features.
- Tumor cells were positive for skeletal muscle markers and TTF-1, mimicking neuroendocrine carcinoma.
- The case highlights the diagnostic difficulty of PPRMS due to its atypical immunophenotype.

## Abstract

Primary pulmonary rhabdomyosarcoma(PPRMS) is an extremely rare neoplasm.Herin,we report a case of a 60-year-old male who presented with a one-week history of progressive chest tightness and non-radiating chest pain. Contrast-enhanced thoracic computed tomography (CT) demonstrated a large,11cm mass in the right inferior lung lobe. Intraoperative frozen-section analysis initially misclassified the lesion as a poorly differentiated carcinoma. The patient subsequently underwent thoracoscopic right lower lobectomy with systematic mediastinal lymph node dissection. Histological examination demonstrated sheets of uniformly sized epithelioid cells arranged in an organoid nesting growth pattern, with areas of geographic necrosis and no definitive morphological features indicative of rhabdomyoblastic differentiation. Immunohistochemical(IHC) staining confirmed diffuse positivity for skeletal muscle lineage markers(desmin, myogenin, MyoD1) in the tumor cells, thereby establishing a definitive diagnosis of rhabdomyosarcoma. Notably, this case showed diffuse expression of thyroid transcription factor 1 (TTF-1) using both the 8G7G3 and SPT24 clones, alongside immunoreactivity for neuroendocrine markers(CD56, PGP9.5) and focal expression of epithelial markers(AE1/AE3, CAM5.2), This immunophenotypic profile may mimic that of poorly differentiated neuroendocrine carcinoma, contributing to diagnostic confusion. The constellation of these unusual histological and immunophenotypic features presents a substantial diagnostic challenge. We herein elaborate on these findings to enhance clinical awareness and facilitate accurate diagnosis in future cases. To the best of our knowledge, To the best of our knowledge, this is the first reported case of epithelioid rhabdomyosarcoma demonstrating diffuse and strong nuclear TTF-1 immunoreactivity across two distinct antibody clones.