Histologically unique case of combined small cell and squamous cell carcinoma in a polypoid bronchial tumor

Presented herein is a case of combined small cell and squamous cell carcinoma in a polypoid bronchial tumor, showing a histologically unique progression, in a 76-year-old Japanese man. A bronchofiberscopic examination revealed that the bronchus (left B3) was occluded by the polypoid tumor. Biopsies were performed, and the pathological diagnosis was poorly differentiated squamous cell carcinoma. The patient consequently underwent a left upper lung lobectomy. The surgical specimen was described as a 24 x 8 x 8 mm soft tumor, emanating from the bronchial wall (left B3). Histologically, the tumor had two distinct components: (i) nearly the entire tumor was composed of atypical small round cells, with a high nuclear-cytoplasmic ratio, in the lamina propria, under the basement membrane; and (ii) the surface of the tumor was composed of poorly differentiated squamous cell carcinoma that had proliferated primarily above the basement membrane but there was also some proliferation, seen as island-like formations, below the basement membrane. The histological diagnosis was combined small cell and squamous cell carcinoma. It was suspected that poorly differentiated squamous cell carcinoma, generated in the bronchial epithelium, had caused small cell carcinoma resulting from neuroendocrine differentiation during its invasion into the lamina propria.