Answer: In-situ adenocarcinoma and small cell carcinoma

Histology: The surface mucosa of the gallbladder has an abnormal papillary configuration. At higher magnification there is a range of cytologic atypia from adenomatous to full thickness cytologic atypia diagnostic of in-situ adenocarcinoma. Focally, there is acute inflammation within the epithelium. There is a large mass of poorly differentiated tumor in the soft tissue surrounding the gallbladder, which extends to the mucosa. At higher magnification, the poorly differentiated tumor consists of cells with high nuclear to cytoplasmic ratios, nuclear molding, and indistinct nucleoli. Mitotic figures and apoptotic bodies are readily identifiable. These features are those of small cell carcinoma. Given the associated in-situ adenocarcinoma and the localization of the infiltrating tumor around the gallbladder without any other primary, it is most likely that the small cell component also arose from the gallbladder.

Discussion: In contrast to infiltrating poorly differentiated adenocarcinoma, the cytologic features are more typical of small cell carcinoma. The cells in poorly differentiated adenocarcinoma would have larger nuclei with more prominent nucleoli and abundant cytoplasm. In contrast to reactive epithelial atypia that consists of uniformly enlarged vesicular nuclei with central eosinophilic prominent nucleoli, the dysplastic nuclei in the mucosa are hyperchromatic and pleomorphic. Also, the atypia is present in areas away from inflammation. The presence of a nested appearance of the tumor with molding of tumor cells is more typical of small cell carcinoma than lymphoma. If necessary, the distinction between these two may be readily performed using epithelial and lymphoid markers.