Answer: Adenomyoma with high grade dysplasia

Histology: see below

Discussion: Sections of the thickened portion of the gallbladder show disorganized smooth muscle bundles admixed with irregular tubular structures lined biliary type epithelium. The surface epithelium is benign, however, some of the crypts/tubular structures within the wall show complex architecture with small papillary fronds that are lined by glandular epithelium exhibiting uniform cytologic atypia with nuclear crowding and mitotic activity. No invasion into the surrounding stroma is identified (no stromal reaction is seen surrounding the round contours of these atypical glandular proliferations).
Dysplasia of the biliary epithelium can be difficult to distinguish from reactive atypia. The best clues are the uniformity of atypical cells in contrast to reac tive atypia where the cells are more heterogeneous. Grading of dysplasia is similar to grading in the colon. In low grade dysplasia the cells are basally located and have crowded, elongated nuclei (mild dysplasia) or show nuclei extend into the luminal portion of the cytoplasm (moderate dysplasia). In high grade dysplasia, the nuclei reach the surface of the cells (severe dysplasia) that may become disorganized and pleomorphic (carcinoma in situ).