Microscopic Description -- A Mass in the Right Ischial Rectal Fossa


Low power hematoxylin and eosin stained histologic sections demonstrate a cellular spindle cell tumor entrapping fat cells. The spindle cells are relatively uniform, occasionally forming fascicles or a herring bone pattern. There is occasional focal nuclear palisading. Higher power reveals two cell types: spindle cells and epithelial cells. The epithelial cells contain large vesicular nuclei. Some epithelial cells form glandular structures which show intraluminal eosinophilic secretions. The secretions stain with PASD. An occasional mitoses can be seen. In certain areas the cells are more pleomorphic. Nucleoli are not prominent.

Immunostaining reveals diffuse positivity for Vimentin. AE1/3 (Images A & B) shows heterogeneity in staining: certain areas are diffusely positive while in other areas, only the glandular structures are positive. Epithelial membrane antigen shows positivity of the glandular structures.

Stains were negative for smooth muscle actin, S100 and desmin.

Other stains which could be used, but were not done on this case, are CD34-which would be negative, and CD99-which is positive in 60-70% of the cases.


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