RESUMEN
A 57-year old man presented with a 3.6 cm submucosal rectal cystic mass detected on colonoscopy and MRI. The lesion was deroofed, with rectal cyst fluid and cyst wall received for examination. Smears contained atypical cells occurring singly and in small aggregates, with epithelioid and spindled cells with hyperchromatic nuclei. Occasional cells exhibited intranuclear cytoplasmic inclusions. The concurrent histopathology specimen from the rectal cyst wall showed good correlation with the cytologic findings. The lesion was confined to the muscularis propria, with predominantly epithelioid cells and occasional spindled forms arranged in sheets, nests, and trabeculae with intervening fibrillary and fibrous stroma. Mitotic figures were scant, with no atypical mitoses or necrosis. An extensive immunohistochemical panel was performed, showing strong and diffuse nuclear and cytoplasmic staining with S100 and moderate to strong intensity nuclear staining with SOX10, in keeping with a benign epithelioid peripheral nerve sheath tumour. The patient remains well 3 years following diagnosis, and two subsequent biopsies at the site show no evidence of recurrent disease. We herein provide the first cytologic description of this lesion, with histopathologic correlation and a brief review of other differential diagnostic possibilities.