RESUMO
Rectal leiomyosarcoma is an uncommon malignancy. Herein, we describe the clinicopathological features and biological behaviour of these tumours, and provide a practical approach to differential diagnosis, particularly with gastrointestinal stromal tumours (GIST). We report two cases in elderly men. In the first, the lesion was 2 cm from the anal sphincter, while it was located in the rectal ampulla in the second case. Histologically, both tumours were characterized by pleiomorphic, large spindle cells, presenting numerous mitoses and marked nuclear atypia. Immunohistochemical analysis showed that tumour cells coexpressed both actin and desmin, whereas CD117 and S100 protein were negative. The final diagnosis was leiomyosarcoma. One of the patients died of pulmonary metastasis within six months. The second patient had bone metastasis, but was lost to follow up. This report underlines the potential diagnostic problems raised by rectal leiomyosarcoma and emphasizes the role of immunohistochemistry in achieving correct diagnosis, which has important clinical, therapeutic and prognostic consequences.