Gastrointestinal stromal tumour of the rectum.

Gastrointestinal stromal tumour (GIST) has been immunohistochemically defined as the tumour lacking differentiation towards either leiomyomatous tumour or schwannoma. We report a 75-year-old man who underwent an abdominoperineal resection of a large submucosal tumour of the rectum. The excised specimen was revealed to be an elastic soft tumour, 8 x 7 x 6 cm in size, which histologically consisted of spindle-shaped cells without nuclear atypia. The mitotic count was fewer than 2 per 10 high-power fields. The tumour cells were positive for staining of CD34 and c-kit protein, while the lesions were negative for alpha-smooth muscle actin, HHF-35, neuron-specific enolase, and S-100 protein. The diagnosis of GIST was confirmed by immunohistochemical examination of the tumour. From these findings, the present case is thought to be potentially malignant, and a long-term follow-up observation is needed for the case.