Diagnostic conundrum of a perineal tumour: a rectal gastrointestinal stromal tumour mimic

Gastrointestinal stromal tumour (GIST) is a common mesenchymal tumour arising in the gastrointestinal tract, but not frequently encountered in the rectum. Herein, we describe a case of a rectal GIST which mimicked histomorphological features of a schwannoma; thus, making intraoperative frozen section evaluation challenging. Although subsequent immunohistochemistry and molecular findings readily confirmed the diagnosis of a GIST, we wish to draw attention to three clues that will help the pathologist steer clear of this potential diagnostic pitfall. One, GISTs are relatively more common than schwannomas in the rectum. Two, schwannomas usually have very little mitoses. Three, rectal GISTs commonly exhibit nuclear palisades. We also discuss the diagnostic, prognostic and therapeutic functions of immunohistochemical and molecular investigations. As the surgical intent for rectal GISTs is for en-bloc excision with wide margins, we surmise that the intraoperative consult should include GIST as a possible differential diagnosis for rectal mesenchymal tumours. In view of the potential for neoadjuvant treatment with imatinib before surgical excision to preserve sphincter function, a multidisciplinary approach is recommended for establishing most effective treatment strategy in these rare complex cases.