What is this condition and how could it be treated?

A 67-year-old, previously fit, woman presented with a history of rectal bleeding.
Per rectum examination revealed a
low rectal mass, which on endoscopy was
demonstrated to be submucosal. MR rectum
confirmed a 5 cm rectal mass displacing the
rectum anteriorly. Chest x-ray and abdominal
CT did not reveal any metastatic disease. Biopsies were consistent with high grade gastrointestinal stromal cell tumour (GIST,) C-Kit was
positive and Ki67 greater >20%. The patient
came forward for an abdominoperineal rectal
resection with permanent stoma and had an
uncomplicated postoperative course.
GISTs are rare tumours that develop from
interstitial cells of Cajal. Only 5% of GIST
tumours arise in the rectum and GIST should
be considered as differential diagnosis for
submucous rectal lesions. Complete resection
is recommended to achieve local disease
control either by local resection or rectal
resection depending on size. The benefit
of imatinib as neoadjuvant treatment is
uncertain.