Background
A gastrointestinal stromal tumour (GIST) is a mesenchymal neoplasm with variable behaviour characterized by differentiation towards the interstitial cells of Cajal. [1] Most GISTs harbor gain-of-function mutations of the KIT or PDGFRA oncogene and progress by the stepwise inactivation of tumour suppressor genes. [1]
GIST can occur anywhere in the gastrointestinal tract (stomach 54%, small bowel 30%, colon and rectum 5%, oesophagus 1%). [1]
Clinical Perspective
The most common presentations include vague abdominal symptoms, an abdominal mass, symptoms related to mucosal ulceration, bleeding and tumour perforation. [1]
Imaging and histological analysis allow the surgeon to plan the best strategy, thus avoiding surgery for diseases for which it is not recommended (e.g. lymphomas or mesenteric fibromatosis) and considering neoadjuvant treatment. [2]
Imaging can identify lesions at risk for peritoneal contamination or bleeding (e.g. cystic and/or mobile masses) and biopsy should only be performed at specialised centres. [2]
Imaging Perspective
Anorectal GISTs are well-circumscribed, non-circumferential, predominantly infralevator, and intramural or exophytic masses. [3]
On MRI, the GISTs show low signal intensity on T1WI, hyper or isointensity with hyperintense areas on T2WI and they exhibit markedly heterogeneous enhancement after gadolinium administration. Data is scarce regarding DWI, but the high cellularity is expected to cause restriction in solid areas. [4]
The invasion of adjacent organs, bowel obstruction and local lymphadenopathy are uncommon. [5]
The standard approach to rectal nodules is represented by biopsy or excision after endorectal ultrasound assessment and pelvic MRI. [2]
The essential diagnostic criteria according to the WHO classification [1] are:
1) An intramural, submucosal or subserosal mass on imaging;
2) Spindle cell, epithelioid or mixed morphology in histological analysis;
3) KIT and/or DOG1 immunopositivity.
There is also a desirable criterion – KIT or PDGFRA gene mutation identification.
Outcome
Histopathological and immunohistochemistry analyses of the biopsy specimen revealed a spindle-cell neoplasm, without mitosis and with CD117 positivity, in keeping with GIST.
The patient underwent surgical resection with free margins.
A widely used risk classification was proposed by the Armed Forces Institute of Pathology, which incorporates the three main prognostic factors in localised GISTs: primary mitotic count, tumour size and tumour site. [2]
The standard treatment of localised GISTs is a complete surgical excision, without dissection of clinically negative lymph nodes. A laparoscopic/robotic approach is discouraged in patients with large tumours because of the risk of rupture, which is an important adverse prognostic factor. [2]
Imatinib is the standard treatment for locally advanced, inoperable, and metastatic patients. [2]
There are no published data to indicate the optimal routine follow-up for surgically treated patients with localised disease. Relapses occur more often in the liver and/or peritoneum. [2]
Take Home Message / Teaching Points
GIST is one of the differential diagnoses in intramural and exophytic rectal masses;
MRI allows for a decision regarding neoadjuvant treatment, surgical planning and biopsy risk stratification;
Written informed patient consent for publication has been obtained.
