Teaching Case

This 58-year-old man presented with a 6-month history of severe rectal pain, especially on sitting. He also reported urinary frequency and urgency. He was unable to tolerate a rectal examination. Examination under anaesthetic revealed a firm rubbery mass in the anterior rectum. Cystoscopy, hormone assays and tumour markers were normal.

CT colonoscopy showed a large mass adjacent to the rectum (Fig. 1), containing gas and appearing to demonstrate a fistula or connection to the rectum (Fig. 2).

Subsequent rectal MRI confirmed a 12 cm tumour with fistulation to the rectum, and lobulated tumour tissue within the rectal lumen, which demonstrates restricted diffusion (Fig. 3). There was a large gas/fluid collection centrally within the tumour (Fig. 4). Anteriorly, the mass appears intimately related to the posterior aspect of the prostate, but can be seen to be separate from the prostatic capsule (Fig. 5).

The mass was markedly FDG avid on PET CT (SUV max 14.2), with central photopenia in keeping with cavitation/necrosis (Fig. 6). FGD avid external iliac and pelvic side wall nodes and moderately avid foci within the femur were noted (Fig. 7).

MRI confirmed the presence of metastatic disease in the left sacrum and both femoral diaphyses (Fig. 8).

Transrectal biopsy confirmed tumour with fascicles of spindle cells, increased mitotic figures (>5/50 mitotic rate per high power field), and cell positivity for DOG-1, CD177 and CD34, in keeping with a high-risk spindle cell type gastrointestinal stromal tumour (GIST). The tumour displayed central cavitation and fistulation into the rectum and posterior prostate gland. It was not thought to be operable, and the patient commenced on palliative imatinib.

GISTs comprise a group of smooth muscle mesenchymal tumours of variable malignancy [1,2], and are thought to account for 5% of all sarcomas.  Diagnosis can be challenging due to a diverse presentation and typical submucosal location. Growth is variable, and may be exophytic or endoluminal.  As such, in the absence of local invasion they can become very large before producing symptoms [3].

Risk of metastasis depends on tumour size, mitotic rate, location and presence of perforation or rupture during surgery [3,4]. 10-30% of GISTS progress to malignancy [5], with GISTS located outside of the stomach demonstrating higher risk [6]. Metastatic spread is usually to the liver or peritoneum; nodal metastases, like those demonstrated here, are rare, occurring in approximately 6% [7].  Bony metastases are also unusual.

CT is the workhorse of diagnosis and staging.  GISTs typically vary in density and show patchy, heterogenous enhancement with contrast. They frequently demonstrate necrosis. Smaller tumours are more homogeneous and can mimic other more common primaries such as pancreatic, oesophageal and small bowel tumours, as well as lymphoma. Differentiating benign from malignant tumours is difficult; malignancy is usually associated with tumours over 5 cm in diameter, central necrosis or extension to other organs.

As in this case, a GIST can become very large and undergo complex cystic change, surrounded by a thick rim of enhancing tumour. Necrosis and fistulation to bowel or adjacent structures may occur. Calcification within the tumour can occur in associated with tumour necrosis [8]. Intratumoral haemorrhage is common.

PET CT is efficient at demonstrating metastatic disease, and has been shown to demonstrate earlier evidence of response to treatment than CT [9]. It is also superior at demonstrating peripheral tumour activity in centrally necrotic tumours.

Due to the spectrum of appearance of these tumours, a high index of suspicious is often required in order to diagnose them accurately and in a timely manner; this is particularly important as prompt treatment with adjuvant chemotherapy has a dramatic effect on prognosis and in reducing recurrence.