Gastrointestinal Stromal Tumor (GIST)

Fast Facts:

• ≈ 3% of all GI cancers (≈ 20% of small bowel cancers)
• Can occur anywhere along GI tract:
  • Stomach (≈ 65%)
  • Small Bowel (≈25%)
  • Uncommon: rectum, esophagus, colon, appendix

Role of PET/CT in the Management of GIST:

PET/CT plays a very important role in the management and evaluation of GIST patients.

Surgical resection is the goal for patients with resectable lesions and no evidence of metastatic disease.  The mainstay of medical therapy is Imatinib, a protein kinase inhibitor (GIST are generally resistant to both chemotherapy and radiation).

The vast majority of GIST lesions (≈ 85%) respond extremely rapidly to Imatinib therapy.  This response, however, is often not accompanied by a decrease in lesion size until very late in therapy.

PET/CT, therefore, plays a unique role in the early assessment of therapy response to Imatinib, as it can demonstrate a significant decrease in a lesion’s metabolic activity, even though the lesion may be unchanged in size (in fact, some lesions may actually increase in size during successful treatment, presumably due to inflammation and/or hemorrhage). 

PET/CT is regularly utilized for:

Initial Staging: 

• Primary lesion: Size, location and metabolic activity.
• Metastatic disease: Present ≈ 10% at time of diagnosis – typically to the liver, omentum, & peritoneum (nodes, bone & lung involvement are less common).

Assessing Response to Therapy & Prognosis: 

• Early Response: Non-responders can be offered alternative therapy.
• Late Response: Assess success or failure of therapy, and ultimate outcome. Can also assess secondary resistance to Imatinib.

Recurrence & Restaging: 

• Restaging suspected recurrence. 
• Distinguishing recurrence from post-therapeutic inflammation.

Criteria for “Active Malignancy”:

Most patients with GIST patients already have a biopsy-proven diagnosis prior to PET/CT scanning. Occasionally, however, an incidental GIST is encountered.

GIST lesions generally present as an FDG-avid soft tissue mass, typically involving the stomach or small bowel (we usually do not specifically include GIST in our differential for FDG-avid colon, rectal, esophageal or appendiceal lesions).

Larger lesions are often accompanied by cystic regions, often representing necrosis.

Recurrent Active Disease: Often appears as a small focus of activity within a larger non-avid treated mass (the recurrence may be centrally or peripherally located).

False Positives:

• “Flare Phenomenon”: Within the first 3 weeks after cessation of Imatinib therapy, there is often a transient increase in the metabolic activity of residual lesions.  For this reason, we generally suggest post-treatment imaging be delayed for at least 6 weeks.

False Negatives:

• An estimated 10-20% of GIST lesions may demonstrate poor-avidity. 
• PET is less sensitive for some small liver metastases.