PET scanning evaluation of response to Imatinib mesylate therapy in gastrointestinal stromal tumor (GIST) patients

Background: Unresectable or metastatic gastrointestinal stromal tumors (GISTs) exhibit a dynamic clinical course, with no evidence of benefit from any standard cytotoxic chemotherapy and an inevitably fatal outcome. With the introduction of Imatinib, an oral drug able to inhibit the KIT receptor tyrosine kinase, new questions arise regarding our ability to monitor treatment response with conventional methods and optimally manage such patients on treatment with new agents. Materials and Methods: Herein we report two cases of patients with a history of GIST in treatment with Imatinib. Results: After 4 weeks from treatment start, CT scan evaluation demonstrated a massive increase in the size of metastatic lesions, but a confirmatory PET excluded, in both patients, the presence of any metabolic activity in the previously known metastatic sites. Imatinib therapy was continued with subjective clinical benefit for 12 further months before a PET scan-confirmed disease progression had occurred in one patient and is still ongoing after 15 months in the other. Conclusion: These cases open the obvious question of whether conventional imaging techniques are adequate to assess the response to Imatinib treatment in GIST patients.