Highlighting the role of FDG PET scan in early response assessment of gastrointestinal stromal tumor treated with imatinib mesylate

A 55-year-old man presented with a progressively increasing mass in the abdomen. The biopsy on debulking surgery demonstrated a gastrointestinal stromal tumor (GIST) with immunopositivity for c-kit. A CT scan done before the initiation of imatinib mesylate therapy demonstrated a generalized solid heterogeneous mass occupying the entire omentum and paracolic gutters and peritoneal spaces with encasement of the entire liver and spleen with peritoneal deposits. An F-18 FDG PET scan was done before the initiation of therapy. The scan demonstrated intense and nonhomogeneous uptake involving almost the entire abdomen. The maximum standardized uptake value (SUVmax) of the mass was 9.59. The FDG PET scan was repeated 24 hours after a single dose of 400 mg of imatinib (approximately 48 hours after the baseline scan), which demonstrated significant reduction in the metabolic activity, observed in the preimatimb scan. The SUVmax was now 2.47. The present case highlights the role of FDG PET scan in assessment of tumor response to imatinib as early as 24 hours.