CT and PET: early prognostic indicators of response to imatinib mesylate in patients with gastrointestinal stromal tumor.
OBJECTIVE
We report results from a pilot study aimed at optimizing the use of CT bidimensional measurements and 18F-FDG PET maximum standardized uptake values (SUVs-(max)) for determining response to prolonged imatinib mesylate treatment in patients with advanced gastrointestinal stromal tumors (GISTs).


METHODS
Sixty-three patients enrolled in a multicenter trial evaluating imatinib mesylate therapy for advanced GIST underwent FDG PET at baseline and 1 month after initiation of treatment. Of these 63 patients, 58 underwent concomitant CT. Time-to-treatment failure (TTF) was used as the outcome measure. Patients were followed up over a range of 23.7 to 37 months (median, 31.7 months). The predictive power of change in CT bidimensional measurements, change in PET SUVmax, and PET SUVmax at 1 month after initiation of treatment were determined, optimized, and compared. The effectiveness of combining metrics was also evaluated.


RESULTS
Both a threshold PET SUVmax value of 2.5 at 1 month (p = 0.04) and the European Organization for Research and Treatment of Cancer (EORTC) criteria for partial response on FDG PET (25% reduction in PET SUVmax) at 1 month (p = 0.004) were predictive of prolonged treatment success. The Southwest Oncology Group (SWOG) criteria for partial response ((3) 50% reduction in CT bidimensional measurements) at 1 month were not predictive (p = 0.55) of TTF. Optimizing metrics improved results performance. An optimized PET SUVmax threshold of 3.4 (p = 0.00002), a reduction in the SUVmax of 40% (p = 0.002), and an optimized CT bidimensional measurement threshold--that is, no growth from baseline to 1 month (p = 0.00005)--outperformed the existing standards (i.e., EORTC and SWOG criteria). Combinations of metrics did not improve performance.


CONCLUSIONS
The two best metrics were the optimized PET SUVmax threshold of 3.4 at 1 month (p = 0.00002) and the optimized CT bidimensional measurement threshold (no growth from baseline to 1 month, p = 0.00005) in this patient group.
Holdsworth
Clay H
CH
Massachusetts College of Pharmacy and Health Sciences, 4 Brook Rd., Unit 11, Salem, NH 03079, USA. clayholdsworth@yahoo.com
Badawi
Ramsey D
RD
Manola
Judith B
JB
Kijewski
Marie F
MF
Israel
David A
DA
Demetri
George D
GD
Van den Abbeele
Annick D
AD
D000970:Antineoplastic Agents / Q000627:therapeutic use; D001549:Benzamides; D005260:Female; D019788:Fluorodeoxyglucose F18; D046152:Gastrointestinal Stromal Tumors / Q000175:diagnosis* / Q000188:drug therapy*; D006801:Humans; D000068877:Imatinib Mesylate; D008297:Male; D010865:Pilot Projects; D010879:Piperazines / Q000627:therapeutic use*; D049268:Positron-Emission Tomography / Q000379:methods*; D011379:Prognosis; D011743:Pyrimidines / Q000627:therapeutic use*; D019275:Radiopharmaceuticals; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D014057:Tomography, X-Ray Computed / Q000379:methods*; D016896:Treatment Outcome; D014481:United States
D000970:Antineoplastic Agents; D001549:Benzamides; D010879:Piperazines; D011743:Pyrimidines; D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18; D000068877:Imatinib Mesylate