New response evaluation criteria using early morphological change in imatinib treatment for patients with gastrointestinal stromal tumor.

838 Background: While the introduction of molecularly targeted drugs, including imatinib, has greatly improved the prognosis of gastrointestinal tumor (GIST), the response evaluation criteria have not been optimized. Early morphological change (EMC) was previously reported as a predictive marker for molecularly targeted drugs in metastatic colorectal cancer. The purpose of the present study was to verify the efficacy of EMC in predicting the outcome in patients with GIST receiving imatinib. Methods: We retrospectively reviewed 55 patients. EMC in computed tomography (CT) image was evaluated, and the patients were categorized into two groups; active MR (morphologic response) (+) group and active MR (-) group, judging from the morphological features including the sharpness of the outline and the attenuation of target lesion. We investigated the association between the presence of active MR and clinical outcomes. Similarly, clinical outcomes classified by response evaluated by RECIST and Choi criteria were analyzed respectively. Results: Thirty-two patients had active MR (+). The median Progression-free survival(PFS) was 7 months vs 31 months (active MR(-) vs active MR(+), P = 0.013) and the median overall survival (OS) was 58 months vs 91 months ( P = 0.086), respectively. Although PFS of RECIST PD (progression disease) or Choi PD patients was shorter than that for PR (partial response) or SD(stable disease) patients ( P = 0.0037 for RECIST, P = 0.0058 for Choi), the number of PD patients was very small (N = 3, both in RECIST and Choi). Conclusions: The evaluation criteria based on EMC could be a sensitive method to predict the clinical outcome of imatinib treatment for patients with inoperative GIST.