2515 Background: Monitoring of treatment efficacy is mainly based on RECIST post-treatment evaluations. We investigated the impact of pre-treatment tumor growth dynamics on the evaluation of response to treatment in phase I trials. METHODS: Response at 12 weeks was evaluated using RECIST criteria in 76 patients treated in phase I trials. All patients had a scannographic evaluation at the start of the experimental treatment (D0), at least 2 evaluations before D0 and one evaluation after D0. The mean time between evaluations was 6 weeks. Tumor growth rate (GR=% change in tumor volume per month) was calculated for the pre-treatment period (from the 1st available evaluation to D0), and the experimental period (after D0). GR was compared to RECIST and to time to progression. Disease control (DC) was defined as stable disease, partial response or complete response. We also classified patients according to the variation in GR. RESULTS: A total of 550 evaluations were performed: 256 before D0, 76 at D0 and 218 after. Tumor GR differed according to the time period: 26.6% per month during pre-treatment period vs. 6.6% in experimental period (P<0.01). The GR was significantly higher in patients without DC during pre-treatment (30% vs. 19%; P=0.003) and experimental time periods (+13% vs. -9%; P=0.0005). There was a significant association between a time to progression > 12 weeks and a decrease in GR during the experimental period (P=0.01). Several discordances however were noted. Eight patients with a time to progression >12 weeks (8/46=17%) had an increased GR with the experimental treatment. Thirteen patients who progressed <12 weeks (13/22=59%) had a decreased GR with the experimental treatment. CONCLUSIONS: Pre-treatment growth dynamic may directly impact RECIST evaluation of tumor response and thus should be carefully documented. Patients with slow growing tumors tend to experience DC, even if treatment has little effect. The evaluation of response to treatment should take into account pre-treatment tumor GR. No significant financial relationships to disclose.