Tumor Response Assessment By Measuring The Single Largest Lesion Per Organ In Patients With Advanced Gastric Cancer Or Colorectal Cancer

766 Background: The Response Evaluation Criteria in Solid Tumors Guidelines version 1.1 (RECIST 1.1) adopted a total of five target lesions to be measured, with a maximum of two lesions per organ. To the best of our knowledge, the criterion of two target lesions per organ in the RECIST 1.1 is arbitrary and has not been supported by any objective evidence. We hypothesized that measuring the single largest lesion in each organ (modified RECIST 1.1; mRECIST 1.1) might show almost the same response classification as measuring two target lesions per organ (RECIST 1.1). Methods: We compared tumor responses according to the modified RECIST 1.1 and RECIST 1.1 using computed tomography in patients with advanced gastric cancer (GC) or colorectal cancer (CRC) who received a first-line chemotherapy. Results: A total of 89 patients who had at least two target lesions in any organ according to the RECIST 1.1 were included: 51 with GC and 38 with CRC. Regardless of the primary sites, the number of target lesions accordi...