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 according to the modified RECIST 1.1 was significantly lower than that according to the RECIST 1.1 (p < 0.001). The assessment of tumor responses showed a high concordance between the two criteria, with a kappa value of 0.906 (95% CI, 0.826-0.986). Only five patients (5.6%) showed disagreement in the tumor response assessment between the two criteria: 3 in GC and 2 in CRC. Four patients showed disagreement between PR and SD, and one showed disagreement between SD and PD. The overall response rates of chemotherapy, which were calculated regardless of the primary sites and anti-cancer treatment, were not significantly different between the two criteria (42.7% versus 42.7%, p = 1.0) (Table). Conclusions: The modified RECIST 1.1, with a decreased number of target lesions, was comparable to the original RECIST 1.1 in the assessment of tumor response in patients with advanced GC or CRC. Our results suggest that it may be possible to measure the single largest lesion per organ for assessing tumor response. [Table: see text]