Ideal number of target lesions per organ to measure in metastatic colorectal cancer.

The Response Evaluation Criteria in Solid Tumors (RECIST 1.1) guideline states that the two largest lesions per organ should be measured as target lesions for assessment of the tumor response. This criterion is considered to be arbitrary and, to the best of our knowledge, has not been supported by any objective evidence. The present study hypothesized that measuring the single largest lesion in each organ into which the cancer had metastasized (termed the modified RECIST; mRECIST 1.1) may yield the same response classification as measuring the two target lesions per organ (as per the RECIST 1.1 guideline). The medical records of patients with metastatic colorectal cancer (CRC), who received first-line chemotherapy between January 2004 and June 2013 were reviewed. The tumor responses of the patients were compared according to the two criteria using computed tomography. A total of 38 patients were included in the present study, all of whom had at least two target lesions in any one organ according to the RECIST 1.1 guidelines. When adopting the mRECIST 1.1, rather than the RECIST 1.1, 18 patients (47.4%) demonstrated an increase in the rate of change of the sum of the tumor measurements. The overall response rates of chemotherapy were 39.4% and 34.2% according to the RECIST 1.1 and the mRECIST 1.1, respectively, and the difference between the two criteria was not identified to be significantly different (P=0.226). The tumor response showed near perfect agreement between the RECIST 1.1 and mRECIST 1.1 criteria (κ=0.905). Only two patients (5.3%) showed a disagreement with regard to the tumor responses between the two criteria. Therefore, it was identified that the mRECIST 1.1 showed a high level of concordance with the original RECIST 1.1 guidelines in the tumor response assessment of metastatic CRC patients to chemotherapy. The present results indicate that the mRECIST 1.1, with a decreased number of target lesions to be measured, may be more convenient in clinical practice for the assessment of tumor response.