The minimum number of target lesions that need to be measured to be representative of the total number of target lesions (according to RECIST).

Response evaluation criteria in solid tumours (RECIST) were introduced as a means to classify tumour response with no definition of the minimum number of lesions. This study was conducted in order to evaluate discrepancies between full assessments based on either all target lesions or fewer lesions. RECIST evaluation was performed on separate occasions based on between one and seven of the target lesions, with simultaneous assessment of non-target lesions. 99 patients were included. 38 patients demonstrated progressive disease, in 61% of whom it was a result of the appearance of new lesions or unequivocal progress in non-target lesions. 32 patients showed stable disease, with 8 having results that differed when 1-3 target lesions were measured. 22 cases were considered as having partial regression, with only 1 case differing when performing 1-3 target lesion assessments. Seven cases demonstrated complete response. The number of discordant cases increased gradually from measuring three lesions to one target lesion. The average number of available target lesions among those with discrepancies was 7.1, which was significantly higher than those demonstrating concordance (4.1 lesions; p<0.05). In conclusion, measuring fewer than four target lesions might cause discrepancies when more than five target lesions are present.