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Tumor shrinkage is a common end point used in screening new cytotoxicagents. The standard World Health Organization criterion for partial response is a 50% ormore decrease in the sum of the products of two measurements (the maximum diameter of atumor and the largest diameter perpendicular to this maximum diameter) of individual tumors.However, theoretically, the simple sum of the maximum diameters of individual tumors is morelinearly related to cell kill than is the sum of the bidimensional products. It has beenhypothesized that the calculation of bidimensional products is unnecessary, and a 30%decrease in the sum of maximum diameters of individual tumors (assuming spherical shape andequivalence to a 50% reduction in the sum of the bidimensional products) was proposed asa new criterion. We have applied the standard response and the new response criteria to the samedata to determine whether the same number of responses in the same patients would result.Data from 569 patients included in eight studies of a variety of cancers werereanalyzed. The two response criteria were separately applied, and the results were comparedusing the κ statistic. The importance of confirmatory measurements and the frequency ofnonspherical tumors were also examined. In addition, for a subset of 128 patients, aunidimensional criterion for disease progression (30% increase in the sum of maximumdiameters) was applied and compared with the standard definition of a 25% increase in thesum of the bidimensional products.Agreement between the unidimensional andbidimensional criteria was generally found to be good. The κ statistic for concordance foroverall response was 0.95.We conclude that one dimensional measurement oftumor maximum diameter may be sufficient to assess change in solid tumors.