Rationale and Objectives. Solid tumor measurements are regularly used in clinical trials of anticancer therapeutic agents and in clinical practice managing patients' care. Consequently studies evaluating the reproducibility of solid tumor measurements are important as lack of reproducibility may directly affect patient management. The authors propose utilizing a modified Bland-Altman plot with a difference metric that lends itself naturally to this situation and facilitates interpretation. Materials and Methods. The modification to the Bland-Altman plot involves replacing the difference plotted on the vertical axis with the relative percent change (RC) between the two measurements. This quantity is the same one used in assessing tumor response to therapeutic agents and is very familiar to radiologists and clinicians working with cancer patients.The distribution of the RC is explored and revised equations for the limits of agreement (LoA) are presented. These methods are applied to positron emission tomography (PET) data studying two radiotracers. Results. The RC can be calculated separately for each lesion measured or at the patient level by summing over lesions within patient. In both cases, the distribution of the RC is highly skewed and is approximated by a negative shifted lognormal distribution. The standard equations for the 95% LoA assume the differences are approximately normally distributed and are not appropriate for the RC. Conclusions. The modified Bland-Altman plot with correctly calculated LoA can aid in evaluating agreement between solid tumor measurements.



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