Imaging is commonly used to assess tumor response to novel therapies in oncology clinical trials. As a surrogate marker to predict the clinically relevant factors of time to progression or patient survival, physicians use the response evaluation criteria in solid tumors (RECIST) 1.1 criteria to measure tumors on computed tomography (CT) scans . RECIST 1.1 assesses changes in linear size of a subset of target lesions. Treatment decisions are based on whether these measurements indicate progressive or stable disease or partial or complete response. Based on these decisions, therapies either pass or fail the clinical trial, leading to either adoption or abandonment of the therapy.
It is well recognized that the RECIST criteria have important limitations. Only a subset of lesions in a given patient are measured. Only a single linear measurement is obtained per lesion. For multiple lesions, the linear measurements of each are summed, compounding the errors and deficiencies inherent in each individual linear measurement. Bone disease is ignored. Large thresholds are used for assessing response or progression, with a 30% reduction in linear size required for response and a 20% minimum increase required for progression. Such large thresholds are meant to compensate for the variability in linear measurements. This variability is a consequence of the difficulty of measuring irregular tumor shapes and poor inter- and intra-observer measurement agreement.
A common sense solution to one of the deficiencies in the RECIST measurement method is to obtain volumetric assessment. A number of studies have been published demonstrating semiautomated and fully automated volumetric measurement techniques for different types of tumors . Although promising, these automated techniques are neither well established yet nor widely available.
In the article by Seyal at al. in this issue of Academic Radiology , the authors compare tumor growth kinetics with RECIST in a group of women with breast cancer liver metastases who have undergone treatment with yttrium-90 radioembolotherapy. Using a commercial semiautomated tumor volume measurement tool, the authors measured 34 liver metastases in 21 patients on pre-and post-treatment CT scans. From the volumetric measurements, they computed the reciprocal of the doubling time (RDT). RDT indicates the number of times a tumor doubles its volume in a year.
To distinguish true volume changes from those within measurement error, the authors computed an absolute percentage error from the volumes measured at two time points. Absolute percentage errors <7.87% were attributed to measurement error, and therefore, the change was considered a stable disease. The 7.87% threshold was determined from a previously published phantom experiment using the same software used in this study . If the change in volume exceeded that expected because of measurement error, the change in volume was considered a true change. In that case, the RDT was assessed. If RDT was negative, the disease was considered a partial response (PR). If positive, the disease was considered a progressive disease (PD). If the lesion disappeared, the disease was considered a complete response.
In RECIST 1.1, multiple lesions are handled by summing the linear dimensions of all lesions (up to 5 per patient). In the article by Seyal et al. , a maximum of two well-defined lesions per treated hepatic lobe were selected for evaluation. Each lesion was assessed independently by RECIST 1.1 and RDT.
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Acknowledgments
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