Tumor response assessment is more robust with sequential CT scanning than external caliper measurements

Rationale and Objectives. Measurements of tumor size are important in assessing response to cancer therapies. To date, preclinical studies of drug development have relied on direct caliper-based measurements of tumor size. We investigated the feasibility of using a human positron emission tomographic (PET)/computed tomographic (CT) scanner to assess tumor size before and after chemotherapy and compared this approach with caliper measurements. Materials and Methods. Fourteen rats with rat mammary tumor underwent high-resolution CT using a PET/CT scanner before and after chemotherapy, and tumor volumes were measured independently by two observers using calipers and CT images. Results. Tumor response could be detected after 1 day of treatment by means of CT imaging, but was not significant until 2 days or later by means of caliper measures because of their greater variability. Independent measurements of tumor size correlated well with one another by means of CT, but correlated less by means of calipers. Conclusion. Tumor size measurements by means of CT from PET/CT were more reliable than caliper measurements because of their smaller variance, allowing earlier assessment of response. It is suggested that CT imaging-based methods of assessing tumor response replace traditional caliper-based measurements, much as CT has become a standard for assessing tumor response in humans.