Assessment of reproducibility in quantitative FDG PET and CT measurements of tumor response to therapy

Objectives To directly compare inter-observer reproducibility of 1) SUV and CT size measurements in malignant tumors pre- and post-therapy and 2) percent change in these parameters in response to therapy in the same patients.

Methods 52 tumors (25 pts) were evaluated by FDG PET/CT pre- and post-therapy. Maximum single-pixel SUV_bw (SUV) and CT size measurements were determined for each tumor independently pre- and post-therapy by 8 readers (4 PET, 4 CT) each using routine non-automated clinical methods. Percent changes in SUV and CT size between pre- and post-therapy scans were calculated. Inter-observer reproducibility is described by the intraclass correlation coefficient (ICC), with ICC >0.6 considered significantly reproducible.

Results ICC for SUV was 0.93 (95% CI 0.90-0.96) pre-therapy and 0.91 (95% CI 0.86-0.94) post-therapy. For the longest CT dimension, ICC was 0.72 (95% CI 0.61-0.82) pre-therapy and 0.85 (95% CI 0.77-0.91) post-therapy. For the two-dimensional CT size, ICC was 0.61 (95% CI 0.48-0.74) pre-therapy and 0.45 (95% CI 0.30-0.61) post-therapy. ICC for percent change in SUV was 0.94 (95% CI 0.90-0.96), for the longest CT dimension 0.70 (95% CI 0.57-0.81) and the two-dimensional size 0.33 (95% CI 0.18-0.50).

Conclusions Inter-observer reproducibility of single-pixel SUV_bw max is high for both untreated and treated tumors and better than inter-observer reproducibility of CT size measurements. Changes in tumor SUV were more highly reproducible measures of tumor response than changes on anatomic imaging, the current standard for assessing tumor response to therapy in the same patient.