First report of the correlation of PET Response Criteria in Solid Tumors (PERCIST) criteria and pathologic change in patients with rectal cancer treated with neoadjuvant radiation.

Abstract

261 Background: High-dose-rate endorectal brachytherapy (Endo-HDR) is a promising technique using a short course of radiation therapy in the neoadjuvant setting for rectal cancer. While clinical exam and standard imaging (CT and MRI) suggest outstanding radiation response in these patients, they cannot reliably predict pathologic complete response (pCR) or high tumor regression grade (TRG). In this study, we investigate the utility of PERCIST criteria to predict pathologic change. Methods: We treated nine patients on a prospective clinical trial investigating Endo-HDR as a single agent neoadjuvant therapy (6.5 Gy x 4 fractions, total dose of 26 Gy) for stage II-III rectal cancer patients. A pre-radiation PET/CT scan was compared to a post-treatment PET/CT scan performed one month following the completion of radiation. All nine patients proceeded to surgical resection with the pathologic specimen analyzed for residual disease. TRG (Mandard et al.) was used to quantify response to treatment. A 50% decrease in SUVpeak by PERCIST criteria was correlated with a TRG 1 or 2 (none or minimal residual cancer cells on the final pathologic specimen). Results: As demonstrated in our table, 3/5 (60%) of patients achieving a TRG 1 or 2 had a SUVpeak that decreased by 50% or more; contrarily, zero of 4 (0%) of patients without a TRG 1 or 2 had a SUVpeak of 50% or more. Despite a trend, the small patient sample limited statistical significance (p=0.17). A minimum decrease of 50% in SUVpeak resulted in a positive predictive value of 100%, a negative predictive value of 67%, a specificity of 100%, and a sensitivity of 60% for predicting TRG 1 or 2. Conclusions: Our results demonstrate the first report of utilizing PERCIST criteria in rectal cancer treated with neoadjuvant Endo-HDR. Reliable clinical predictors following Endo-HDR for excellent pathologic response may identify patients in whom a non-operative rectal cancer treatment is possible. [Table: see text]