Role of 18FDG PET-CT & MRI for Predicting Response to Neo-Adjuvant Therapy in Rectal Cancer

Neoadjuvant chemoradiotherapy (NACRT) is standard for locally advanced rectal cancer. Accurate response assessment is essential, particularly to identify complete responders eligible for organ-preserving strategies. While MRI is widely used, 18 F-FDG PET-CT has been proposed as a complementary modality. This study compared MRI-based Tumor Regression Grade (mrTRG) and PET-CT parameters in predicting pathological response.

A retrospective analysis was conducted at Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, from January 2013 to May 2024. A total of 268 patients underwent MRI and PET-CT before and after NACRT, followed by surgery. Response was categorized using mrTRG and pathological TRG (pTRG). PET-CT response was assessed by changes in SUVmax. Diagnostic accuracy of mrTRG and PET-CT was evaluated using statistical tests, kappa agreement, and ROC curves.

Of 268 patients, 27.2% were classified as good responders (mrTRG 1-2), whereas 46.2% had good pathological response (pTRG 0-1). The correlation between mrTRG and pTRG was weak (kappa = 0.215). PET-CT showed a mean SUVmax reduction of 58.2%, with greater decline in responders. However, ROC analysis demonstrated poor discriminative ability (AUC = 0.502), indicating no advantage over MRI. Combining MRI and PET-CT imaging slightly improved accuracy in matching pathological grading (κ = 0.36).

MRI remains the preferred modality for post-NACRT response assessment in rectal cancer, showing moderate predictive accuracy. PET-CT, despite reflecting metabolic changes, is not reliable in distinguishing complete responders. A combined multimodal approach may improve evaluation and support individualized treatment planning.