Role of [18F]FDG-PET/CT in Evaluation of Tumor Response to Chemoradiation Therapy for Advanced Colorectal Cancer

Objectives: The objective is to evaluate the efficacy of ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG-PET) computed tomography (CT) in the evaluation of tumor response to preoperative/palliative chemoradiotherapy (CRT) for advanced colorectal cancer; including metastatic cancer at primary presentation and recurrent cancers with local and/or distant metastasis.

Materials and methods: Fifty patients with advanced rectal cancer underwent two point imaging with 18 FDG PET-CT before and after 3 weeks of completion of preoperative/palliative CRT in between 2016 and 2022. Patients with locally recurrent cancer also underwent radical surgery. The assessment consisted of the evaluation of the following metabolic PET parameters: Maximum standardized uptake value (SUV_max), SUV_ratio, metabolic tumor volume (MTV), and total lesion glycolysis (TLG). Response was assessed among the followed patients using RECIST 1.1 criteria.

Observations and results: There was a significant decline in the mean post therapy SUV_max and SUV_ratio as compared to baseline (P = 0.0001). Twenty-six out of 50 (52%) patients were classified as responders. A significant decrease in all parameters (SUV_max, SUV_ratio, TLG, and MTV) from baseline was observed in responders of the study when comparing with nonresponders (P < 0.05). Besides SUV_max and SUV_ratio, the mean posttherapy TLG was significantly reduced in responders than nonresponders (P = 0.0065).

Conclusion: PET-CT is a useful combined anatomic and functional imaging modality in monitoring tumor response to preoperative/palliative CRT in advanced rectal cancer, whether primary or recurrent, including metastatic cancers at presentation. Posttherapy SUV and TLG in particular are significantly associated with treatment response.