Abstract Aim Positron emission tomography (PET)/CT can be used tomonitor themetabolic changes that occur after intensified treatment with induction chemotherapy and chemo(re) irradiation for locally recurrent rectal cancer (LRRC). This study aimed to analyse the correlation between the PET/CT response and final histopathological outcomes. Methods All LRRC patients who underwent induction chemotherapy prior to surgery between January 2010 and July 2020 andweremonitoredwith pretreatment and post-treatment PET/CT were included. Visual qualitative analysis was performed, and patients were scored as having achieved a complete metabolic response (CMR), partial metabolic response (PMR), or no response (NR). The histopathological response was assessed according to the Mandard tumour regression (TRG) score and categorized as major (TRG 1‑2), partial (TRG 3), or poor (TRG 4‑5). The PET/CT and TRG categories were compared, and possible confounders were analysed. Results A total of 106 patients were eligible for analysis; 24 (23 percent) had a CMR, 54 (51 percent) had a PMR, and 28 (26 percent) had NR. PET/CT response was a significant predictor of the negative resection margins rate, achieving 96 percent for CMR, 69 percent for PMR, and 50 percent for NR. The overall accuracy between PET score and pathological TRG was 45 percent, and the positive predictive value for CMR was 63 percent. A longer interval between post-treatment PET/CT and surgery negatively influenced the predictive value. Conclusion Metabolic PET/CT response evaluation after neoadjuvant treatment proves to be a complementary diagnostic tool to standardMRI in assessing tumour response, andmay play a role for treatment planning in LRRC patients. Novel findings Metabolic PET/CT response evaluation in LRRC patients predicts histopathological outcomes and can be used as an additional tool for decision-making during multidisciplinary tumour board meetings. However, the validity of the response evaluation results is limited to a certain time frame, as the ongoing response or progression of the tumour reduces the predictive value.
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