The role of sequential 18 F ‐ FDG PET / CT in predicting tumour response after preoperative chemoradiation for rectal cancer

Aim The aim of this study was to investigate the potential of sequential positron emission tomography ( PET )/ CT standardized uptake value ( SUV )/metabolic area variation in predicting the pathological response to preoperative chemoradiotherapy ( CRT ) for rectal cancer. Method Fifty‐three patients diagnosed with clinical T 3‐4 and/or N + rectal cancer were enrolled. All patients received CRT followed by radical surgery after 6–8 weeks. A PET / CT scan was performed before ( PET / CT 1) initiation of treatment and a second scan ( PET / CT 2) was performed within 1 week after the completion of CRT . Thirty‐five of 53 patients also underwent a third ( PET / CT 3) scan within 1 week before surgery. Maximal SUV within the tumour ( SUV max), average SUV within the tumour ( SUV mean), metabolic tumour volume ( MV ), total lesion glycolysis ( TLG ) and response indices (∆%, i.e. the percentage difference between two different PET / CT scans for SUV max, SUV mean, MV and TLG ) were calculated. The different metabolic parameters were analysed and correlated with the tumour regression grade ( TRG ) score. Results When patients were regrouped as responders ( TRG 3‐4) and nonresponders ( TRG 0‐2), significant differences were observed in the percentage differences between PET / CT 1 and PET / CT 3 for MV (∆% MV (1‐3); 91.08% vs 75.43%) and for TLG (∆% TLG (1‐3); 94.00% vs 82.02%). As demonstrated by receiver–operating characteristics analysis, ∆% MV (1‐3) and ∆% TLG (1‐3) both had a strong capability to discriminate between responders and nonresponders. Patients classified as having a pathological complete response ( pCR ) and a non‐ pCR showed significant differences in the percentage difference between PET / CT 1 and PET / CT 3 in SUV max (∆% SUV max(1‐3); 69.17% vs 57.77%), SUV mean (∆% SUV mean(1‐3); 44.20% vs 30.19%), ∆% MV (1‐3) (90.93% vs 80.30%) and ∆% TLG (1‐3) (94.22% vs 85.63%). ∆% TLG (1‐3) was a more powerful discriminator than the others. Conclusion Differences in the SUV /metabolic area with 18 F ‐fluorodeoxyglucose (18 F ‐ FDG ) PET / CT have the potential to predict a response to preoperative CRT for rectal cancer.