The value of 18F-FDG PET/CT for assessing the response to neoadjuvant therapy in locally advanced rectal cancer

Neoadjuvant radiochemotherapy (RCT) is an accepted treatment for locally advanced rectal cancer (LARC) that improves surgical outcomes. If a pathological complete response is achieved, conservative surgery can be considered. The objective of our study was to assess the reliability of F-18-FDG PET/CT for evaluating the response to neoadjuvant RCT in LARC. We prospectively studied 41 patients diagnosed with LARC and candidates for neoadjuvant RCT. PET/CT was performed before RCT and again 7 weeks later. A visual and semiquantitative analysis was carried out. The pathological response was classified according to the Mandard tumour regression grade (TRG). We analysed: (a) the relationship between TRG and the result of the posttreatment PET/CT scan, and (b) the correlation between the percentage of pathological response and the percentage decrease in SUVmax according to the response index (RI). The mean SUVmax of the rectal lesions at diagnosis was 13.6 and after RCT 3.96. The mean RI was 65.32 %. Sensitivity was 88.88 %, specificity 92.86 %, positive predictive value 96 %, negative predictive value 81 %. Of the 41 patients, 8 had TRG I (all negative PET/CT); 6 had TRG II (5 negative, 1 positive PET/CT); 16 had TRG III (13 positive, 3 negative PET/CT); 9 had TRG IV (all positive PET/CT); 2 had TRG V (all positive PET/CT). Of the 14 patients classified as responders (TRG I, II), 13 (92.86 %) had negative PET/CT. Of the 27 patients classified as nonresponders (TRG III-V), 24 (88.88 %) had positive PET/CT. Differences were statistically significant (p < 0.0001). The RI in responders was 79.9 % and in nonresponders was 60.3 %. Differences were statistically significant (p < 0.037). PET/CT is a reliable technique for assessing response to neoadjuvant RCT in LARC, with a view to considering more conservative surgical treatment. The combination of the visual and semiquantitative analysis increases the diagnostic validity of PET/CT.