The value of PET/CT in early predicting pathological response of locally advanced rectal cancer after neoadjuvant radiochemotherapy

Objectives The objective of our prospective study was to access the value of PET/CT for evaluating the response to Neoadjuvant radiochemotherapy (RCT) in locally advanced rectal cancer (LARC).

Methods 52 patients diagnosed with LARC and treated with neoadjuvant RCT were studied. In all the patients, 18F-FDG PET/CT was performed before and after RCT (PET/CT 1 and 2). Among them, there were 32 patients accepted additional PET/CT before surgery (PET/CT 3). The images were analyzed visually and semiquantitatively. The pathological response was classified according to tumor regression grade (TRG). All the patients were divided into two groups: nonresponders (TRG1) and responders (TRG2~4). We analyzed the relationship between TRG and the percentage decrease in SUVmax, SUVmean and volume in consideration of radiation enteritis.

Results Differences of SUVmax, SUVmean and volume among 3 PET/CT scans were statistically significant (p <0.001). Of 52 patients, 10 had TRG1; 21 had TRG2; 10 had TRG3; 11 had TRG4. There was no difference of the percentage decrease between nonresponders and responders without regard to radiation enteritis. However, we analyzed separately according to with or without radiation enteritis in PET/CT 2. In 31 patients with radiation enteritis, only the percentage decrease in volume between PET/CT 1 and 3 was significantly different (p=0.017) between nonresponders (0.01±0.12) and responders (0.37±0.22). In 21 patients without radiation enteritis, only the percentage decrease in SUVmean between PET/CT 1 and 2 was significantly different (p=0.029) between nonresponders (0.36±0.19) and responders (0.54±0.09).

Conclusions PET/CT is capable to access the pathological response to neoadjuvant RCT in LARC. The percentage decrease in volume between PET/CT 1 and 3 in patients with radiation enteritis and the percentage decrease in SUVmean between PET/CT 1 and 2 in those without radiation enteritis may be reliable parameters.