Prospective analysis of 18F-FDG PET/CT predictive value in patients with low rectal cancer treated with neoadjuvant chemoradiotherapy and conservative surgery

Abstract

This study prospectively assessed (18)F-FDG PET/CT in predicting the response of locally advanced low rectal cancer (LRC) to neoadjuvant chemoradiation (nCRT).

Methods: 56 patients treated with chemoradiation underwent two (18)F-FDG PET/CT scans (baseline and 5-6 weeks post-nCRT). (18)F-FDG uptake (SUVmax and SUVmean) and differences between baseline (SUV1) and post-nCRT (SUV2) scans (ΔSUV and RI%) were evaluated. Results were related to the Mandard's TRG and (y)pTNM.

Results: (18)F-FDG PET/CT sensitivity, specificity, accuracy, PPV and NPV resulted in 88.6%, 66.7%, 83.92%, 90.7%, and 61.5%. SUV2 resulted in better than SUV1 to predict nCRT response by TRG, with no significant statistical difference between the SUVmax2 and SUVmean2 AUC (0.737 versus 0.736; P = 0.928). The same applies to the (y)pTNM (0.798 versus 0.782; P = 0.192). In relation to the TRG, RI values had a higher AUC than ΔSUV, with no significant difference between RImax and RImean (0.672 versus 0.695; P = 0.292). The same applied to the (y)pTNM (0.742 versus 0.741; P = 0.940). In both cases ΔSUV does not appear to be a good predictive tool. Logistic regression confirmed the better predictive role of SUVmax2 for the (y)pTNM (odds ratio = 1.58) and SUVmean2 for the TRG (odds ratio = 1.87).

Conclusions: (18)F-FDG PET/CT can evaluate response to nCRT in LRC, even if more studies are required to define the most significant parameter for predicting pathologic tumor changes.

Figure 1

A 77-year-old male with a vegetans eccentric ulcerated lesion, 45 mm in length, localized 3 cm from the anal verge (cT3N0). Baseline ¹⁸F-FDG PET/CT MIP (a) and sagittal images (b) showed the rectal lesion, with value of 18.9 for SUVmax, 10.4 for SUVmean, and 3.0 SD (green arrows). The post-nCRT ¹⁸F-FDG PET/CT MIP (c) and sagittal images (d) did not show a pathological uptake of ¹⁸F-FDG (SUVmax = 2.5, SUVmean = 1.1, and std = 0.3). Histological specimen analysis showed (y)pT0N0 M0, TGR1, and R0 and the patient was classified as a complete responder. In this patient ΔSUVmax, ΔSUVmean, RImax, and RImean results were 16.4, 9.3, 86.77, and 89.42%, respectively.

Figure 2

A 76-year-old male with an eccentric lesion, 30 mm in length, localized 2 cm from the anal verge (G3, cT3N0). Baseline ¹⁸F-FDG PET/CT MIP (a) and sagittal images (b) showed the rectal lesion with a 12.9 value for SUVmax, 5.4 for SUVmean, and 1.5 SD (green arrows). The post-nCRT ¹⁸F-FDG PET/CT MIP (c) and sagittal images (d) showed, in the same site (green arrows), the persistence of ¹⁸F-FDG pathological uptake (SUVmax 11.8, SUVmean 5.4, and SD 1.5). Histological specimen analysis showed (y)pT3N0 M0, TGR4, and R0 and the patient was classified as a nonresponder. In this patient ΔSUVmax, ΔSUVmean, RImax, and RImean results were 1.10, 0, 8.5, and 0%, respectively.

Figure 3

ROC curves for ¹⁸F-FDG PET/CT assessment of SUVmax1, SUVmean1, SUVmax2, and SUVmean2 in predicting response to CRT based on Mandard's TRG (a) and the (y)pTNM (b) criteria.

Figure 4

ROC curves for ¹⁸F-FDG PET/CT assessment of ΔSUVmax, ΔSUVmean, RImax, and RImean in predicting response to CRT based on Mandard's TRG (a) and the (y)pTNM (b) criteria.