Early prediction of histopathological response of rectal tumors after one week of preoperative radiochemotherapy using 18 F-FDG PET-CT imaging. A prospective clinical study

Abstract

Background: Preoperative radiochemotherapy (RCT) is standard in locally advanced rectal cancer (LARC). Initial data suggest that the tumor's metabolic response, i.e. reduction of its 18 F-FDG uptake compared with the baseline, observed after two weeks of RCT, may correlate with histopathological response. This prospective study evaluated the ability of a very early metabolic response, seen after only one week of RCT, to predict the histopathological response to treatment.

Methods: Twenty patients with LARC who received standard RCT regimen followed by radical surgery participated in this study. Maximum standardized uptake value (SUV-MAX), measured by PET-CT imaging at baseline and on day 8 of RCT, and the changes in FDG uptake (ΔSUV-MAX), were compared with the histopathological response at surgery. Response was classified by tumor regression grade (TRG) and by achievement of pathological complete response (pCR).

Results: Absolute SUV-MAX values at both time points did not correlate with histopathological response. However, patients with pCR had a larger drop in SUV-MAX after one week of RCT (median: -35.31% vs -18.42%, p = 0.046). In contrast, TRG did not correlate with ΔSUV-MAX. The changes in FGD-uptake predicted accurately the achievement of pCR: only patients with a decrease of more than 32% in SUV-MAX had pCR while none of those whose tumors did not show any decrease in SUV-MAX had pCR.

Conclusions: A decrease in ΔSUV-MAX after only one week of RCT for LARC may be able to predict the achievement of pCR in the post-RCT surgical specimen. Validation in a larger independent cohort is planned.

Figure 1

The histopathological appearance of representative cases of a complete responder and a non-responder (H&E, magnification X4). In the case of a complete responder (Figure 1A), the mucosa was eroded at the tumor site and there was marked fibrosis with focal microcalcifications. No residual tumor was found. In the case of a non-responder (Figure 1B), nearly no signs of response were noted (tumor regression grade [TRG] IV). Abundant tumor was found, with minimal fibrosis and regressive changes between the invasive tumor.

Figure 2

Distribution of SUV-max values at baseline and after one week of RCT according to histopathological response. Histopathological response was evaluated by the presence or absence of pCR (Figure 2A) or tumor regression grade (Figure 2B). Extreme values are represented by red circles.

Figure 3

Distribution of ΔSUV-MAX between baseline and after one week of RCT according to histopathological response. Histopathological response was evaluated by the presence or absence of pCR (Figure 3A) or tumor regression grade (Figure 3B). Extreme values are represented by red circles.

Figure 4

Prediction of histological response using early metabolic response.