Predicting residual rectal adenocarcinoma in the surgical specimen after preoperative brachytherapy with endoscopic ultrasound.

Abstract

A novel brachytherapy (BT) protocol evaluated at McGill University has shown promise in terms of downstaging and achieving high tumour sterilization rates in rectal cancer. Endoscopic ultrasound (EUS) has emerged as the imaging modality of choice for local staging of rectal cancer. However, external beam radiotherapy appears to decrease the accuracy of EUS from 85% to 40%. The aim of the present study was to prospectively evaluate the accuracy of EUS in assessing the response of rectal cancer to BT.
Thirty-three patients with locally advanced (stage T2 or T3) operable rectal carcinomas were included in an experimental protocol involving a novel conformal technique, using three-dimensional planning, to administer high-dose rate preoperative BT. The 18 patients who were able to have a post-BT EUS exam arranged within two weeks before surgery (eg, four to eight weeks post-BT) were included in this study. Tumour (T)- and lymph node (N)-staging on radial EUS, as well as interpretation of the residual tumour, were assessed prospectively. Pathologists were blinded to the post-BT EUS results.
The mean age was 70 years (SD +/- 11; range, 52 to 93 years) and 78% of the patients were male. Pre-BT EUS indicated that 16 patients (89%) were stage T3, and two were stage T2. Five patients (28%) had positive nodes (N1) by ultrasound. With BT, the mean maximal wall thickness on EUS decreased from 14 mm to 9.4 mm (P<0.001). At the time of surgery, seven of the 18 patients (39%) had no detectable tumour in the resected specimen; one had carcinoma in situ, one was stage T1, one was stage T2, and eight were stage T3. Eleven patients (61%) underwent an abdominoperineal resection, including four of the 11 (36%) with no ultimate evidence of residual carcinoma. Eight patients (44%) were node-positive. The sensitivity, specificity, and positive and negative predictive values of post-BT EUS in predicting residual tumour were 82%, 29%, 64% and 50%, respectively. The post-BT EUS accurately predicted the T-stage in eight (44%) patients; most errors were due to overstaging.
Rectal cancer T-staging by EUS post-BT is inaccurate, and although it appears sensitive in predicting the presence or absence of residual tumor in rectal adenocarcinoma after preoperative BT, the low predictive values in this setting limit its utility at this time.