Changes in Mesorectal CT Values and Therapeutic Effect of Preoperative Chemoradiotherapy in Rectal Cancer

The efficacy of preoperative chemoradiotherapy (CRT) in lower rectal cancer is determined by its effects on the primary tumor. However, the effects on the mesorectum have not been investigated. Furthermore, edema in the dissection planes is frequently observed after postoperative CRT. Herein, we aimed to evaluate the changes in mesorectal computed tomography (CT) values (CTVs) before and after CRT and its relationship with treatment efficacy.

We retrospectively examined 46 patients with lower rectal cancer who had undergone preoperative long-course CRT before radical surgery. The CTVs were measured at four predetermined points within the mesorectum on the coronal and sagittal sections. The difference in the average CTVs before and after CRT (ΔCTV) was recorded for each patient. Associations between the ΔCTV and clinicopathological factors were investigated via univariate and multivariate analyses. Tumor regression grade (TRG) was divided into "poor response" (TRG 1a/1b) and "good response" (TRG 2/3) according to the Japanese Classification of Colorectal Carcinoma.

The mesorectal CTVs increased after CRT in 28 patients and decreased in 18 patients. The ΔCTV was higher in small tumors (p=0.007), ypT0-2 stage (p=0.01), and good TRG response (p=0.002). Multivariate analysis demonstrated an independent association between the increase in ΔCTV and good TRG response (p=0.050) or earlier ypT stage (p=0.029).

The ΔCTV, which varied among patients with lower rectal cancer, was associated with ypT stage and TRG. ΔCTV may be useful in predicting the tumor response to CRT.