Effects of Neoadjuvant Treatments on Mesorectal Computed Tomography Values and Tumor Regression in Rectal Cancer

The efficacy of preoperative chemoradiotherapy (CRT) for lower rectal cancer is primarily determined by its effects on the primary tumor. We recently reported an association between the change in mesorectal CT values (ΔCTVs) during chemoradiotherapy (CRT) and histological tumor regression grade (TRG). However, the influence of neoadjuvant systemic therapy (ST) on the mesorectum remains unknown. This study aimed to determine the association between ΔCTV before and after neoadjuvant therapy including systemic therapy (ST) and other preoperative, surgical and pathological factors.

Patients with lower rectal cancer who underwent CRT (93 cases) or ST (16 cases) before surgery were examined. Clinicopathological factors, including ΔCTV at predetermined points within the mesorectum were compared between the two neoadjuvant treatments. Multivariate analysis was also conducted to identify factors associated with good TRG (TRG 2/3) based on the Japanese Classification of Colorectal Carcinoma.

ΔCTV was higher with CRT than with ST (median: +8.7 vs. -6.0 Hounsfield Units; p=0.002). However, operative time (p=0.003) and blood loss (p=0.03) during rectal surgery were higher with ST than with CRT. Multivariate analysis identified CRT and ΔCTV >6.5 Hounsfield Units as independent predictors of good TRG.

Both CRT and high ΔCTV were associated with good TRG in rectal cancer. CRT generally increased ΔCTV, suggesting the induction of mesorectal edema. Despite this unfavorable condition for dissecting the surgical plane, short-term surgical outcomes were better with CRT than with ST owing to greater tumor shrinkage.