50P - Linear tumor regression of rectal cancer in daily MRI during preoperative chemoradiotherapy: An insight of tumor regression velocity for personalized cancer therapy

Background

Neoadjuvant chemoradiotherapy (CCRT) is the current standards of care for locally advanced rectal cancer. Precise observation of tumor during CCRT with daily MRI can provide real-time treatment sensitivity and tumor biology, enabling patient-driven treatment decision at early phase.

Methods

Patients with cT3-4 and/or cN+ rectal adenocarcinoma undergoing preoperative CCRT with capecitabine on the pelvis up to 50 Gy in 25 fractions from 2018 to 2019 were consecutively included. Rectal tumor volume was measured by single physician (YKK) using daily 0.35T MRI of an MR-guided linear accelerator. Primary endpoint was to assess the pattern of tumor volume regression throughout the full course of CCRT using daily registration MRI. Secondary endpoint was to assess the effect of tumor regression velocity on disease-free survival (DFS). Tumor regression velocity (cc) per fraction of each patient was calculated using the simple regression analysis of tumor volumes from fractions 1 to 25.

Results

Twenty patients were included. Daily tumor volumetry demonstrated linear tumor regression during CCRT. Tumor regression velocity of 20 patients was 2.40 cc per fraction (R2 = 0.93; P < 0.001). Median tumor regression velocity was 1.52 cc per fraction. Rapid regressors had tumor regression velocity > 1.52 cc per fraction (N = 9). Slow regressors had tumor regression velocity < 1.52 cc per fraction (N = 11). Rapid regressors had significantly greater tumor regression velocity (4.58 cc per fraction) than slow regressors (0.78 cc per fraction; P < 0.001). Rapid regressors had numerically superior DFS rate compared to slow regressors. The 2-year DFS was 88.9% for rapid regressors and 72.7% for slow regressors, respectively (P = 0.400).

Conclusions

This study is the first observation of linear tumor regression in daily MRI during preoperative CCRT of locally advanced rectal cancer. Daily tumor regression velocity numerically discriminated DFS. Although this study is hypothesis-generating, the potential of CCRT from therapeutics to a newer level, the theranostics, has been sufficiently suggested. Further validation studies for the value of daily tumor volumetry on treatment decisions are warranted.

Clinical trial identification

Editorial acknowledgement

Legal entity responsible for the study

The authors.

Funding

Has not received any funding.

Disclosure

All authors have declared no conflicts of interest.