Differential Response Rates of Gastrointestinal Tumors During Real-time MRI-Guided Radiation Therapy: Opportunities for Treatment Individualization

Abstract

Changes in diagnostic MRI at a single mid-treatment time point during neoadjuvant chemoradiation (CRT) have potential to predict treatment response in locally advanced gastroesophageal junction (GEJ) and rectal cancer patients. While using intra-treatment imaging as a biomarker may allow personalization of therapy, selecting the optimal timing of response assessment for these patients is unclear. We hypothesized that real-time MRI-guided radiotherapy would allow for repetitive temporal assessment of volumetric changes in gross tumor volume (GTV) for gastrointestinal cancer patients (pts). GEJ (n = 5) and rectal (n = 4) adenocarcinoma pts who underwent neoadjuvant MRI-guided radiotherapy (CRT) were selected for this study. CRT regimen for GEJ ca was weekly carboplatin and paclitaxel with RT, 41.4-50.4 Gy in 23-28 fractions (fxns). Rectal cancer pts underwent pelvic CRT with daily oral capecitabine and RT, 50-50.4 Gy in 25-28 fractions. Pts underwent MRI during simulation and at each treatment fraction. The GTV was contoured on MRI at baseline (B) and every fifth fraction for all pts. For this analysis, fractions 23 or 25 were designated as the final evaluation time point. Change in GTV was expressed as a percentage volume difference (ΔGTV = [GTVB-GTVfxn]/GTVB*100). A threshold ΔGTV of 50% was selected for both cohorts to identify relevant time points. All pts in this analysis had adenocarcinoma histology. A total of 54 imaging data sets were available for ΔGTV analysis. For GEJ pts, a mean ΔGTV of 50.2% (SD 3.5%) was seen at fraction 10 when compared to baseline. The mean ΔGTV at fraction 23 (final fraction) when compared to baseline was 64.7% (SD 8.5%). In comparison, rectal pts experienced a mean ΔGTV of 51.6% (SD 7.0%) at fraction 20 when compared to baseline. A mean ΔGTV of 59.5% (SD 3.5%) was seen at fraction 25 when compared to baseline. At fraction 10, the mean ΔGTV was only 32.9% (SD 9.5%) for the rectal cohort compared to 50.2% (SD 3.5%) for GEJ pts. The number of patients, clinical outcomes, and correlation with pathological response will be updated at the time of presentation. Real-time MRI-guided radiotherapy allows for the temporal assessment of volumetric changes in tumor during treatment. GEJ tumors responded rapidly to CRT while rectal cancer showed a more protracted regression demonstrating inherent differences in tumor responsiveness despite similar histologies. By assessing rates of response, we hope to correlate volumetric changes with pathological response outcome and thereby tailor treatment to individual tumor biology in future studies.