P. Singh1, J. M. Bryant2, S. Choo1, J. M. Frakes2, R. F. Palm3, S. Hoffe2, and G. Redler2; 1University of South Florida Morsani College of Medicine, Tampa, FL, 2H. Lee Moffitt Cancer Center and Research Institute, Department of Radiation Oncology, Tampa, FL, 3University of Toledo Medical Center, Toledo, OH
Purpose/Objective(s): Young onset locally advanced colorectal cancer is increasing in incidence, highlighting the need for strategies to decrease the risk of late pelvic toxicity. The recently reported PROSPECT trial has identified selective RT as a non-inferior option for patients responding appropriately to neoadjuvant FOLFOX systemic therapy. Current standard of care (SOC) clinical target volume (CTV) volumes include: mesorectum, presacral, internal iliac (T3) and external iliac nodes (T4). A reduced Total Mesorectal Irradiation (TMI) volume has been proposed to reduce toxicity and has been incorporated into the randomized phase II STAR TREC trial for patients with =T3bN0M0 disease. We hypothesized that TMI can significantly decrease the mean dose to organs at risk (OAR) for long term sequelae. Materials/
Methods: In this IRB approved retrospective study, we reviewed consecutive patients (n=20) with T3/4 and or node positive adenocarcinoma of the rectum who received neoadjuvant long course chemoradiation from 2021 to 2023. New TMI volumes as described in the STAR TREC trial were created as follows: total MRI detectible gross tumor volume (GTV) and CTV defined by the mesorectal fascia and presacral nodes with the superior border of the CTV defined by the cutoff of the pelvic brim, posteriorly at S2. A new TMI planning target volume (PTV) was created with a uniform 5 mm CTV expansion, smaller than the 10-15 mm margin in the STAR TREC trial facilitated by daily image guidance and potential online adaptation. Comparison plans were generated with both SOC and TMI targets for each patient, using fixed-field IMRT in a CBCT-guided online adaptive platform. All plans were normalized to provide 95% PTV coverage by 45 Gy in 25 fractions. The average organ at risk (OAR) radiation doses to the bladder, bowel, genitalia, iliac crests, and femoral heads were evaluated and compared. Results: CTV and PTV volumes with the TMI approach decreased compared to SOC by 43.3% and 39.4%, respectively. This resulted in appreciably lower OAR doses: mean bowel, bladder, and genitalia dose decreased by 58.7%, 23.2 %, and 19.2%, respectively. The bowel, bladder, and femoral heads max dose decreased by 19.2%, 23.2%, and 24.1%, respectively. The V10 iliac crest dose decreased by 40.1% and the V40 dose by 83.4%. Conclusion: Reduced volume TMI fields, planned with an online adaptive platform, improved the dosimetry profile with respect to bone marrow, bowel, bladder, and genitalia sparing while maintaining homogeneous/conformal target coverage by prescribed dose as compared to SOC fields on locally advanced colorectal cancer plans. Strategies incorporating reduced volume irradiation such as TMI for patients with locally advanced rectal cancer warrant further clinical investigation, especially given the option of online adaptive radiation integration to account for daily anatomical variation in the context of smaller targets and minimal margins.
