3308 - Daily Online Adaptive Radiotherapy for Rectal Cancer Utilizing CT-Linac: Assessing Feasibility, Toxicities and Optimal Margins

W. Wang¹, Y. Zhang², Z. Gu², F. Zhang², C. Wang², K. Hu¹, J. Tang², and P. Peng³; ¹Department of Radiation Oncology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China, ²Department of radiation oncology, Peking Union Medical College Hospital. Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China, ³Shenzhen United Imaging Research Institute of Innovative Medical Equipment, Shenzhen, China

Purpose/Objective(s): Radiotherapy for rectal cancer requires a 8-10mm margin around the clinical target volume (CTV) to account for variability in rectal and bladder filling, tumor regression. Online adaptive radiotherapy (oART), with daily re-planning, enables precise dose delivery and potential margin reduction. Although current oART strategies commonly employ CBCT or MRI guidance, the former may offer inferior image quality, whereas the latter can be time consuming. In contrast, CT-Linac-based oART, utilizing diagnostic-quality fan-beam CT (FBCT) guidance, strikes a balance between image quality and treatment efficiency. This study reports on the feasibility and toxicities of oART for rectal cancer with a CT-Linac and analyzes the optimal margins needed. Materials/

Methods: A Phase 2 trial enrolled 13 patients with locally advanced rectal cancer. All patients received long-course neoadjuvant radiotherapy with a 45Gy/25 fractions dose to the CTV. To form the planning target volume (PTV), 5mm margins were added to the CTV. Daily oART was administered using a CT-Linac. Pre-treatment FBCT scans were conducted, and the CTV and OARs were automatically contoured, subject to physician review and confirmation. Treatment plans were then automatically generated. Post-treatment FBCT scans were also obtained. Acute toxicities were graded using CTCAE v5.0. To analyze the margins, CTV contours were drawn on both pre- and post-treatment FBCT. The study employed uniform expansions to PTVpre to estimate the minimum margin necessary to cover CTVpost. Relative distance (RD), target covering rate (TCR), and false coverage rate (FCR) for OARs were used to identify the optimal margin. RD quantifies the distance between PTVpre and CTVpost, where negative values denote overlap and positive values indicate insufficient coverage. TCR and FCR assess target coverage and avoidable OAR irradiation, respectively.
Results: All patients successfully completed radiotherapy with an average treatment duration of 18 minutes. The incidences of =Grade 2 acute gastrointestinal and genitourinary toxicities were 23.0% and 7.7%, respectively, with no = Grade 3 acute toxicity. Mean RD, TCR and FCR values were shown in the Table. Statistical significance (p<0.01) was observed between all margin comparisons. Most RD values for 1mm and 2mm margins were positive, suggesting incomplete CTV coverage by the PTV. Conversely, results from 3mm to 5mm showed negative RD values, indicating that the majority of the CTV was adequately covered by the PTV. Thus, balancing target coverage and OAR protection, a 3mm margin was deemed sufficient for the CTV when applying daily oART.
Conclusion: Daily online adaptive radiotherapy for rectal cancer using a CT-Linac is not only feasible but also associated with a low incidence of acute toxicities. With daily oART, the margin for CTV can be safely reduced to 3mm for rectal cancer.

Abstract 3308 – Table 1

Margins	RD	TCR	FCR
1mm	1.57	0.982	0.114
2mm	-0.06	0.991	0.1667
3mm	-2.31	0.997	0.292
4mm	-3.50	0.998	0.370
5mm	-4.59	0.999	0.449