2143 - Dual-Isocenter MR-Linac Workflow for Cervical Cancer: Reducing GI Toxicity in Large-Volume Irradiation via Daily Adaptive Planning
Presenter(s)
K. Li1, Y. Yin2, C. Wang3, Z. Li4, and Y. Chen5; 1Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, 2Department of Radiation Oncology, Shandong University Cancer Center, Jinan, Shandong, China, 3Department of Gynecologic Oncology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, 4Department of Radiation physics,Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China, 5Elekta Instrument Ltd. Beijing Branch, Shanghai, China
Purpose/Objective(s): To present a clinical workflow implementing a dual-isocenter technique on an MR-Linac for daily adaptive cervical cancer treatment, enabling the irradiation of extended target volumes while improving organ-at-risk (OAR) sparing.
Materials/Methods: This workflow accounts for inter-fraction anatomical changes by adjusting structures on the MRI scan for the first isocenter plan. The first isocenter, aligned with the center of the planning image set, was positioned (1) near the midpoint of the total planning target volume (PTV), with one edge at the maximum field length, and (2) to encompass PTV regions and OARs prone to intra-fraction motion as much as possible. The second isocenter was positioned to cover the remaining PTV while ensuring an acceptable composite dose. Its reference plan was optimized based on bias-dose distribution from the treated first isocenter plan. A pre-developed template divided the total PTV into sub-PTVs using three slab structures to create a wedged dose distribution at the plan junction, ensuring composite dose robustness. After shifting the couch by the inter-isocenter distance, the second isocenter plan was treated using either the “adapt-to-position” (ATP) or a simplified “adapt-to-shape” (AST) workflow without additional PTV modifications. Ten cervical cancer patients with nodal involvement (PTV length: 23.7 ± 1.5 cm) were treated with 50 Gy to the clinical target volume (CTV) and 45 Gy to the PTV in 25 fractions. Composite doses to the PTV and OARs (rectum, bladder, and small bowel) were evaluated.
Results: On a 1.5T MR-Linac (Elekta AB, Stockholm, Sweden), the dual-isocenter workflow extended the treatable target length from 22 cm to 25 cm with a modest time increase, enabling treatment for most cervical cancer cases. An MRI field of view (FOV) exceeding 30 cm is necessary to further extend treatment beyond 25 cm. Daily adaptive planning revealed significant inter- and intra-fraction motion. Dosimetric analysis demonstrated improved target coverage and reduced OAR doses compared to conventional IGRT, as simulated by two ATP treatments. No radiation enteritis was observed in the ten treated patients.
Conclusion: The dual-isocenter workflow on an MR-Linac is a feasible and effective approach for treating cervical cancer. Daily adaptive replanning enhances target coverage and OAR sparing, reducing GI toxicity. This workflow expands the treatable patient population on MR-Linac and has the potential to improve clinical outcomes in cervical cancer radiotherapy.