Dosimetric study on the effect of minimum distance between optimization structures and planning target volume on small intestine dose in cervical cancer VMAT plans

Cervical cancer radiotherapy often faces challenges in managing gastrointestinal toxicities, particularly due to the dose received by the small intestine. This study investigates the impact of optimization structure margins (1.2-2.4 cm) on reducing small intestine dose and improving plan quality in patients with mean small intestine doses (Dmean_INT) exceeding 25 Gy. A total of 27 cervical cancer patients treated with volumetric modulated arc therapy were retrospectively analyzed. Iterative constraint adjustments were applied to optimization structures at varying distances from the planning target volume (PTV). Five optimization margins (0.6, 1.2, 1.8, 2.4, and 3.0 cm) were created by volumetrically excluding small intestine regions proximal to PTV, with iterative dose constraints. Each plan was named RrIi, where r represents the cutting distance of the optimization structure and i represents the iteration number (ranging from 4-8). Results demonstrated that all optimized plans achieved significant reductions in Dmean_INT compared to the original plans (ORI group), with reductions ranging from 1.21 to 1.83 Gy (P < .001). The R_1.2I_5.5 protocol achieved the most favorable balance, reducing Dmean_INT by 1.23 Gy, corresponding to a 15% to 22% relative risk reduction for Grade ≥ 2 enteritis. Smaller margins (1.2 cm) effectively reduced intermediate-high-dose regions (e.g., V40Gy), while larger margins (2.4 cm) better controlled low-dose regions (e.g., V15Gy), supporting tailored clinical decision-making based on patient characteristics. In conclusion, optimization structure margins of 1.2 to 2.4 cm provide a clinically meaningful framework for reducing small intestine dose while preserving PTV coverage, advancing cervical cancer radiotherapy planning.