2952 - Dosimetric Comparison of VMAT with Dynamic Collimator Rotation and Modulated Ports (VMATp) vs. Conventional VMAT for Bowel Sparing In Cervical Cancer Extended-Field Radiotherapy

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 26

POSTER

Presenter(s)

Kansiree Benjangkhaprasert, MD - King Chulalongkorn Memorial Hospital, Bangkok, Krung Thep

K. Benjangkhaprasert¹, T. Sanghangthum², I. Israngkul Na Ayuthay³, M. Keawsamur², S. Kingkaew³, N. Chatchumnan³, J. Suntiruamjairucksa¹, K. Thonglert¹, C. Chakkabat², and N. Amornwichet²; ¹Division of Radiation Oncology, Department of Radiology, King Chulalongkorn Memorial Hospital, Thai Red Cross Society, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand, ²Division of Radiation Oncology, Department of Radiology, Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand, ³Division of Radiation Oncology, Department of Radiology, King Chulalongkorn Memorial Hospital, Bangkok, Thailand

Purpose/Objective(s):

To compare dosimetric outcomes of conventional volumetric modulated arc therapy (VMAT) and VMATp, which integrates dynamic collimator rotation and static angle modulated ports, for cervical cancer extended-field radiotherapy (EFRT). The study utilized a standardized multi-planner workflow to evaluate whether VMATp reduces bowel dose while maintaining target coverage.

Materials/Methods:

Five medical physicists established consensus configurations for arc geometry and static angle ports. Inter-planner reproducibility was verified on a single standardized case using the coefficient of variation (CV) for PTV D95% and bowel bag V30Gy/V40Gy for both techniques. Subsequently, twenty EFRT cases were replanned with balanced workloads and blinded comparator identities. All plans used identical contours and EMBRACE II–based dose constraints. The prescription was 45 Gy in 25 fractions, with simultaneous integrated boosts of 55 Gy to pelvic lymph nodes and 57.5 Gy to paraaortic lymph nodes. Primary endpoints were bowel V30Gy and V40Gy (cc). Secondary endpoints included PTV D95%, PTV Dmax, and other organ-at-risk (OAR) metrics. Paired comparisons performed using Wilcoxon signed-rank tests with exact p-values.

Results:

The consensus configurations used VMAT three arcs for VMAT and two arcs for VMATp (one static angle modulated ports at 0°, 90°, 135°, 225°, and 270° and one planner-selected setting). Reproducibility testing demonstrated adequate standardization (CV <5%). In the 20 paired cases, VMATp significantly reduced median bowel dose compared to VMAT: median bowel V30Gy was 381.2 cc (IQR 306.8–446.4) vs 450.9 cc (383.7–509.6) (median paired difference -73.5 cc; improved in 85% of patients; p<0.001), and median bowel V40Gy was 169.0 cc (124.7–245.8) vs 215.0 cc (150.4–276.2) (median paired difference -25.8 cc; improved in 85% of patients; p=0.002). PTV coverage and Dmax were comparable between both techniques (p>0.3). Rectum dose was also lower with VMATp: median rectum V30Gy was 80.43% (IQR 73.97–90.98) vs 87.05% (78.93–96.42) with VMAT (p=0.036), and median rectum V40Gy was 69.84% (57.67–76.56) vs 73.04% (67.21–79.37) (p=0.090). Bladder dose showed no significant difference, and all other OARs met established constraints.

Conclusion:

In cervical cancer EFRT, a two-arc VMATp strategy provides meaningful bowel sparing compared with conventional three-arc VMAT while preserving target coverage and meeting all constraints, supporting its use as a reproducible bowel-sparing planning approach.