2273 - Orthogonal vs. Parallel Dual-Layer Multileaf Collimator in Volumetric Modulated Arc Therapy with Simultaneous Integrated Boost for Stage IIIc Cervical Cancer: A Dosimetric Comparison

Purpose/Objective(s): This study evaluated the dosimetric differences between the orthogonal and parallel dual-layer multileaf collimator (MLC) in volumetric modulated arc therapy with simultaneous integrated boost (VMAT-SIB) for lymph node metastasis (LNM) in cervical cancer (FIGO 2018 stage IIIC).

Materials/Methods: This retrospective study analyzed dosimetric data from 45 stage IIIC cervical cancer patients (three 15-patient subgroups each with unilateral pelvic, bilateral pelvic, and para-aortic LNM) treated with VMAT-SIB at a single institution (Feb 2022–Mar 2024). The prescribed dose was 45–50.4 Gy in 25–28 fractions (1.8–2.0 Gy/fraction, 5 fraction/week) for the planning target volume (PTV) and 54–63 Gy (2.1–2.3 Gy/fraction) for positive lymph nodes (PGTVnd) with SIB. Orthogonal (TiGRT MC TPS v2.0/VenusX) and parallel (Eclipse v15.0 / Halcyon 2.0) plans were generated, with orthogonal MLC using a single arc (179 - 181° counterclockwise) and Monte Carlo Algorithm, while parallel MLC used 2 arcs (179 - 181° ) and the Anisotropic Analytic Algorithm (AAA). SPSS 26.0 were used to conduct paired t-tests to compare dosimetric parameters.

Results: Both orthogonal and parallel MLC plans achieved clinically acceptable target coverage across all 3 subgroups. For target coverage, the gradient index of PTV was significantly lower with orthogonal MLC in the bilateral pelvic LNM subgroup (P < 0.05), while other dosimetric parameters (D₉₅, V₉₅, V₁₁₀, conformity index, and homogeneity index of the PTV and PGTVnd) showed no significant differences. Regarding organ-at-risk (OAR) sparing, orthogonal plans reduced radiation exposure to the small intestine, colon, rectum, bladder, left femoral head, cauda equina, and spinal cord compared to parallel plans (P < 0.05). These OAR-sparing advantages were consistent across the entire cohort and within 3 subgroups. After comparing the 2 groups, the key dosimetric parameters with statistically significant differences in OAR among the 45 patients are summarized in the table 1.

Conclusion: Dosimetric analysis confirmed that both MLC types meet clinical target coverage requirements. In more complex cases, orthogonal MLC provides superior OAR sparing. Further large-scale studies are needed to validate these findings and assess their clinical significance. Ethics Approval: Approved by the Ethics Committee, Peking University Third Hospital (No. 2023-633-02).