3212 - Enhanced Dosimetric Performance of Staircase Volumetric Modulated Arc Therapy in Lattice Radiotherapy for Bulky Tumors

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 14

POSTER

Presenter(s)

Changyou Zhong, - Radiotherapy Center of Meizhou People’s Hospital (Huangtang Hospital), meizhou,

C. Zhong¹, Z. Jiang¹, and Z. Dai²; ¹Radiotherapy Center of Meizhou People’s Hospital (Huangtang Hospital), meizhou, China, ²National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital & Shenzhen Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Shenzhen, China

Purpose/Objective(s): Lattice radiation therapy (LRT) is an emerging dose-modulation strategy for bulky, unresectable tumors. The peak-to-valley dose ratio (PVDR) is a critical dosimetric metric defining the unique dose distribution of LRT. Conventional VMAT (c-VMAT) for LRT is limited by multileaf collimator (MLC) leakage and the “island blocking problem” between adjacent lattice vertices, degrading PVDR. The piecewise VMAT technique established by Xu et al. has shown robust modulation for spatially discrete targets. Herein, we rename it Staircase VMAT (St-VMAT) for its stepwise longitudinal field design, and investigate its dosimetric performance in LRT for bulky tumors, compared head-to-head with c-VMAT.

Materials/Methods: Nineteen patients with bulky tumors were retrospectively included. For each patient, 6–34 spherical lattice vertices (lattice target volume, LTV) were automatically generated within the gross tumor volume (GTV) using an in-house close-packing algorithm. A prescription dose of 36 Gy in 3 fractions was prescribed to the LTV. Two LRT plans were generated: St-VMAT and c-VMAT. For St-VMAT plans, the collimator was rotated to 90°, and each arc was limited to a maximum field width of 2.5 cm, with a 0.5 cm overlap between adjacent longitudinal arcs. Dosimetric evaluation included: (1) LTV metrics: D_mean, D_2%, V_110%, gradient index (GI); (2) GTV parameters: D_mean, equivalent uniform dose (EUD), and D₂/D₅/D₁₀/D₅₀/D₉₀/D₉₅; (3) PVDR (D₁₀/D₉₀, D₅/D₉₅, D₂/D₅₀); (4) mean dose to normal tissue (Body–GTV); (5) plan complexity/delivery: monitor units (MU), modulation complexity score (MCS), delivery time.

Results: Compared with c-VMAT, St-VMAT achieved significantly higher PVDR for all 3 definitions (all p = 0.001). For LTV, St-VMAT yielded higher V_110% (64.06% ± 8.08% vs. 60.41% ± 13.03%, p = 0.018) and steeper GI (7.19 ± 2.31 vs. 8.26 ± 3.69, p < 0.001), with no differences in LTV D_mean or D_2% (p > 0.05). For GTV, St-VMAT reduced D_mean (11.93 ± 2.34 Gy vs. 12.69 ± 2.48 Gy, p < 0.001), EUD (1.39 ± 0.85 Gy vs. 1.65 ± 1.07 Gy, p < 0.001), and D₁₀/D₅₀/D₉₀/D₉₅ (all p = 0.001). Mean normal tissue dose was lower (2.31 ± 2.48 Gy vs. 2.52 ± 2.72 Gy, p < 0.001). St-VMAT increased complexity: higher MU (16019.42 ± 3095.68 vs. 7690.01 ± 1450.27, p < 0.001), longer delivery time (11.44 ± 2.21 min vs. 5.49 ± 1.04 min, p < 0.001), and lower MCS (0.25 ± 0.018 vs. 0.28 ± 0.028, p < 0.001).

Conclusion: This study represents the first application of the Staircase VMAT technique to LRT for bulky tumors. By mitigating MLC leakage and island blocking, this strategy significantly improves the defining dosimetric metrics of LRT – including enhanced PVDR, higher LTV V_110%, steeper perilesional dose fall-off, and reduced normal tissue irradiation – without compromising baseline LTV dose coverage. Despite a modest increase in plan complexity and delivery time, St-VMAT is a feasible, effective, and clinically valuable planning strategy to optimize the dosimetric performance of LRT for bulky, unresectable tumors.