Dosimetric comparison of pipe-joint versus conventional VMAT for lattice radiotherapy in bulky tumors: a retrospective planning study.

BACKGROUND: Lattice radiotherapy (LRT) is a promising spatially fractionated technique for bulky tumors. However, when lattice vertices extend over a large superior-inferior distance, conventional volumetric modulated arc therapy (c-VMAT) may require broad jaw apertures, which may compromise longitudinal dose modulation, increase leakage-related low-dose exposure, and exacerbate island-blocking-like effects. To address these limitations, we developed pipe-joint VMAT (PJ-VMAT), a beam's-eye-view-guided longitudinal segmentation strategy. METHODS: Nineteen consecutive patients with bulky tumors were retrospectively included. Spherical lattice vertices were generated within the gross tumor volume (GTV) using an in-house sphere-packing algorithm. For each patient, four plans were generated: lattice stereotactic body radiotherapy (L-SBRT) and standalone lattice radiotherapy (S-LRT), each implemented using c-VMAT or PJ-VMAT. Plans were normalized such that 95% of the lattice target volume received 100% of the prescription dose. The primary dosimetric endpoints were dose fall-off around the lattice vertices and the peak-to-valley dose ratio (PVDR). Secondary endpoints included background-dose reduction, normal-tissue sparing, plan complexity, and delivery efficiency. An exploratory geometry-based association analysis was performed to examine whether target-geometry descriptors were associated with the magnitude of dosimetric benefit from PJ-VMAT. No formal multiplicity adjustment was applied, and all reported values were nominal. RESULTS: Compared with c-VMAT, PJ-VMAT achieved steeper dose fall-off, with lower gradient index values in both L-SBRT (PJ-VMAT: 10.55 vs. c-VMAT: 12.72) and S-LRT (PJ-VMAT: 7.12 vs. c-VMAT: 8.33). PJ-VMAT improved most PVDR metrics, with larger gains observed in the S-LRT setting. In addition, PJ-VMAT reduced background dose to the GTV-LTV region, Body-GTV region, and GTV shell. These dosimetric advantages were accompanied by higher monitor units, greater plan complexity, and longer delivery times. Several associations between target geometry and dosimetric benefit were observed, particularly in the L-SBRT setting; however, these findings were exploratory and hypothesis-generating. CONCLUSIONS: In this in silico planning study, PJ-VMAT demonstrated favorable dosimetric performance compared with c-VMAT for LRT in bulky tumors.The observed advantages may reflect improved longitudinal dose modulation through segment-specific jaw restriction, resulting in enhanced peak-to-valley dose separation and reduced background-dose exposure. The exploratory geometry-related findings should not be used to guide individual patient selection and require validation in larger cohorts.