Dosimetric feasibility of lattice radiotherapy for bulky tumors: a paired VMAT vs. helical tomotherapy comparison - the STRYKE trial

Background Lattice Radiotherapy (LRT) delivers ablative doses to intratumoral vertices alternating with low-dose valleys to treat bulky tumors. While VMAT-LRT is widely adopted, Helical Tomotherapy (HT-LRT) may offer enhanced spatial dose modulation. This study provides the first paired dosimetric comparison of HT-LRT versus VMAT-LRT across scalar, directional, and DVH-based endpoints. Methods Paired HT-LRT and VMAT-LRT plans were evaluated in a cohort of 22 patients with bulky tumors using a staggered triangular lattice geometry (5.0 cm inter-vertex spacing, 1.0 cm diameter). Spatial dose modulation was characterized using three scalar Valley-to-Peak Dose Ratio (VPDR) computational methods (VPDR Wu , VPDR D90/D10 , VPDR median ) and a directionally resolved decomposition along principal and diagonal spatial axes. Target coverage, OAR sparing, and morphological correlations were systematically analyzed. Results HT-LRT demonstrated statistically significant superiority over VMAT-LRT across all scalar VPDR metrics ( p < 0.05). Directional analysis revealed that HT-LRT advantage is anisotropic, concentrating along intra-plane radial and principal axes, whereas oblique inter-plane diagonals showed comparable modulation between techniques. HT-LRT yielded significantly cooler valley doses and improved low-dose OAR sparing without compromising target coverage ( D 95% ). Target elongation positively correlated with radial VPDR in both modalities due to inter-plane dose summation, while compactness influenced radial VPDR exclusively in HT-LRT. Conclusion HT-LRT should provide superior, directionally selective spatial dose modulation compared with VMAT-LRT in bulky tumors. Directionally resolved VPDR analysis exposes technique-specific geometric strengths obscured by scalar summary metrics, supporting HT-LRT as a promising platform for LRT execution, while the relative contribution of delivery technique and planning strategy warrants further investigation.