Dosimetrical and geometrical parameters in single-fraction lattice radiotherapy for the treatment of bulky tumors: Insights from initial clinical experience

•Lattice radiotherapy is feasible for a wide range of bulky tumor volumes.•Reporting lattice radiotherapy geometrical and dosimetrical parameters is essential.•Valley and peak dose definitions should be unified to improve data reporting.•New valley-to-peak dose ratio definition represents spatially fractionated pattern.•Correlations among variables help understanding lattice planning and results. This study aims to investigate lattice radiotherapy (LRT) for bulky tumor in 10 patients, analyzing geometrical and dosimetrical parameters and correlations among variables. Patients were prescribed a single-fraction of 18 Gy to 50 % of each spherical vertex (1.5 cm diameter). Vertices were arranged in equidistant planes forming a triangular pattern. Center-to-center distance (Dc-c) between vertices was varied from 4 to 5 cm. A new method for calculating the valley-to-peak dose ratio (VPDR) was proposed and compared to other two from existing literature. GTV volumes (VGTV), vertex number (Nvert), low-dose related parameters and vertex D99%, D50%, and D1% were recorded. Beam-on time and Monitor Units (MU) were also evaluated. Correlations were assessed using Spearman’s coefficient, with significant differences analyzed using Mann-Whitney U test. Tumor volumes ranged from 417 to 3615 cm3. Median vertex number was 14.5 (IQR:11.3–17.8). VPDR ranged from 0.16 to 0.28. Median D99% spanned from 10.0 to 13.7 Gy, median D50% exceeded 18.0 Gy, and median D1% surpassed 23.3 Gy. Periphery dose remained under 4.0 Gy. Plans exhibited high modulation, with median beam-on time and MU of 8.8 min (IQR:8.2–10.1) and 13,069 MU (IQR:11574–13639). Significant correlations were found between Nvert and VGTV (p