Presenter(s)
J. L. Mora, and A. J. Frankart; Department of Radiation Oncology, University of Cincinnati, Cincinnati, OH
Purpose/Objective(s): Lattice radiation therapy (LRT) is a form of spatially fractionated treatment involving delivery of ablative high-dose spheres within the gross tumor volume (GTV) to provide better control of bulky tumors while sparing adjacent normal tissue. It is unclear whether dosimetric parameters such as conformality are impacted as an increasing number of peak dose spheres are utilized for very large tumor volumes. This interim dosimetric analysis of a prospective phase 2 clinical trial was performed to characterize dosimetry of LRT planning over a multi-disease site retrospective cohort to evaluate the hypothesis that similar dosimetric outcomes are achieved across tumor volumes.
Materials/Methods: Twenty-two patients treated with LRT between 2024 and 2025 at a single academic medical center were retrospectively reviewed. The median patient age at treatment was 62 years. Disease sites included head and neck, lung, chest wall, liver, kidney, and pelvic regions, all treated with palliative intent. Treatment plans were created using VMAT-based approaches in standard departmental treatment planning software with prescribed doses of 50 Gy to the peak sphere volume and 20 Gy to the remaining GTV, all given over 5 fractions. Peak dose spheres were 1 cm in diameter with 3 cm center-to-center spacing between spheres and generated with automated Python scripting in the treatment software. Dosimetric parameters collected for each patient included GTV volume (cc), number of peak dose spheres, and V50, V40, and V30 for the GTV. Conformality index (CI) was defined as the volume of the GTV receiving 50 Gy peak dose divided by the total volume of the peak dose sphere structures. The relationships between variables were analyzed using correlation analysis and descriptive statistics.
Results: GTV volumes ranged from 74.7 to 3299.5 cc (median 272.3 cc). The median number of spheres was 7.5 (range 2-74) with an expected linear relationship between GTV volume and the number of spheres (r = 0.98). Dose-volume metrics V50, V40, and V30 were all strongly correlated with GTV volume (r = 0.98, r = 0.97, and r = 0.98, respectively) with outliers only present for tumor volumes greater than 1000 cc. The median CI was 1.5 (range 1.4 – 2.2) with tight clustering of CI values between 1.4 and 1.6 for tumors less than 500 cc in volume. The two cases with CI values =2 involved GTV volumes greater than 1000 cc.
Conclusion: Across a range of GTV sizes, volumetric measures of peak dose (V50) and moderate dose fall-off from peak dose spheres (V30) as well as conformality indices were tightly clustered for tumor volumes =500 cc with outliers only present for tumor volumes over 1000 cc. This suggests preservation of true spatial fractionation effects for bulky tumors up to 500 cc and the need for special attention in planning for larger tumors. With an increased understanding of the biologic effects of peak and valley dose, the importance of preserving these dose regions will grow and could necessitate differential planning approaches by tumor volume.