Purpose: Spatially fractionated radiation therapy (SFRT) delivered via volumetric lattices with suggested radiobiological mechanisms (abscopal or immune effects) has enabled effective radiation therapy for bulky tumors. However, at present, lattice placement is a manual and suboptimal process. Here, we propose an automated and efficient solution to optimize patient-specific lattice geometry to achieve maximum target overlap in SFRT.
Methods: Spherical boosting and avoidance volumes were created in a three-dimensional, body-centered cubic lattice pattern (1.5 cm diameter, 4.2 cm in-plane spacing) for five clinical patients with bulky (390-2210 cc) tumors. Using automation, this base lattice was iteratively repositioned through a search space of three-dimensional translations and rotations, at each position calculating the volumetric union of repositioned boost lattice and fixed target core (1 cm retraction of GTV), defined as the lattice-core overlap volume (LCOV). Scripted automatic planning infrastructure was created to produce fifty-two SFRT treatment plans for each patient, ten each for lattice orientations near the 25th, 50th, 75th, 90th, and 95th percentiles of LCOV, as well as for minimum and maximum LCOV. LCOV was analyzed for correlation to the achievable PTV V₃₀Gy as a surrogate for dose heterogeneity.
Results: LCOV was extremely sensitive to lattice position, ranging from <25% to >200% of a patient’s median LCOV. Searching LCOV in 3 mm translation and 30° rotation steps produced robust approximations in less than one hour of computation. Despite the PTV V₃₀Gy varying widely among the treatment plans created for a patient, the average V₃₀Gy observed in each LCOV percentile subgroup was within one standard deviation of all other subgroups.
Conclusion: Lattice orientation influences the intermediate dose levels in SFRT treatment plans. However, the LCOV correlates poorly with the relative volume of achievable dose heterogeneity. When coupled with further radiobiological evaluation, automated optimization of lattice orientation will reduce variability in lattice SFRT.
Optimization, Treatment Planning, Treatment Techniques
TH- External Beam- Photons: Development (new technology and techniques)