Purpose: To validate a novel and practical volumetric modulated arc therapy (VMAT) planning method for Biology Spatial Fractionated (SF) therapy (GRID).
Methods: Spherical contours measuring 1.5 cm in diameter are distributed in a bulky tumor ranging in sizes from 500 to 3000 cm3. Each contour must be at least 3 cm apart, center to center and at least 1 cm from any organ at risk (OAR). The spheres are then grouped together and planned as one target. Three concentric ring structures (0.5, 1 and 3 cm) are created to optimize the dose to the center of the spheres and minimize the dose surrounding the spheres. The result is alternating regions of high and low dose to the tumor and minimal dose to the OAR’s The number of spheres varied from 12 to 30 depending on the size of the tumor. High dose rate filter-free (FFF) beams are used to deliver the entire treatment, typically in less than 20 minutes.
Results: The validation is performed on 5 previously treated bulky liver or stomach patients using anonymized data. The spheres were placed by expanding the ROI size to 3 cm arranging them to not overlap, until the tumor was completely filled. Once the tumor was filled, the sphere size was reduced to 1.5cm diameter. The planning was optimized using similar objectives as other VMAT multi-target SRS planning techniques and then delivered on a Varian TrueBeam STX. Dose measurements were collected using a standard multidetector planar array. The treatments varied from 2-8 arcs. All five treatment plans passed 95% gamma at 3%/3mm 10% threshold.
Conclusion: We have validated a novel approach to planning GRID therapy patients. VMAT can be used to treat bulky tumors in a SF approach both as a single treatment or as a boost, without sacrificing dose to surrounding tissue.