Purpose: To investigate the impact of vertex size to the dose metrics of lattice spatially fractionated radiation therapy (L-SFRT) in treating bulky pelvic tumors.
Methods: Two patients previously treated with 3DCRT to the pelvis for large bulky disease were retrospectively planned using Pinnacle (Version 16.2.1) treatment planning system VMAT techniques with Varian Truebeam 6 MV FFF beam. The plan was optimized to deliver prescription dose to a lattice of spheres throughout each delineated target volume. Each patient had 3 separate plans generated using 3 sphere diameters (1.5 cm, 2.0 cm, and 2.5 cm) while maintaining 3.0 cm center-to center spacing in any direction (generated in MIM Version 7.1.4). A total of 3-4 full coplanar arcs were used to deliver the prescription dose of 400 cGy in 5 fractions, to the alternating “PTV_hot” spheres. Dose volume histograms of those six plans were analyzed. Equivalent uniform dose (EUD) and valley/peak dose ratios (VPDR) were derived; the therapeutic ratios (TR) were calculated by assuming tumors as radiosensitive, moderately radiosensitive and radioresistant respectively.
Results: Equivalent uniform dose was found from 5.6 to 10.9 Gy; a larger vertex size tends to give a larger EUD. VPDRs derived from D90/D10 were found consistent. For patient 1 the VPDRs for vertex sizes of 1.5, 2 and 2.5 cm are 0.31, 0.30 and 35; for patients 2, they are 0.41, 0.38 and 0.40 respectively for the same sizes of vertexes. TRs are from 1.7 to 584.8. A larger vertex size and radioresistant tumor give a larger TR.
Conclusion: L-SFRT is advantageous to treat radioresistant bulky tumors, because it shows a larger TR comparing radiosensitive tumor. A larger vertex size should be considered because it delivers a larger EUD, but VPDRs are consistent regardless of their vertex size. More studies are needed to confirm our dosimetry findings.