Phantom-Based Dosimetric Comparison of Helical and Fixed-Beam TomoTherapy for Spatially Fractionated Radiotherapy...

IntroductionSpatially Fractionated Radiotherapy (SFRT) delivers intentionally heterogeneous dose distributions with alternating high- and low-dose regions and has been widely applied in the management of bulky tumors. In SFRT, high-dose sub-volumes (referred to as vertices) are distributed within the gross tumor volume (GTV) to create a spatial peak-valley dose pattern. However, studies of SFRT using the TomoTherapy platform remain limited. This study evaluates the dosimetric performance of GRID and lattice vertex designs using TomoHelical and TomoDirect techniques.MethodsA phantom-based planning study was performed with two simulated GTV representing medium- and large-sized tumors. Cylindrical GRID vertices and spherical lattice vertices were created with diameters of 1.00 cm and 1.25 cm and corresponding center-to-center spacings of 3.0 cm and 2.5 cm, respectively. Treatment plans were created using TomoHelical and TomoDirect techniques with identical prescription criteria, requiring at least 50% of the target volume to receive 15 Gy in a single fraction. Beam-on time, normal tissue dose metrics (V30% and V50%), homogeneity index, conformity index, peak-to-edge dose ratio (PEDR), and peak-to-valley dose ratio (PVDR) were evaluated. Delivery accuracy was assessed using ArcCHECK measurements with a 3%/2 mm gamma criterion.ResultsAll plans met prescription and delivery accuracy requirements, with gamma passing rates exceeding 95%. TomoHelical produced higher PVDR and PEDR values, improved conformity, and lower V50% compared with TomoDirect. TomoDirect achieved shorter beam-on times but showed greater variability in vertex mean dose. Lattice configurations yielded higher PVDR values than GRID, while vertex diameter had minimal impact on most dosimetric parameters.ConclusionTomoHelical delivery combined with lattice designs provided superior dose modulation and normal tissue sparing for SFRT, while requiring longer delivery times. In contrast, GRID designs enabled faster treatment delivery. These findings provide practical guidance for optimizing SFRT planning using the TomoTherapy platform.