Purpose: Current studies report the protocol for the use of TTF in treatment of glioblastoma as CT simulation using a foam pad as a surrogate for the high-density TTF transducer array followed by the delivery of external beam radiotherapy with the array in place. This work assesses the need for a dosimetric correction factor using these conditions.
Methods: Measurements were taken on an Elekta VersaHD accelerator with and without the transducer array for depths of 0–20 cm using solid water slabs for the following energies: 6 MV, 10 MV, 18 MV, 6 MV FFF, and 10 MV FFF. Surface measurements were taken with OSLDs, measurements up to 1 cm were taken with a parallel plate chamber, and measurements 1 cm and greater were taken with a cylindrical chamber. For each depth and energy, 100 MU was delivered with a 10x10 cm^2 open field at 100 cm SAD, and ratios of the readings with the array to readings without the array were determined.
Results: Ratios ranged from 0.96–2.35 for 6 MV, 0.97–3.34 for 10 MV, 0.97–4.08 for 18 MV, 0.96–2.43 for 6 MV FFF, and 0.96–2.87 for 10 MV FFF. The deviations between readings with and without the transducer array were greatest for shallow depths for each energy. The ratios decreased with increasing depth throughout the build-up region before stabilizing near d_max for each energy.
Conclusion: Utilizing lower energy beams for radiation treatments with TTF transducer arrays will reduce the discrepancies in the build-up region between dose calculated without the array and dose delivered with the array in place. It is recommended to apply a correction factor for targets located at shallow depths, as differences for measurements taken with and without the transducer arrays were most pronounced for depths shallower than d_max for each energy.