Temporal Resolution Analysis of a 6 MV Photon Beam from Two Linac Vendors Using a Novel 3D Cubic Electronic Detector Array
Abstract
Purpose
To experimentally quantify a radiotherapy beam ramp-up to treatment output, de-tuning effects, and linac dose rates for a 6 MV photon beam from two radiotherapy linac vendors.
Methods
An improved 3D cubic electronic detector array comprising a 30x30x15 cm3 PMMA phantom with 5 active-matrices (AMs), of 529 detectors per AM (2645 total), at 1.5 cm, 3.5 cm, 5.5 cm, 7.5 cm, and 9.5 cm depths; and 44.16 ms data readout time, was used. Measurements consisted of 50 MU irradiations with 6 MV beams from an Elekta Infinity and Varian Trilogy linacs with dose rates of 78 - 630 MU/min for Infinity, and 100 - 600 MU/min for Trilogy. Irradiation setup was 100 cm SSD and 10x10 cm2 field size. Several measurements per linac were collected for quality assessment.
Results
Infinity showed a consistent 0.8-2.1 (mean≈1.5) second ramp-up for all dose rates prior to reaching treatment outputs attributed to RF injection into its traveling waveguide. No comparable ramp-up was observed with Trilogy, likely due to its standing waveguide use. On the other hand, Infinity had a uniform treatment output while Trilogy showed a de-tuning effect of 0.7-2.8 (mean≈1.4) second range before reaching treatment output, resulting in a 11-40% output deficit within that time range. De-tuning effects were evident for all dose rates. Finally, measured dose rates, compared to a selected/dialed dose rate in a linac console, differed by 1.6% for Infinity and 0.9% for Trilogy while, the measured cumulative dose difference was ≤1.0% for both linacs and dose rates.
Conclusion
This analysis quantified subtle differences in 6 MV beam generation and treatment output produced by an Infinity and Trilogy linacs, and not readily available to a user. Also, it showed the temporal resolution capabilities of a new 3D electronic detector system that could be useful in a clinic.