Abstract
Purpose: The energy spectrum of a linear accelerator used for dose calculations is determined during beam commissioning by iteratively adjusting the spectrum and comparing calculated and measured percent depth-dose curves. Direct measurement of the energy spectrum using pulse mode detectors is particularly challenging because of the high-energy, high fluence nature of these beams and limitations of the detector systems. This work implements a Compton scattering (CS) spectroscopy setup and presents detector corrections and spectral unfolding techniques to measure the spectrum of a 6 MV linear accelerator using a pulse mode detector. Methods: Spectral measurements were performed using a Varian Clinac 21EX linear accelerator and a high-purity germanium (HPGe) detector. To reduce fluence to the detector, a custom-built lead shield and a CS spectrometry setup were used. The detector was placed at CS angles of 46°, 89°, and 125°. At each of these locations, a detector response function was generated to account for photon interactions within the experimental geometry. Gold’s deconvolution algorithm was used to unfold the energy spectrum. The measured spectra were compared to simulated spectra, which were obtained using an experimentally benchmarked model of the Clinac 21EX in MCNP6. Results: Measurements were acquired and detector response corrections were calculated for all three CS angles. A comparison of spectra for all CS angles showed good agreement with one another. The spectra for all three angles were averaged and showed good agreement with the MCNP6 simulated spectrum, with all points above 400 keV falling within 4%, which was within the uncertainty of the measurement and statistical uncertainty. Conclusions: The measurement of the energy spectrum of a 6 MV linear accelerator using a pulse-mode detector is presented in this work. For accurate spectrum determination, great care must be taken to optimize the detector setup, determine proper corrections, and to unfold the spectrum.
Highlights
Dose calculation engines uses in clinical radiation therapy treatment planning for photons primarily use model based dose calculation algorithms to determine dose to the patient [1] [2]
Linear accelerator output over the course of 60 minutes is stable, and the pulse-height distribution (PHD) measurements used for comparison were identical in time
Three independent measurements were taken at different Compton scattering (CS) angles and were uniquely corrected for the response and probability of scattered photon to determine the spectrum along the central axis
Summary
Dose calculation engines uses in clinical radiation therapy treatment planning for photons primarily use model based dose calculation algorithms to determine dose to the patient [1] [2]. For these algorithms, the knowledge of the energy spectrum is important for accurate dose determination, especially around high-Z material interfaces and heterogeneities [3] [4] [5] [6] [7]. The spectrum is typically estimated by an iterative tuning approach [8] using beam data measured during linear accelerator commissioning [9]. It is known that there are differences among these codes including electron transport, bremsstrahlung cross sections, and variance reduction techniques [15] [16] [21], and as such it is recommended that these models are validated against measured beam data [22]
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