Comparison of measured electron energy spectra for six matched, radiotherapy accelerators

This study compares energy spectra of the multiple electron beams of individual radiotherapy machines, as well as the sets of spectra across multiple matched machines. Also, energy spectrum metrics are compared with central-axis percent depth-dose (PDD) metrics.

A lightweight, permanent magnet spectrometer was used to measure energy spectra for seven electron beams (7-20 MeV) on six matched Elekta Infinity accelerators with the MLCi2 treatment head. PDD measurements in the distal falloff region provided R₅₀ and R_80-20 metrics in Plastic Water^® , which correlated with energy spectrum metrics, peak mean energy (PME) and full-width at half maximum (FWHM).

Visual inspection of energy spectra and their metrics showed whether beams on single machines were properly tuned, i.e., FWHM is expected to increase and peak height decrease monotonically with increased PME. Also, PME spacings are expected to be approximately equal for 7-13 MeV beams (0.5-cm R₉₀ spacing) and for 13-16 MeV beams (1.0-cm R₉₀ spacing). Most machines failed these expectations, presumably due to tolerances for initial beam matching (0.05 cm in R₉₀ ; 0.10 cm in R_80-20 ) and ongoing quality assurance (0.2 cm in R₅₀ ). Also, comparison of energy spectra or metrics for a single beam energy (six machines) showed outlying spectra. These variations in energy spectra provided ample data spread for correlating PME and FWHM with PDD metrics. Least-squares fits showed that R₅₀ and R_80-20 varied linearly and supralinearly with PME, respectively; however, both suggested a secondary dependence on FWHM. Hence, PME and FWHM could serve as surrogates for R₅₀ and R_80-20 for beam tuning by the accelerator engineer, possibly being more sensitive (e.g., 0.1 cm in R_80-20 corresponded to 2.0 MeV in FWHM).

Results of this study suggest a lightweight, permanent magnet spectrometer could be a useful beam-tuning instrument for the accelerator engineer to (a) match electron beams prior to beam commissioning, (b) tune electron beams for the duration of their clinical use, and (c) provide estimates of PDD metrics following machine maintenance. However, a real-time version of the spectrometer is needed to be practical.