Abstract
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.MethodsA 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 R 50 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).ResultsVisual 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 R 90; 0.10 cm in R 80–20) and ongoing quality assurance (0.2 cm in R 50). 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 R 50 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 R 50 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).ConclusionsResults 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.
Highlights
A lightweight, permanent magnet spectrometer and data analysis techniques were developed by McLaughlin et al for the measurement of energy spectra of therapeutic electron beams.[1]
We evaluated the agreement between peak mean energy (PME) and full-width at half maximum (FWHM) of the energy spectra necessary for R50 values to agree within 0.05 cm and R80À20ðR20 À R80Þ values to agree within 0.10 cm for matched machines
Measured energy spectra for seven electron beams on six matched Elekta Infinity accelerators. (a–d): Four accelerators in group A with beams for machines A-2 to A-4 matched to beams of machine A-1. (e–f) Two accelerators in group B with beams for machine B-2 matched to beams of machine B-1
Summary
A lightweight, permanent magnet spectrometer and data analysis techniques were developed by McLaughlin et al for the measurement of energy spectra of therapeutic electron beams.[1] The 4-kg spectrometer (16.5 cm long by 5.3 cm wide by 7.8 cm high) contains a dipole, neodymium permanent magnet with a 1.43 cm air separation, producing a 0.54 T field. The magnetic field disperses the energy distributed electrons onto a computed radiography (CR) strip, whose measured spatial distribution transforms to an energy spectrum (cf Fig. 1). The aims of the present study were to (a) demonstrate its potential utility for beam matching by comparing electron energy spectra for six matched Elekta accelerators and (b) study the correlation of measured energy spectra metrics with percent depth-dose curve metrics, showing the potential of the former for estimating percent depth-dose metrics for quality assurance. Results are reported for a set of seven electron beams on six Elekta Infinity radiotherapy accelerators with the MLCi2 treatment head
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