Abstract
A new megavoltage (MV) energy was recently introduced on Varian TrueBeam linear accelerators for imaging applications. This work describes the experimental characterization of a 2.5 MV inline portal imaging beam for commissioning, routine clinical use, and quality assurance purposes. The beam quality of the 2.5 MV beam was determined by measuring a percent depth dose, PDD, in water phantom for 10×10 cm2 field at source‐to‐surface distance 100 cm with a CC13 ion chamber, plane parallel Markus chamber, and GafChromic EBT3 film. Absolute dosimetric output calibration of the beam was performed using a traceable calibrated ionization chamber, following the AAPM Task Group 51 procedure. EBT3 film measurements were also performed to measure entrance dose. The output stability of the imaging beam was monitored for five months. Coincidence of 2.5 MV imaging beam with 6 MV therapy beam was verified with hidden‐target cubic phantom. Image quality was studied using the Leeds and QC3 phantom. The depth of maximum dose, dmax, and percent dose at 10 cm depth were, respectively, 5.7 mm and 51.7% for CC13, 6.1 mm and 51.9% for Markus chamber, and 5.1 mm and 51.9% for EBT3 film. The 2.5 MV beam quality is slightly inferior to that of a 60Co teletherapy beam; however, an estimated kQ of 1.00 was used for output calibration purposes. The beam output was found to be stable to within 1% over a five‐month period. The relative entrance dose as measured with EBT3 films was 63%, compared to 23% for a clinical 6 MV beam for a 10×10 cm2 field. Overall coincidence of the 2.5 MV imaging beam with the 6 MV clinical therapy beam was within 0.2 mm. Image quality results for two commonly used imaging phantoms were superior for the 2.5 MV beam when compared to the conventional 6 MV beam. The results from measurements on two TrueBeam accelerators show that 2.5 MV imaging beam is slightly softer than a therapeutic 60Co beam, it provides superior image quality than a 6 MV therapy beam, and has excellent output stability. These 2.5 MV beam characterization results can serve as reference for clinics planning to commission and use this novel energy‐image modality.PACS number(s): 87.57.‐s, 87.59.‐e, 06.20.fb, 87.53.Bn
Highlights
Low-energy megavoltage imaging with new target designs attained a renewed interest
When taking into account the effective measurement depth, this latter value compares well with the literature values ranging from 16.4%–20% for Varian TrueBeam machines for a 10 × 10 cm2 field size.[12,13,14] The 2.5 MV entrance dose is somewhat lower than the Monte Carlo simulated value of 81% reported by Parsons et al[1] A comparison of the percentage depth-dose (PDD)
In this paper we report our initial experiences with the 2.5 MV imaging beam available on the TrueBeam 2.0 linear accelerator
Summary
Low-energy megavoltage imaging with new target designs attained a renewed interest. A new low-energy 2.5 megavoltage (MV) imaging beam was made available for in-line, portal-image verification of patient setup on the TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA). This beam possesses a lower average photon energy and in theory should produce better soft-tissue contrast due to a larger proportion of diagnosticquality photons in the beam spectrum compared to therapy beams (e.g., 6 MV). The 2.5 MV beam is in line with the treatment beams having the potential to remove some of the isocentre coincidence uncertainties that can occur with orthogonal kV imagers mounted on linear accelerators. Some details of development of this type of beam can be found in references.[1,2]
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