Characterization of a 2.5 MV inline portal imaging beam

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, normaldtruemax, 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 normalkQ 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

Fiducial markers visibility and artefacts in prostate cancer radiotherapy multi-modality imaging

“…However, PM FMs were not visible on EPID MV or on lateral kV planar images. Even when using the low-MV imaging beam (2.5 MV) option [29] that provided better image contrast for all gold FMs, PM visibility on planar images did not improve. When EPID verification or kV imaging was used for image guidance, GMs with large diameters were the most visible FMs on both LAT and AP planar images.…”

Validating kQ=1.0 assumption in TG51 with PTW 30013 farmer chamber for Varian TrueBeam's 2.5 MV imaging beam

“…We initially performed the calibration of this beam using the TG51 protocol with an assumed k Q value of 1.0. A recently published paper by Gräfe et al6 showed a similar calibration again with the assumed k Q of 1.0, using the 2.5 MV imaging beam and 0.64 cc Exradin A12 (Standard Imaging Inc., Middleton, WI, USA) ionization chamber.…”

Calculating dose from a 2.5 MV imaging beam using a commercial treatment planning system

“…The image quality from the 2.5 MV beam has been investigated in the literature . Although image quality was not a primary motivator of this work, several properties of the beam commissioning parameters indicate the potential for higher image quality of the 2.5 MV beam compared to 6 MV.…”

“…Measured PDDs were postprocessed using a least‐squares smoothing filter (5 mm width, 0.2 mm resolution) and interpolation to a 1 mm grid using a cubic spline. Measured beam characteristics such as d max and %dd(10 cm) were compared to those reported in the literature for a consistency check . Table shows a list of the measured beam data used for commissioning.…”

“…The TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) is equipped with a 2.5 MV unflattened beam option that is used for image guidance. This beam has shown to have improved image quality in comparison to higher energy MV imaging beams, such as 6 MV . The 2.5 MV beam is currently not used for delivering a therapeutic dose.…”