Dosimetric and mechanical equivalency of Varian TrueBeam linear accelerators

Abstract

To investigate and improve the level of equivalency of Varian TrueBeam linear accelerators (linacs) in energy-, dosimetric leaf gap-(DLG) and jaw calibration. Methods: Eight linacs with four photon energies: 6 MV, 6 MV FFF, 10 MV FFF, and 15 MV, and three electron energies (on two linacs): 6, 9, and 12 MeV were commisioned and beam-matched. Initially, symmetry of lateral profiles was calibrated for maximum field size. Energy-matching was then performed for photons by adjusting diagonal profiles at maximum field size and depth of maximum dose to coincide with the reference linac, and for electrons by matching the range at percentage depth of ionization of 90%, 80%, and 50%. Calibration of DLG was performed for 6 MV and evaluated among the linacs. The relationship between DLG and the Gap value was investigated. A method using electronic portal imaging device (EPID) was developed and implemented for jaw calibration. Results: Symmetry calibration for photons (electrons) was within 1% (0.7%), further improving the vendor's acceptance criteria. Photon and electron energy-matching was within 0.5% and 0.1 mm, respectively. Calibration of DLG was within 0.032 mm among the linacs and utilizing the relationship between DLG and the Gap value resulted in an empirical calibration method which was implemented to simplify DLG adjustment. Using EPID-based method of calibration, evaluation of the jaw-positioning among the linacs for 30 cm × 30 cm field size was within 0.4 mm and in the junction area within 0.2 mm. Dose delivery error of VMAT-plans were at least 99.2% gamma pass rate (1%, 1 mm). Conclusions: High level of equivalency, beyond clinically accepted criteria, of True-Beam linacs could be achieved which reduced dose delivery systematic errors and increased confidence in interchanging patients among linacs.

Golden beam data provided by linear accelerator manufacturers should be used in the commissioning of treatment planning systems

“…Second, some studies have confirmed that the locally acquired beam data are either equivalent within 1% to that provided by the manufacturer [ 6 , 7 ] or to beam data for linacs of the same model [ 8 , 9 ]. Glide-Hurst et al [ 6 ] reported excellent agreement between five Varian TrueBeam data from three institutions.…”

Matched linac stereotactic radiotherapy: An assessment of delivery similarity and distributive patient‐specific quality assurance feasibility

“…The ability to match linacs at a level sufficient for routine treatments and PSQA has been assessed in several studies 1–8 . Initial investigations focus on the ability to achieve a high level of matching between characteristics of the linac output, such as beam profiles, depth dose curves, and output factors 1,2,4 .…”

“…Initial investigations focus on the ability to achieve a high level of matching between characteristics of the linac output, such as beam profiles, depth dose curves, and output factors 1,2,4 . Further investigations consider the resultant similarity within composite dose deliveries for volumetric‐modulated arc therapy (VMAT) or intensity‐modulated radiotherapy (IMRT) 3,5,8 . More recently investigations have begun to consider the possibility of linac matching at the level required for stereotactic radiotherapy 6,7 …”

“…The ability to match linacs at a level sufficient for routine treatments and PSQA has been assessed in several studies. 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 Initial investigations focus on the ability to achieve a high level of matching between characteristics of the linac output, such as beam profiles, depth dose curves, and output factors. 1 , 2 , 4 Further investigations consider the resultant similarity within composite dose deliveries for volumetric‐modulated arc therapy (VMAT) or intensity‐modulated radiotherapy (IMRT).…”

Transferability of patients for radiation treatment between unmatched machines

“… 8 Linac beam‐matching process is also elaborated on in the literature for three‐dimensional conformal radiation therapy (3DCRT) and intensity‐modulated radiation therapy (IMRT) for the ease of patient transferability. 6 , 9 , 10 , 11 …”