Dosimetric characteristics of 6MV flattening filter free and flattened beams among beam-matched linacs: a three-institutional study

Abstract

Background Beam matching is a concept in radiotherapy applied to clinics where more than one linac is employed to harmonise beam characteristics across linacs for allowing patients interchange without replanning. In view of this, the current study analyzes and compares dosimetric characteristics of 6MV flattening filter free and flattened beams of three beam-matched linear accelerators (linacs) from three different clinics with the aim to evaluate the matching under tight criteria for gamma analysis. Methods Three Elekta linacs from three different clinics were included. The linacs have the same collimator assembly, Elekta Agility. Beam data were collected during commissioning process using PTW dosimetry systems. Dose profiles and percentage depth doses (PDD) were analyzed using 1D gamma analysis (1 mm/1%) as well as the following parameters: depth of maximum dose, PDD10, flatness, unflattnes, symmetry, penumbra, output factors. Additionally, five stereotactic treatment plans were optimized in one clinic and calculated by all three planning systems (Monaco) for a dosimetric comparison. Results Gamma analysis of dose profiles and PDDs showed clinically acceptable results of 96.3% passing rate for profiles and 100% passing rate for PDDs. All dosimetric parameters were in good agreement with the reference data. Furthermore, dosimetric comparisons between stereotactic treatment plans showed a maximum standard deviation of 0.48 Gy for the maximum dose to PTV, and a maximum standard deviation of 0.1 Gy for the dose to the organs at risk. Conclusions All three linacs showed a strong agreement between parameters and passed the gamma analysis using 1% DD/1mm DTA criteria. This study confirmed the matching between linacs, offering the possibility to interchange patients with no replanning.

A Study of the TPS Based Beam-Matching Concept for Medical Linear Accelerators at a Tertiary Hospital

“…This TPS beam model is availed by the vendor during acceptance testing. Specific beam profiles and percentage depth dose (PDD) measurements are undertaken at acceptance testing to verify the matching for broad square fields such as 10 × 10 cm 2 and 30 × 30 cm 2 [6]. For example, Elekta stipulates that for the purpose of matching, a photon beam requires the percentage depth dose at 10 cm (PDD 10 ) to be within ±1% for beam-matched LINACs.…”

“…Furthermore, Elekta stipulates that for beam profiles of 10 × 10 cm 2 and 30 × 30 cm 2 field sizes at a depth of 10 cm, the averaged point dose within the region covering 80% of the full width half maximum (FWHM) ought to be within a 2% difference in comparison to the same points from beam profiles of beam-matched LINACs [1]. A confirmation or agreement between dosimetric data is an assurance that a single beam model can be used for a cohort of LINACs from the same vendor [6] [7]. However, the beam-matching often defined by the vendor in most cases are not strict enough to guarantee optimal beam-match [4].…”