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ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
Beam modelling and matching of eight Varian TrueBeam linear accelerators
Mohammed Ghazal, Sweden
PO-1364

Abstract

Beam modelling and matching of eight Varian TrueBeam linear accelerators
Authors: Mohammed Ghazal.(Karolinska University Hospital, Department of Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden), Tobias Pommer.(Karolinska University Hospital, Department of Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden), Tobias Pommer.(Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden), Lars Södergren.(Karolinska University Hospital, Department of Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden), Julia Söderström.(Karolinska University Hospital, Department of Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden), Mathias Westermark.(Karolinska University Hospital, Department of Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden)
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Purpose or Objective

Eight Varian TrueBeam linear accelerators (linacs) were accepted for photon energies: 6X, 15X, 6XFFF and 10XFFF and modelled in Analytical Anisotropic Algorithm (AAA). The beam-model was optimized in terms of Dosimetric Leaf Gap (DLG) and Spot Size. All linacs were matched to achieve equivalency. The scope of this report is to present the level of agreement achievable: (i) between the beam-model and the beam-data, (ii) among the linacs.

Material and Methods

Beam-data measured at the first linac were set as reference for matching and beam modelling. The photon energies were matched among the linacs by measuring 40 x 40 cm2 diagonal profiles at Dmax in water and comparing the region within the penumbra with the reference profile. The energy matching was verified by Tissue Phantom Ratio (TPR20,10) measurements according to IAEA TRS-398 (6X and 15X) and TRS-483 (6XFFF and 10XFFF). An inhouse imager-based method was developed for the calibration of the jaw-collimators using jaw-position encoders and mathematical prediction model. The DLG was matched among the linacs.

The Spot Size in AAA was optimized by calculating plans with varying Spot Size and comparing them to measured lateral dose profiles in water. DLG was optimized in AAA by calculating several dynamic representative plans with varying DLG in AAA and compare the dose difference to measured plans using Delta4 and Quasar phantoms.

Verification of both the matching among the linacs and the beam-model were performed by profile measurements in different geometries in water.
Results

The regions within the penumbra of the 40 x 40 cm2 diagonal profiles for all linacs and energies were matched to < 0.5 %. The TPR20,10 measurements were within 0.5 % for all energies. The global average gamma evaluation index (2 %, 2 mm) of all matching verification measurements was 100 %. The DLG for 6X was matched to 1.40±0.03 mm resulting in a DLG of 1.56±0.04 mm (15X), 1.25±0.03 mm (6XFFF) and 1.45±0.04 mm (10XFFF). The jaws were calibrated in each linac with a precision of 0.3 mm, and the jaw junction at isocenter was calibrated with a precision of 0.2 mm, significantly improving the calibration method provided by the vendor (Table 1).

Varying the Spot Size in AAA resulted in a negligible effect on output. The Spot Size was therefore optimized based on penumbra widths of small MLC-defined lateral dose profiles and resulted in 0.5 mm and 0 mm in X- and Y-axis respectively for all energies (vendor recommendation is X=1 mm and Y=0 mm). The global average gamma evaluation index (2 %, 2 mm) of all AAA verification measurements was 100 % (Figure 1). DLG in AAA was optimized for 6X (6XFFF and 10XFFF) to 1.8 mm (1.6 mm, 1.8 mm) to match a 1.4 mm measured DLG (1.25 mm and 1.45 mm). No DLG optimization was done for 15X because no dynamic treatments are used clinically for this energy.



Conclusion

Optimization is required to achieve excellent agreement between the AAA beam-model and the beam-data. High-precision matching is feasible among eight Varian TrueBeam linacs.