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Commissioning and quality assurance of the Pinnacle³ radiotherapy treatment planning system for external beam photons

¹ Joint Department of Physics, The Institute of Cancer Research and the Royal Marsden NHS Trust, Downs Road, Sutton, Surrey SM2 5PT and ² National Physical Laboratory, Centre for Ionising Radiation Metrology, Dosimetry Group, Kaye Building, Queens Road, Teddington TW11 0LW, UK.

The commissioning of a Pinnacle³ treatment planning system is described. Four Elekta linear accelerators were commissioned for external beam photons. Measured data were used to derive parameter values for the Pinnacle³ beam model by (1) fitting a Monte Carlo model of the accelerator head to measured data and then extracting the parameters for the Pinnacle³ beam model, and by (2) using the auto-modelling facility within Pinnacle³. Both of these methods yielded dose distributions in accord with published recommendations. A separate small-field beam model, customized for an in-house compact blocking system, was also created, which satisfied appropriate acceptance criteria for stereotactically guided conformal brain treatments. Inhomogeneous, oblique, asymmetrical and irregular fields were also assessed, with calculated and measured doses agreeing to within ±3%. Dose–volume histogram calculation was found to be accurate to within ±5% dose or volume for a grid size of 4 mm x 4 mm x 4 mm, with better accuracy being achieved for finer grids. Isocentric doses were compared between Pinnacle³'s collapsed cone convolution algorithm and the Bentley–Milan algorithm within the Target-2 treatment planning system. Dose differences were generally less than 3% in the dose prescribed, with larger values for breast plans, where the Pinnacle³ algorithm calculated scatter more accurately. Pelvic and thoracic plans were also verified using an anthropomorphic phantom, with local dose differences between calculated and delivered dose of up to 8%, but mainly less than 3%, and with no systematic difference. Ionization chamber verifications using START and RT-01 trial procedures demonstrated differences between calculated and measured doses of less than 2%. Following satisfactory performance in the commissioning process, Pinnacle³ has now been introduced into routine clinical use.

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