BackgroundMachine Performance Check (MPC) is an application to verify geometry and beam performances of TrueBeam Linacs, through automated checks based on their kV-MV imaging systems. In this study, preliminary tests with MPC were analyzed using all photon beam energies of our TrueBeam, comparing whenever possible with external independent checks.MethodsData acquisition comprises a series of 39 images (12 with kV and 27 with MV detector) acquired at predefined positions without and with the IsoCal phantom in the beam, and with particular MLC pattern settings. MPC performs geometric and dosimetric checks. The geometric checks intend to test the treatment isocenter size and its coincidence with imaging devices, the positioning accuracy of the imaging systems, the collimator, the gantry, the jaws, the MLC leaves and the couch position. The dosimetric checks: refer to a reference MV image and give the beam output, uniformity and center change relative to the reference. MPC data were acquired during 10 repetitions on different consecutive days.Alternative independent checks were performed. Geometric: routine mechanical tests, Winston-Lutz test for treatment isocenter radius. Dosimetric: the 2D array StarCheck (PTW) was used just after the MPC data acquisition.ResultsResults were analyzed for 6, 10, 15 MV flattened, and 6, 10 MV FFF beams. Geometric checks: treatment isocenter was between 0.31 ± 0.01 mm and 0.42 ± 0.02 mm with MPC, compared to 0.27 ± 0.01 mm averaged on all energies with the Winston-Lutz test. Coincidence of kV and MV imaging isocenters was within 0.36 ± 0.0 and 0.43 ± 0.06 mm, respectively (0.4 ± 0.1 mm with external tests). Positioning accuracy of MLC was within 0.5 mm; accuracy of jaws was 0.04 ± 0.02, 0.10 ± 0.05, −1.01 ± 0.03, 0.92 ± 0.04 mm for X1, X2, Y1, Y2 jaws, respectively, with MPC. Dosimetric tests: the output stability relative to the baseline was in average 0.15 ± 0.07% for MPC to compare with 0.3 ± 0.2% with the independent measurement.ConclusionsMPC proved to be a reliable, fast and easy to use method for checking the machine performances on both geometric and dosimetric aspects.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-015-0381-0) contains supplementary material, which is available to authorized users.

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The agreement between Monte Carlo simulations and experimental data proved that the evaluated Varian phase-space files for FFF beams from TrueBeam can be used as radiation sources for accurate Monte Carlo dose estimation, especially for field sizes up to 10 × 10 cm(2), that is the range of field sizes mostly used in combination to the FFF, high dose rate beams.

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Delivery times of intensity-modulated proton therapy (IMPT) can be shortened by reducing the number of spots in the treatment plan, but this may affect clinical plan delivery. Here, we assess the experimental deliverability, accuracy and time reduction of spot-reduced treatment planning for a clinical case, as well as its robustness. For a single head-and-neck cancer patient, a spot-reduced plan was generated and compared with the conventional clinical plan. The number of proton spots was reduced using the iterative ‘pencil beam resampling’ technique. This involves repeated inverse optimization, while adding in each iteration a small sample of randomly selected spots and subsequently excluding low-weighted spots until plan quality deteriorates. Field setup was identical for both plans and comparable dosimetric quality was a prerequisite. Both IMPT plans were delivered on PSI Gantry 2 and measured in water, while delivery log-files were used to extract delivery times and reconstruct the delivered dose via Monte-Carlo dose calculations. In addition, robustness simulations were performed to assess sensitivity to machine inaccuracies and errors in patient setup and proton range. The number of spots was reduced by 96% (from 33 855 to 1510 in total) without compromising plan quality. The spot-reduced plan was deliverable on our gantry in standard clinical mode and resulted in average delivery times per field being shortened by 46% (from 51.2 to 27.6 s). For both plans, differences between measured and calculated dose were within clinical tolerance for patient-specific verifications and Monte-Carlo dose reconstructions were in accordance with clinical experience. The spot-reduced plan was slightly more sensitive to machine inaccuracies, but more robust against setup and range errors. In conclusion, for an example head-and-neck case, spot-reduced IMPT planning provided a deliverable treatment plan and enabled considerable shortening of the delivery time (∼50%) without compromising plan quality or delivery accuracy, and without substantially affecting robustness.

The two algorithms analyzed showed encouraging results in predicting out-of-field region dose for clinical use.

The step-and-shoot method of pencil beam scanning delivers the dose on a threedimensional grid in the target volume, with one dimension defined by the proton energy. While the dose per pencil beam may vary substantially within an iso-energy layer, the beam current typically remains constant. In this static operation mode, the inherent latency of the beam switchoff mechanism results in a lower limit for the deliverable spot dose, which may prevent the application of some of the low-weighted spots prescribed by the treatment planning system. To overcome this limitation, we introduced dynamic beam current control at the PSI Gantry 2, an innovative new approach successfully commissioned and in clinical operation since fall 2017. The control system was enhanced with a direct link to the vertical deflector located at the centre of the cyclotron. This connection allows much faster beam current changes (~ 0.1 ms) and hence opens up the possibility of dynamically reducing the current for individual low-dose spots. We demonstrate that with this new dynamic operation mode, all spots are delivered as planned without compromising treatment time. We show by two independent and complementary methods that the delivered dose distribution is improved.

The parameters proposed for the characterization and routine control of stability of profiles of FFF beams appear to be a viable solution with a strong similarity to the conventional parameters used for flattened beams. The results from three different TrueBeams and the cross-validation against a Clinac-iX suggested the possible generalization of the methods and the possibility to use common tolerances for the parameters. The data showed also the reproducibility of beam characteristics among different systems (of the same vendor) and the resulting parameter values could therefore be possibly generalized.