Dosimetric accuracy assessment of a treatment plan verification system for scanned proton beam radiotherapy: one-year experimental results and Monte Carlo analysis of the involved uncertainties

Abstract

During one year of clinical activity at the Italian National Center for Oncological Hadron Therapy 31 patients were treated with actively scanned proton beams. Results of patient-specific quality assurance procedures are presented here which assess the accuracy of a three-dimensional dose verification technique with the simultaneous use of multiple small-volume ionization chambers. To investigate critical cases of major deviations between treatment planning system (TPS) calculated and measured data points, a Monte Carlo (MC) simulation tool was implemented for plan verification in water. Starting from MC results, the impact of dose calculation, dose delivery and measurement set-up uncertainties on plan verification results was analyzed. All resulting patient-specific quality checks were within the acceptance threshold, which was set at 5% for both mean deviation between measured and calculated doses and standard deviation. The mean deviation between TPS dose calculation and measurement was less than ±3% in 86% of the cases. When all three sources of uncertainty were accounted for, simulated data sets showed a high level of agreement, with mean and maximum absolute deviation lower than 2.5% and 5%, respectively.

Dosimetric characterization of a microDiamond detector in clinical scanned carbon ion beams

“…Full technological and clinical details about the CNAO are reported elsewhere. 18,[30][31][32] In particular, for the absorbed dose to water determination under reference conditions and, consequently, the beam monitor calibration, the IAEA-TRS 398 recommendations 17 were used, adapted following the formalism proposed by Jäkel et al 33 to take into account the specificities of particle pencil beam scanning. Reference conditions included the use of a Farmertype ionization chamber calibrated under Co-60 beams at a standard dosimetry laboratory (SSDL), monoenergetic beams, homogeneous 6 × 6 cm 2 scanned fields, and the measurement depth of 2 cm in water.…”

“…To allow the comparison of the calculated depth dose distribution curves with the experimental data, the sizes of the Advanced Markus chamber's collecting electrode (radius = 2.5 mm) and the microDiamond (radius = 1.1 mm) were taken into account in the simulations. Similarly to the method reported by Molinelli et al, 18 in the adopted cylindrical scoring mesh inside a pure water tank, the radial scoring was restricted to the specific active radius of the measuring device, hence deriving dose-towater in voxels of the same size as the detector volumes. The MC results were normalized at the depth in water of 20 mm.…”

“…The use of a single-element dosimeter, requiring delivery of the whole 3D treatment field for each point, would be highly time-consuming, and multiple measurement points would be preferable. 18 The pretreatment verification measurements of patient-specific plans by scanned high energy pencil ion beams are routinely performed by simultaneous measurements of the dose at a representative sample of points, by means of a limited number of pinpoint ionization chambers (24, typically) arranged in a 3D stack, 19 calibrated in terms of dose to water. 17 Such ionization chamber arrays are considered as a suitable instrument for 3D dosimetry in scanning ion beam radiotherapy, thanks to their energy independence and small detector size.…”

The CNAO dose delivery system for modulated scanning ion beam radiotherapy

“…Each 150 layer is irradiated by several overlapping iso-energetic beams arranged in a grid of 2÷3 mm pitch, called spots, each delivering a dose in small volumes and characterized by the number of particles and by the position in the transverse plane. A typical treatment, composed by more than 15÷20 daily fractions, each characterized by two or more fields [15,16], consists of 30 to 100 slices, for a total of 10000 to 50000 spots used to cover 155 the tumour volume.…”

Nuclear physics and particle therapy

“…An example of actual longitudinal dose deposition in water is given in Figure 1, where we show a comparison of experimental data and simulation for different proton beam energies [10] as measured at the CNAO facility in Italy [11].…”