Dosimetric impact of the low-dose envelope of scanned proton beams at a ProBeam facility: comparison of measurements with TPS and MC calculations

Abstract

Due to the low-dose envelope of scanned proton beams, the dose output depends on the size of the irradiated field or volume. While this field size dependence has already been extensively investigated by measurements and Monte Carlo (MC) simulations for single pencil beams or monoenergetic fields, reports on the relevance of this effect for analytical dose calculation models are limited. Previous studies on this topic only exist for specific beamline designs. However, the amount of large-angle scattered primary and long-range secondary particles and thus the relevance of the low-dose envelope can considerably be influenced by the particular design of the treatment nozzle. In this work, we therefore addressed the field size dependence of the dose output at the commercially available ProBeam(®) beamline, which is being built in several facilities worldwide. We compared treatment planning dose calculations with ionization chamber (IC) measurements and MC simulations, using an experimentally validated FLUKA MC model of the scanning beamline. To this aim, monoenergetic square fields of three energies, as well as spherical target volumes were studied, including the investigation on the influence of the lateral spot spacing on the field size dependence. For the spherical target volumes, MC as well as analytical dose calculation were found in excellent agreement with the measurements in the center of the spread-out Bragg peak. In the plateau region, the treatment planning system (TPS) tended to overestimate the dose compared to MC calculations and IC measurements by up to almost 5% for the smallest investigated sphere and for small monoenergetic square fields. Narrower spot spacing slightly enhanced the field size dependence of the dose output. The deviations in the plateau dose were found to go in the clinically safe direction, i.e. the actual deposited dose outside the target was found to be lower than predicted by the TPS. Thus, the moderate overestimation of dose to normal tissue by the TPS is likely to result in no severe consequences in clinical cases, even for the most critical cases of small target volumes.

Impact of varying planning parameters on proton pencil beam scanning dose distributions in four commercial treatment planning systems

“…In a study by Lee, it was found that, provided the spot spacing was equal to or smaller than two‐thirds of the spot size (FWHM), a plan with 100% uniformity could be achieved. It has been shown previously that spot spacing has an impact on the field sizes . This was not found in our work, with the extent of spot positions remaining within the field sizes as expected, however, an optimal spot spacing should be found to balance dose homogeneity with output factor for a given field size …”

“…It has been shown previously that spot spacing has an impact on the field sizes . This was not found in our work, with the extent of spot positions remaining within the field sizes as expected, however, an optimal spot spacing should be found to balance dose homogeneity with output factor for a given field size …”

“…27 This was not found in our work, with the extent of spot positions remaining within the field sizes as expected, however, an optimal spot spacing should be found to balance dose homogeneity with output factor for a given field size. 27 It was found that reducing the spot spacing improves the dose homogeneity (Table III), but not for values less than 4 mm. Although our work was based on uniform material phantoms and simple cubic targets, it has been shown that similar values are suitable for patient geometries.…”

Nuclear halo measurements for accurate prediction of field size factor in a Varian ProBeam proton PBS system

“…These measurements have previously been reported for an IBA PBS nozzle . Wurl et al used ion chambers for measurement and validated a FLUKA Monte Carlo (MC) simulation to model the halo effect for a Varian ProBeam system . The results obtained by Wurl et al were used to influence analytical dose calculations.…”

“…However, Monte Carlo algorithms do not model the halo that is created upstream from the treatment volume, for example, in a range shifter . This can lead to discrepancies for low‐energy proton beams as the halo is generated in both the treatment head and through scattering in air . In commissioning our Varian ProBeam PBS system, we found the largest disagreement between TPS and measurement was 8% for a 2 × 2 cm 2 monoenergetic field at 70 MeV.…”