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

Abstract

Purpose: For pencil-beam scanning proton therapy systems, in-air non-Gaussian halo can significantly impact output at small field sizes and low energies. Since the low-intensity tail of spot profile (halo) is not necessarily modeled in treatment planning systems (TPSs), this can potentially lead to significant differences in patient dose distribution. In this work, we report such impact for a Varian ProBeam system. Methods: We use a pair magnification technique to measure two-dimensional (2D) spot profiles of protons from 70 to 242 MeV at the treatment isocenter and 30 cm upstream of the isocenter. Measurements are made with both Gafchromic film and a scintillator detector coupled to a CCD camera (IBA Lynx). Spot profiles are measured down to 0.01% of their maximum intensity. Field size factors (FSFs) are compared among calculation using measured 2D profiles, calculation using a clinical treatment planning algorithm (Raystation 8A clinical Monte Carlo), and a CC04 small-volume ion chamber. FSFs were measured for square fields of proton energies ranging from 70 to 242 MeV.Results: All film and Lynx measurements agree within 1 mm for full width at half maximum beam intensity. The measured radial spot profiles disagree with simple Gaussian approximations, which are used for modeling in the TPS. FSF measurements show the magnitude of disagreements between beam output in reality and in the TPS without modeling halo. We found that the clinical TPS overestimated output by as much as 6% for small field sizes of 2 cm at the lowest energy of 70 MeV while the film and Lynx measurements agreed within 4% and 1%, respectively, for this FSF.Conclusions: If the in-air halo for low-energy proton beams is not fully modeled by the TPS, this could potentially lead to under-dosing small, shallow treatment volumes in PBS treatment plans.

Beam characteristics of the first clinical 360° rotational single gantry room scanning pencil beam proton treatment system and comparisons against a multi‐room system

“…From the illustration, one can see that the largest correction factors or dose discrepancies are observed in lower energies with field sizes smaller than 4 × 4 cm 2 . In the 2 × 2 cm 2 field‐sized 70 MeV proton plan, the AcurosPT model in current version overestimated the near surface dose up to +7.0% (as listed in Table 1), similar to the result of +6% published by Harms at al 8 …”

“…Employment of a range shifter when applicable to avoid lowest energies can also further minimize the surface dose deviations for a shallow small target. As observed in Table 4, the surface dose disagreement for small fields (≤3.2°× 3.2 cm 2 ) due to ignored halo created in the nozzle 8 with our current beam model is reduced to below 1.4% by using a range shifter. For open field above 4.0°× 4.0 cm 2 , there is no clinically meaningful difference (≤1.5%) between measurement and AcurosPT for both open fields and fields with range shifter.…”

“…In the current commissioning, the proton beam spot profiles are derived from single Gaussian fitting, partially ignored the minor dose deviations in the profile low dose tails or halo region. To overcome the modeling and algorithm deficiencies, 8 an additional tuning for dose outputs was applied, particularly for AcurosPT model. Since AcurosPT is exclusively used as the final plan dose computing algorithm, a more careful output fine‐tuning was for improving the overall dosimetric accuracy within the scope of routine clinically used field sizes or 2.4 cm or greater in each dimension.…”

“…Validation of the modeling comprises the verifications of the computed IDDs and calculated dose in difference conditions and field sizes with both PCS and AcurosPT models 11–16 ; the overall accuracy of the AcurosPT‐computed treatment planning and dose delivery using realistic clinical plans as well as the end‐to‐end tests for different open beams and beams with different range shifters and air gaps 17,18 . The halo effects of the beams beyond the modeled profile were examined by comparison between the AcurosPT computed and measured doses with different field sizes and energies at near the surface 8 . A PTW Semiflex 2.4 mm × 4.8 mm ion chamber (TN31021) was used for the measurements at ISO with 8 mm WET, matching the inherent buildup of PTW Octavius 1500XDR for the dose conversion.…”

“…The principle beam dosimetric components of the commissioning typically comprised of integrated depth dose curves (IDDs), absolute dose calibration for given MUs (or dose output), in‐air beam spot size or profiles 3–6 . Other essential elements are virtual source position relative to ISO, beam spot accuracy and dose uniformity, field size factors (or halo effect of spot profile), MU linearity, mechanical accuracy, OBI and CBCT quality and accuracy, CT stoichiometric calibration, WET measurement for the range shifters, table support and inserts, as well as immobilization and physics accessories 7–9 . However, only beam dosimetric characteristics will be discussed in this report.…”