Physical and biological impacts of collimator‐scattered protons in spot‐scanning proton therapy

Ueno, Koki; Matsuura, Tetsuya; Hirayama, S.; Sugiyama, Takao; Ueda, Hideaki; Matsuo, Yuto; Yoshimura, Takaaki; Umegaki, Kikuo

doi:10.1002/acm2.12653

Cited by 7 publications

(6 citation statements)

References 35 publications

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“…This trend was also supported by experimental results, which showed an increase of 1.5 keV μm −1 in LET F in water further away from the field edge. Although no comparison was made with uncollimated fields, Ueno et al also showed through MC simulations that the LET D of collimator-scattered protons was several keV μm −1 higher than that of unscattered ones, in a set of investigated positions (Ueno et al 2019). The reported increase is also in agreement with the results found in this work in terms of LET D , although a direct comparison cannot be made given the differences in the irradiated fields, scored quantities and positions.…”

Section: Discussionsupporting

confidence: 81%

Biophysical characterization of collimated and uncollimated fields in pencil beam scanning proton therapy

et al. 2023

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Objective: The lateral dose fall-off in proton pencil beam scanning (PBS) technique remains the preferred choice for sparing adjacent organs at risk as opposed to the distal edge due to the proton range uncertainties and potentially high relative biological effectiveness. However, because of the substantial spot size along with the scattering in the air and in the patient, the lateral penumbra in PBS can be degraded. Combining PBS with an aperture can result in a sharper dose fall-off, particularly for shallow targets. Approach: The aim of this work was to characterize the radiation fields produced by collimated and uncollimated 100 and 140 MeV proton beams, using Monte Carlo simulations and measurements with a MiniPIX-Timepix detector. The dose and the linear energy transfer (LET) were then coupled with published in silico biophysical models to elucidate the potential biological effects of collimated and uncollimated fields. Main results: Combining an aperture with PBS reduced the absorbed dose in the lateral fall-off and out-of-field by 60%. However, the results also showed that the absolute frequency-averaged LET (LETF) values increased by a maximum of 3.5 keV/µm in collimated relative to uncollimated fields, while the dose-averaged LET (LETD) increased by a maximum of 7 keV/µm. Despite the higher LET values produced by collimated fields, the predicted DNA damage yields remained lower, owing to the large dose reduction. Significance: This work demonstrated the dosimetric advantages of combining aperture with PBS coupled with lower DNA damage induction. A methodology for calculating dose in water derived from measurements with a silicon-based detector was also presented. This work is the first to demonstrate experimentally the increase in LET caused by combining PBS with aperture, and to assess the potential DNA damage which is the initial step in the cascade of events leading to the majority of radiation-induced biological effects.

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Section: Discussionsupporting

confidence: 81%

Biophysical characterization of collimated and uncollimated fields in pencil beam scanning proton therapy

et al. 2023

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“…A slightly increased dose averaged LET of the protons scattered at the aperture could be detected by Ueno et al 35 Although the increased LET was negligible in the overall LET, it should be considered especially for the application with small fields. These secondary effects contributing to absorbed dose require particular emphasis in subsequent works.…”

Section: Discussionmentioning

confidence: 90%

Optimization of proton pencil beam positioning in collimated fields

et al. 2023

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Background: The addition of static or dynamic collimator systems to the pencil beam scanning delivery technique increases the number of options for lateral field shaping. The collimator shape needs to be optimized together with the intensity modulation of spots. Purpose: To minimize the proton field's lateral penumbra by investigating the fundamental relations between spot and collimating aperture edge position. Methods: Analytical approaches describing the effect of spot position on the resulting spot profile are presented. The theoretical description is then compared with Monte Carlo simulations in TOPAS and in the RayStation treatment planning system, as well as with radiochromic film measurements at a clinical proton therapy facility.In the model,one single spot profile is analyzed for various spot positions in air. Further, irradiation setups in water with different energies, the combination with a range shifter, and two-dimensional proton fields were investigated in silico. Results:The further the single spot is placed beyond the collimating aperture edge ('overscanning'), the sharper the relative lateral dose fall-off and thus the lateral penumbra. Overscanning up to 5 mm reduced the lateral penumbra by about 20% on average after a propagation of 13 cm in air. This benefit from overscanning is first predicted by the analytical proofs and later verified by simulations and measurements.Corresponding analyses in water confirm the benefit in lateral penumbra with spot position optimization as observed theoretically and in air. The combination of spot overscanning with fluence modulation facilitated an additional improvement. Conclusions: The lateral penumbra of single spots in collimated scanned proton fields can be improved by the method of spot overscanning. This suggests a better sparing of proximal organs at risk in smaller water depths at higher energies, especially in the plateau of the depth dose distribution. All in all, spot overscanning in collimated scanned proton fields offers particular potential in combination with techniques such as fluence modulation or dynamic collimation for optimizing the lateral penumbra to spare normal tissue.

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“…RayStation TPS accurately predicted the dose bump as well as lateral penumbra. Ueno et al 18 reported the maximum dose increase of 22% for all the tested condition from collimator‐scattered protons. This result was consistent with our finding that maximum surface dose increased by AA scattered protons of 25%.…”

Section: Discussionmentioning

confidence: 94%

“…A Monte Carlo simulation study of the lateral penumbra of a PBS proton system was conducted by Winterhalter et al 17 They studied various beam conditions and recommended that the collimated edge enhancement with variable preabsorber be an optimal way to reduce the lateral fall‐off for the PBS proton therapy system. Ueno et al 18 conducted a Monte Carlo simulation of the impact of collimator‐scattered protons in PBS proton therapy. They reported increased surface dose by 3.0–22.0% due to collimator‐scattered protons.…”

Section: Introductionmentioning

confidence: 99%

Characterization of penumbra sharpening and scattering by adaptive aperture for a compact pencil beam scanning proton therapy system

2021

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To quantitatively access penumbra sharpening and scattering by adaptive aperture (AA) under various beam conditions and clinical cases for a Mevion S250i compact pencil beam scanning proton therapy system. Methods: First, in-air measurements were performed using a scintillation detector for single spot profile and lateral penumbra for five square field sizes (3 9 3 to 18 9 18 cm 2 ), three energies (33.04, 147.36, and 227.16 MeV), and three snout positions (5, 15, and 33.6 cm) with Open and AA field. Second, treatment plans were generated in RayStation treatment planning system (TPS) for various combination of target size (3-and 10-cm cube), target depth (5, 10, and 15 cm) and air gap (5-20 cm) for both Open and AA field. These plans were delivered to EDR2 films in the solid water and penumbra reduction by AA was quantified. Third, the effect of the AA scattered protons on the surface dose was studied at 5 mm depth by EDR2 film and the RayStation TPS computation. Finally, dosimetric advantage of AA over Open field was studied for five brain and five prostate cases using the TPS simulation. Results: The spot size changed dramatically from 3.8 mm at proton beam energy of 227.15 MeV to 29.4 mm at energy 33.04 MeV. In-air measurements showed that AA substantially reduced the lateral penumbra by 30% to 60%. The EDR2 film measurements in solid water presented the maximum penumbra reduction of 10 to 14 mm depending on the target size. The maximum increase of 25% in field edge dose at 5 mm depth as compared to central axis was observed. The substantial penumbra reduction by AA produced less dose to critical structures for all the prostate and brain cases. Conclusions: Adaptive aperture sharpens the penumbra by factor of two to three depending upon the beam condition. The absolute penumbra reduction with AA was more noticeable for shallower target, smaller target, and larger air gap. The AA-scattered protons contributed to increase in surface dose. Clinically, AA reduced the doses to critical structures.

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Physical and biological impacts of collimator‐scattered protons in spot‐scanning proton therapy

Cited by 7 publications

References 35 publications

Biophysical characterization of collimated and uncollimated fields in pencil beam scanning proton therapy

Biophysical characterization of collimated and uncollimated fields in pencil beam scanning proton therapy

Optimization of proton pencil beam positioning in collimated fields

Characterization of penumbra sharpening and scattering by adaptive aperture for a compact pencil beam scanning proton therapy system

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