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Purpose To evaluate the impact of beam quality in terms of distal fall off DFO, 90 10 and lateral penumbra LP, 80 20 of single beam ocular proton therapy OPT and to derive resulting ideal requirements for future systems. Methods Nine different beam models with DFO varying between 1 and 4 mm and LP between 1 and 4 mm were created. Beam models were incorporated into the RayStation with RayOcular treatment planning system version 10 B RaySearch Laboratories, Stockholm, Sweden . Each beam model was applied for eight typical clinical cases, covering different sizes and locations of uveal melanoma. Plans with and without an additional wedge were created, resulting in 117 plans with a total prescribed median dose of 60 Gy RBE to the clinical target volume. Treatment plans were analyzed in terms of V20 V80 penumbra volume, D1 dose to 1 of the volume for optic disc and macula, optic nerve V30 volume receiving 30 Gy RBE , i.e., 50 of prescription , as well as average dose to lens and ciliary body. An LP dependent aperture margin was based on estimated uncertainties, ranging from 1.7 to 4.0 mm. Results V20 V80 showed a strong influence by LP, while DFO was less relevant. The optic disc D1 reached an extra dose of up to 3000 cGy RBE , comparing the defined technical limit of DFO LP 1 mm with DFO 3 mm LP 4 mm. The latter may result from a pencil beam scanning PBS system with static apertures. Plans employing a wedge showed an improvement for organs at risk sparing. Conclusion Plan quality is strongly influenced by initial beam parameters. The impact of LP is more pronounced when compared to DFO. The latter becomes important in the treatment of posterior tumors near the macula, optic disc or optic nerve. The plan quality achieved by dedicated OPT nozzles in single or double scattering design might not be achievable with modified PBS systems

Purpose : In light-ion beam dosedelivery with the scanning technique the spacing between adjacent spots is an important parameter during treatment planning. In order to study the effect of spot spacing on dose conformity and robustness for single field uniform dose configurations, fundamental geometrical properties of placement of Gaussian beamlets are explored. In particular, the dependence of penumbra width and flatness on spot width and spot spacing is investigated. Methods : Infinitesimal calculus and analytical methods are used to derive simple expressions for the lateral penumbra and the flatness of one-dimensional dose profiles in continuous scanning and uniform discrete spot scanning. In the same way expressions for the fundamental modes of perturbation of the spot sequence are developed. A numerical, matrix-based approach is followed to optimize weights spot-by-spot. Results : Generally the lateral penumbra widths lie between 1.13 σ b and 1.68 σ b with σ b being the standard deviation of the beam spot profile. For regularly placed spots of equal weight with spot spacing λ the lateral penumbra is given by 1.68 σ ′ where σ ′ results from quadratic subtraction of λ / 12 from σ b . The quantization error is identified as additional parameter describing the lateral dose conformity. It’s variance is given by λ 2 / 12 for a bunch of spots with uniform weights. The matrix-based optimization of weights for a one-dimensional dose box results in a lateral penumbra of typically 1.4 σ b . This value reduces to about 1.3 σ b if also the positions of the beam spots are optimized for the considered field size. The analytical formulas for uniform discrete scanning can be used as rough approximations of the best-case scenarios for weight-optimized dose profiles if the spot spacing is defined as effective spot spacing. Conclusions : The trade-off between flatness, quantization error, and robustness on the one side and penumbra width on the other side can be described analytically for equally weighted spots. Treatment planning systems often perform a least-squares optimization of the individual spot weights which results in smaller lateral penumbras and smaller quantization errors than for uniform discrete scanning. However, the benefit of this weight optimization decreases with increasing λ (in the regime λ > σ b ). The spot spacing, which is obtained from the scenario that the optimization objective is met by uniform discrete scanning, poses a sharp upper limit for the spot spacing λ in weight optimization methods.

In light-ion beam dose delivery with the scanning technique the spacing between adjacent spots is an important parameter during treatment planning. In order to study the effect of spot spacing on dose conformity and robustness for single field uniform dose configurations, fundamental geometrical properties of placement of Gaussian beamlets are explored. In particular, the dependence of penumbra width and flatness on spot width and spot spacing is investigated.Infinitesimal calculus and analytical methods are used to derive simple expressions for the lateral penumbra and the flatness of one-dimensional dose profiles in continuous scanning and uniform discrete spot scanning. In the same way expressions for the fundamental modes of perturbation of the spot sequence are developed. A numerical, matrix-based approach is followed to optimize weights spot-by-spot.Generally the lateral penumbra widths lie between 1.13 sigma(b) and 1.68 sigma(b) with sigma(b) being the standard deviation of the beam spot profile. For regularly placed spots of equal weight with spot spacing lambda the lateral penumbra is given by 1.68 sigma' where sigma' results from quadratic subtraction of lambda/square root of 12 from sigma(b). The quantization error is identified as additional parameter describing the lateral dose conformity. It's variance is given by lambda2/12 for a bunch of spots with uniform weights. The matrix-based optimization of weights for a one-dimensional dose box results in a lateral penumbra of typically 1.4 sigma(b). This value reduces to about 1.3 sigma(b) if also the positions of the beam spots are optimized for the considered field size. The analytical formulas for uniform discrete scanning can be used as rough approximations of the best-case scenarios for weight-optimized dose profiles if the spot spacing is defined as effective spot spacing.The trade-off between flatness, quantization error, and robustness on the one side and penumbra width on the other side can be described analytically for equally weighted spots. Treatment planning systems often perform a least-squares optimization of the individual spot weights which results in smaller lateral penumbras and smaller quantization errors than for uniform discrete scanning. However, the benefit of this weight optimization decreases with increasing lambda (in the regime lambdasigma(b)). The spot spacing, which is obtained from the scenario that the optimization objective is met by uniform discrete scanning, poses a sharp upper limit for the spot spacing lambda in weight optimization methods.

Purpose: To describe the design and initial testing of a large diameter multilayerionization chamber (MLIC) system for water equivalent range measurements of pristine proton fields. Method and Materials: The MLIC uses a large 120 mm physical diameter collecting cross‐section in a stacked vented parallel plate geometry of 180 layers spaced 0.8 mm apart. The plates are fabricated from printed circuit board material where copper traces have been etched to form the conductive elements for the ionization chambers. In addition a dedicated software package was developed to assist with device calibration data acquisition and analysis. Computer modeling of the MLIC was performed with the Monte Carlo (MC) code FLUKA. Radiological testing was performed in a dedicated proton pencil beam scanning (PBS) therapeutic delivery system. The energies tested varied from 100–230 MeV. A series of 10 pristine depth profiles were first acquired for reference within this energy range by longitudinally scanning a discrete parallel plate ionization chamber of 84 mm diameter in a water phantom. After a calibration routine was applied to the MLIC the same energies were tested with the MLIC and the results compared. Additional tests were performed at different fluence rates to evaluate changes in response to detector bias and charge quantum (QC) settings. Results: The analyzed data from the MC simulation and MLIC physical data compared within one mm to the water phantom reference data under the conditions tested. The MLIC results were not sensitive to bias setting but depended with fluence rate on the QC setting. Conclusion: The initial test of the new MLIC prototype provided satisfactory results with regard to water equivalent range accuracy in comparison to reference measurements from the PBS system. Some flexibility is provided with different fluence rates by adjusting the QC. The detailed results will be presented and discussed.

Background and purpose: Monte Carlo simulations as well as analytical computations of proton transport in material media require accurate values of multiple Coulomb scattering (MCS) angles. High-quality experimental data on MCS angles in the energy range for proton therapy are, however, sparse. In this work, MCS modeling in proton transport was evaluated employing an experimental method to measure these angles on a medical proton beamline in clinically relevant materials. Results are compared to Monte Carlo simulations and analytical models. Materials and methods: Aluminum, brass, and lucite (PMMA) scatterers of clinically relevant thicknesses were irradiated with protons at 100, 160, and 220 MeV. Resulting spatial distributions of individual pencil beams were measured with a scintillating screen. The MCS angles were determined by deconvolution and a virtual point source approach. Results were compared to those obtained with the Monte Carlo codes PENH, TOPAS, and RayStation Monte Carlo, as well as the analytical models RayStation Pencil Beam Algorithm and the Molière/Fano/Hanson variant of the Molière theory. Results: Experimental data obtained with the presented methodology agree with previously published results within 6%, with an average deviation of 3%. The combined average uncertainty of the experimental data yielded 1.8%, while the combined maximum uncertainty was below 4%. The obtained Monte Carlo results for PENH, TOPAS, and RayStation deviate on average for all considered energies, materials and thicknesses, by 2.5%, 3.4%, and 2.8% from the experimental data, respectively. For the analytical models, the average deviations amount to 4.5% and 2.9% for the RayStation Pencil Beam Algorithm and the Molière/Fano/Hanson model, respectively. Conclusion: The experimental method developed for the present work allowed to measure MCS angles in clinical proton facilities with good accuracy. The presented method permits to extend the database on experimental MCS angles which is rather limited. This work further provides benchmark data for lucite in thicknesses relevant for clinical applications. The data may serve to validate dose engines of treatment planning systems and secondary dose check software. The Monte Carlo and analytical algorithms studied are capable of reproducing MCS data within the required accuracy for clinical applications.

Background and purpose PENH is a recently coded module for simulation of proton transport in conjunction with the Monte Carlo code PENELOPE. PENELOPE applies class II simulation to all type of interactions, in particular, to elastic collisions. PENH uses calculated differential cross sections for proton elastic collisions that include electron screening effects as well as nuclear structure effects. Proton-induced nuclear reactions are simulated from information in the ENDF-6 database or from alternative nuclear databases in ENDF format. The purpose of this work is to benchmark this module by simulating absorbed dose distributions from a single finite spot size proton pencil beam in water. Materials and methods Monte Carlo simulations with PENH are compared with simulation results from TOPAS Monte Carlo (v3.1p2) and RayStation Monte Carlo (v6). Different beam models are examined in terms of mean energy and energy spread to match the measured profiles. The phase-space file is derived from experimental measurements. Simulated absorbed dose distributions are compared to experimental data obtained with the ionization chamber array MatriXX 2D detector (IBA Dosimetry) in a water tank. The experiments were conducted with a clinical IBA pencil beam scanning dedicated nozzle. In all simulations a Fermi-Eyges phase-space representation of a single finite spot size proton pencil beam is used. Results In general, there is a good agreement between simulated results and experimental data up to a distance of 3 cm from the central axis. In the core region (region where the dose is more than 10% of the maximum dose) PENH shows, overall, the smallest deviations from experimental data, with the largest radial rms (root mean square) smaller than 0.2. The results achieved by TOPAS and RayStation in that region are very close to those of PENH. For the halo region, that is the area of the dose distribution outside the core region reaching down to 0.01% of the maximum intensity, the largest rms achieved by TOPAS is always smaller than 0.5, yielding better results than the rest of the codes. Conclusion The physics modeling of the PENELOPE/PENH code yields results consistent with measurements in the dose range relevant for proton therapy. The discrepancies between PENH appearing at distances larger than 3 cm from the central-beam axis are accountable to the lack of neutron simulation in this code. In contradistinction, TOPAS has a better agreement with experimental data at large distances from the central-beam axis because of the simulation of neutrons.