Design and characterization of an aperture system and spot configuration for ocular treatments with a gantry-based spot scanning proton beam.

Proton therapy is a key modality used in the treatment of ocular melanoma. Traditionally ocular sites are treated using a dedicated eyeline with a passively scattered proton beam and a brass aperture. This work aims to design and characterize a beam-collimating aperture to treat ocular targets with a gantry-based spot scanning proton beam. A plastic aperture system that slides into the gantry nozzle of a spot scanning proton beam was designed and constructed. It consists of an intermediate scraper layer to attenuate stray protons and a 3D-printed patient-specific aperture positioned 5.7 cm from the surface of the eye. The aperture system was modeled in TOPAS and Monte Carlo simulations were validated with film measurements. Two different spot configurations were investigated for treatment planning and characterized based on lateral penumbra, central axis (CAX) dose and relative efficiency. Alignment and leakage were investigated through experimental film measurements. Range was verified using a multi-layer ionization chamber. Reference dose measurements were made with a PinPoint 3D ion chamber. Neutron dose was evaluated through Monte Carlo simulations. Aperture alignment with radiation isocenter was determined to be within 0.31 mm at a gantry angle of 0°. A single-spot configuration with a 10 mm diameter aperture yielded film-measured lateral penumbras of 1 mm to 1.25 mm, depending on depth in the spread-out Bragg peak. TOPAS simulations found that a single spot configuration results in a flat dose distribution for a 10 mm diameter aperture and provides a CAX dose of less than 106% for apertures less than 14 mm in diameter. For larger targets, adding four corner spots to fill in the dose distribution is beneficial. Trade-offs between lateral penumbra, CAX dose and relative efficiency were characterized for different spot configurations and can be used for future clinical decision-making. The aperture was experimentally determined to not affect proton beam range, and no concerning leakage radiation or neutron dose was identified. Reference dose measurements with a PinPoint ion chamber were within 2.1% of Monte Carlo calculated doses. The aperture system developed in this work provides a method of treating ocular sites on a gantry-based spot scanning proton system. Additional work to develop compatible gaze tracking and gating infrastructure is ongoing.

Hickling SV ，Corner S ，Kruse JJ ，Deisher AJ ... -  《-》

Design and commissioning of the non-dedicated scanning proton beamline for ocular treatment at the synchrotron-based CNAO facility.

Only few centers worldwide treat intraocular tumors with proton therapy, all of them with a dedicated beamline, except in one case in the USA. The Italian National Center for Oncological Hadrontherapy (CNAO) is a synchrotron-based hadrontherapy facility equipped with fixed beamlines and pencil beam scanning modality. Recently, a general-purpose horizontal proton beamline was adapted to treat also ocular diseases. In this work, the conceptual design and main dosimetric properties of this new proton eyeline are presented. A 28 mm thick water-equivalent range shifter (RS) was placed along the proton beamline to shift the minimum beam penetration at shallower depths. FLUKA Monte Carlo (MC) simulations were performed to optimize the position of the RS and patient-specific collimator, in order to achieve sharp lateral dose gradients. Lateral dose profiles were then measured with radiochromic EBT3 films to evaluate the dose uniformity and lateral penumbra width at several depths. Different beam scanning patterns were tested. Discrete energy levels with 1 mm water-equivalent step within the whole ocular energy range (62.7-89.8 MeV) were used, while fine adjustment of beam range was achieved using thin polymethylmethacrylate additional sheets. Depth-dose distributions (DDDs) were measured with the Peakfinder system. Monoenergetic beam weights to achieve flat spread-out Bragg Peaks (SOBPs) were numerically determined. Absorbed dose to water under reference conditions was measured with an Advanced Markus chamber, following International Atomic Energy Agency (IAEA) Technical Report Series (TRS)-398 Code of Practice. Neutron dose at the contralateral eye was evaluated with passive bubble dosimeters. Monte Carlo simulations and experimental results confirmed that maximizing the air gap between RS and aperture reduces the lateral dose penumbra width of the collimated beam and increases the field transversal dose homogeneity. Therefore, RS and brass collimator were placed at about 98 cm (upstream of the beam monitors) and 7 cm from the isocenter, respectively. The lateral 80%-20% penumbra at middle-SOBP ranged between 1.4 and 1.7 mm depending on field size, while 90%-10% distal fall-off of the DDDs ranged between 1.0 and 1.5 mm, as a function of range. Such values are comparable to those reported for most existing eye-dedicated facilities. Measured SOBP doses were in very good agreement with MC simulations. Mean neutron dose at the contralateral eye was 68 μSv/Gy. Beam delivery time, for 60 Gy relative biological effectiveness (RBE) prescription dose in four fractions, was around 3 min per session. Our adapted scanning proton beamline satisfied the requirements for intraocular tumor treatment. The first ocular treatment was delivered in August 2016 and more than 100 patients successfully completed their treatment in these 2 yr.

Ciocca M ，Magro G ，Mastella E ，Mairani A ，Mirandola A ，Molinelli S ，Russo S ，Vai A ，Fiore MR ，Mosci C ，Valvo F ，Via R ，Baroni G ，Orecchia R ... -  《-》

Commissioning and beam characterization of the first gantry-mounted accelerator pencil beam scanning proton system.

To present and discuss beam characteristics and commissioning process of the first gantry-mounted accelerator single room pencil beam scanning (PBS) proton system. The Mevion HYPERSCAN employs a design configuration with a synchrocyclotron mounted on the gantry to eliminate the traditional beamline and a nozzle that contains the dosimetry monitoring chambers, the energy modulator (Energy Selector (ES)), and an Adaptive Aperture (AA). To characterize the beam, we measured the integrated depth dose (IDDs) for 12 energies, from highest energy of 227 MeV down to 28 MeV with a range difference ~ 2 cm between the adjacent energies, using a large radius Bragg peak chamber; single-spot profiles in air at five locations along the beam central axis using radiochromic EBT3 film and cross compared with a scintillation detector; and determined the output using a densely packed spot map. To access the performance of AA, we measured interleaf leakage and the penumbra reduction effect. Monte Carlo simulation using TOPAS was performed to study spot size variation along the beam path, beam divergence, and energy spectrum. This proton system is calibrated to deliver 1 Gy dose at 5 cm depth in water using the highest beam energy by delivering 1 MU/spot to a 10 × 10 cm2 map with a 2.5 mm spot spacing. The spot size in air varies from 4 mm to 26 mm from 227 MeV to 28 MeV at the isocenter plane with the nozzle retracted 23.6 cm from isocenter. The beam divergence of 28 MeV beam is ~ 52.7 mrad, which is nearly 22 times that of 227 MeV proton beam. The binary design of the ES has resulted in shifts of the effective SSD toward the isocenter as the energy is modulated lower. The peaks of IDD curves have a constant 80%-80% width of 8.4 mm at all energies. The interleaf leakage of the AA is less than 1.5% at the highest energy; and the AA can reduce the penumbra by 2 mm to 13 mm for the 227 and 28 MeV energies at isocenter plane in air. The unique design of the HYPERSCAN proton system has yielded beam characteristics significantly different from that of other proton systems in terms of the Bragg peak shapes, spot sizes, and the penumbra sharpening effect of the AA. The combination of the ES and AA has made PBS implementation possible without using beam transport line and range shifter devices. Different considerations may be required in treatment planning optimization to account for different design and beam characteristics.

Kang M ，Pang D 《-》

Development and characterization of the first proton minibeam system for single-gantry proton facility.

Minibeam represents a preclinical spatially fractionated radiotherapy modality with great translational potential. The advantage lies in its high therapeutic index (compared to GRID and LATTICE) and ability to treat at greater depth (compared to microbeam). Proton minibeam radiotherapy (pMBRT) is a synergy of proton and minibeam. While the single-gantry proton facility has gained popularity due to its affordability and compact design, it often has limited beam time available for research purposes. Conversely, given the current requirement of pMBRT on specific minibeam hardware collimators, necessitates a reproducible and fast setup to minimize pMBRT treatment time and streamline the switching time between pMBRT and conventional treatment for clinically translation. The contribution of this work is the development and characterization of the first pMBRT system tailored for single-gantry proton facility. The system allows for efficient and reproducible plug-and-play setup, achievable within minutes. The single room pMBRT system is constructed based on IBA ProteusONE proton machine. The end of nozzle is attached with beam modifying accessories though an accessory drawer. A small snout is attached to the accessory drawer and used to hold apertures and range shifters. The minibeam aperture consists of two components: a fitting ring and an aperture body. Three minibeam apertures were manufactured. The first-generation apertures underwent qualitatively analysis with film, and the second generation aperture underwent more comprehensive quantitative measurement. The reproducibility of the setup is accessed, and the film measurements are performed to characterize the pMBRT system in cross validation with Monte Carlo (MC) simulations. We presented initial results of large field pMBRT aperture and the film measurements indicates the effect of source-to-isocenter distance = 930 cm in Y proton scanning direction. Consistent with TOPAS MC simulation, the dose uniformity of pMBRT field <2 cm is demonstrated to be better than 2%, rendering its suitability for pre-clinical studies. Subsequently, we developed the second generation of aperture with five slits and characterized the aperture with film dosimetry studies and compared the results to the benchmark MC. Comprehensive film measurements were also performed to evaluate the effect of divergence, air gap and gantry-angle dependency and repeatability and revealing a consistent performance within 5%. Furthermore, the 2D gamma analysis indicated a passing rate exceeding 99% using 3% dose difference and 0.2 mm distance agreement criteria. We also establish the peak valley dose ratio and the depth dose profile measurements, and the results are within 10% from MC simulation. We have developed the first pMBRT system tailored for a single-gantry proton facility, which has demonstrated accuracy in benchmark with MC simulations, and allows for efficient plug-and-play setup, emphasizing efficiency.

Lin Y ，Li W ，Johnson D ，Prezado Y ，Gan GN ，Gao H ... -  《-》

Optimization of proton pencil beam positioning in collimated fields.

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. To minimize the proton field's lateral penumbra by investigating the fundamental relations between spot and collimating aperture edge position. 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. 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 $5\,\text{mm}$ reduced the lateral penumbra by about 20% on average after a propagation of 13 cm $13\,\text{cm}$ 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. 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.

Behrends C ，Bäumer C ，Verbeek NG ，Wulff J ，Timmermann B ... -  《-》