Dual ring multilayer ionization chamber and theory-based correction technique for scanning proton therapy

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

BACKGROUND AND PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Physical characterization of therapeutic proton delivery through common dental materials

PURPOSE

Dental fixtures are commonplace in an aging, radiation treatment population. The current, local standard of practice in particle therapy is to employ treatment geometries to avoid delivery through implanted dental fixtures. The present study aims to observe the physical effect of delivering therapeutic proton beams through common dental fixture materials as prelude to an eventual goal of assessing the feasibility of using treatment geometries not specified for avoidance of oral implants. A sampling of common dental materials was selected based on prosthodontic consult and was evaluated in terms of relative stopping power and three-dimensional (3D) dose perturbation.

METHODS

Amalgams, porcelain-fused-to-metal (PFM) crowns consisting of zirconia and non-noble base metals, and lithium disilicate implants were chosen for analysis. Theoretical stopping power (S) and mass stopping power (S/ρ) were calculated using the Stopping and Range of Ions in Matter (SRIM) application, basing stoichiometric compositions of each fixture on published materials data. S and S/ρ were calculated for a range of historically available compositions of amalgams from 1900 until the current era. The perturbance of S and S/ρ as a function of clinically relevant ranges of amalgam compositions for the modern era was analyzed. Water equivalent thickness (WET) and relative stopping power (S_rel ) of each material was measured for a clinical spot-scanning proton beam with monoenergies of 159.9 and 228.8 MeV with a multi-layer ionization chamber (MLIC). Subsequently, 3D dose perturbation was assessed by delivering proton beams through a custom phantom designed to simulate both en-face and on-edge treatment geometries through the selected materials. A treatment plan mimicking the experimental delivery was constructed in the institutional treatment planning system and calculated using TOPAS based Monte Carlo Simulation (MCS). Experimental results were used to validate the MCS. Finally, TPS outputs were compared to MCS to determine the accuracy of the dose calculation model.

RESULTS

Historical compositions of amalgams ranged in S from 44.8 to 42.9 MeV/cm, with the greatest deviation being observed for the 1900-1959 era. Deviation as a function of amalgam composition from the modern era was most sensitive to proportion of Hg, accounting for deviations up to -4.2% at the greatest clinically relevant concentration. S/ρ was not found to vary greatly between each porcelain and metal alloy material for porcelain-fused-to-metal (PFM) type crowns. Relative stopping powers ranged between 1.3 and 5.4 for all studied materials, suggesting substantial changes in proton range with respect to water. Film measurements of pristine spots confirm dose perturbance and shortening of proton range, with an upstream shift of each Bragg peak being observed directly behind the installed fixture. At high energies, cold spots were found in all cases directly behind each material feature with a medial fill-in of dose occurring distally. Qualitative agreement of spot perturbance was confirmed between film measurements and MCS. Finally, when comparing integrated depth doses (IDD) by summing over all axial directions, good agreement is observed between TPS and MCS.

CONCLUSIONS

All dental materials studied substantially perturbed the dosimetry of pristine proton spots both in terms of WET/S_rel as well as the spatial distribution of dose. Proton range was quantifiably shortened, and each dental material affected a cold spot directly behind the object with medial dose back-filling was observed distally. Monte Carlo simulations and Eclipse dose calculations exhibited good agreement with measurements, suggesting that treatment planning without employing avoidance strategies may be possible with further investigation. This article is protected by copyright. All rights reserved.

Dosimetric Assessment of a High Precision System for Mouse Proton Irradiation to Assess Spinal Cord Toxicity

The uncertainty associated with the relative biological effectiveness (RBE) in proton therapy, particularly near the Bragg peak (BP), has led to the shift towards biological-based treatment planning. Proton RBE uncertainty has recently been reported as a possible cause for brainstem necrosis in pediatric patients treated with proton therapy. Despite this, in vivo studies have been limited due to the complexity of accurate delivery and absolute dosimetry. The purpose of this investigation was to create a precise and efficient method of treating the mouse spinal cord with various portions of the proton Bragg curve and to quantify associated uncertainties for the characterization of proton RBE. Mice were restrained in 3D printed acrylic boxes, shaped to their external contour, with a silicone insert extending down to mold around the mouse. Brass collimators were designed for parallel opposed beams to treat the spinal cord while shielding the brain and upper extremities of the animal. Up to six animals may be accommodated for simultaneous treatment within the restraint system. Two plans were generated targeting the cervical spinal cord, with either the entrance (ENT) or the BP portion of the beam. Dosimetric uncertainty was measured using EBT3 radiochromic film with a dose-averaged linear energy transfer (LETd) correction. Positional uncertainty was assessed by collecting a library of live mouse scans (n = 6 mice, two independent scans per mouse) and comparing the following dosimetric statistics from the mouse cervical spinal cord: Volume receiving 90% of the prescription dose (V90); mean dose to the spinal cord; and LETd. Film analysis results showed the dosimetric uncertainty to be ±1.2% and ±5.4% for the ENT and BP plans, respectively. Preliminary results from the mouse library showed the V90 to be 96.3 ± 4.8% for the BP plan. Positional uncertainty of the ENT plan was not measured due to the inherent robustness of that treatment plan. The proposed high-throughput mouse proton irradiation setup resulted in accurate dose delivery to mouse spinal cords positioned along the ENT and BP. Future directions include adapting the setup to account for weight fluctuations in mice undergoing fractionated irradiation.

A High-Precision Method for In Vitro Proton Irradiation

Purpose

Although proton therapy has become a well-established radiation modality, continued efforts are needed to improve our understanding of the molecular and cellular mechanisms occurring during treatment. Such studies are challenging, requiring many resources. The purpose of this study was to create a phantom that would allow multiple in vitro experiments to be irradiated simultaneously with a spot-scanning proton beam.

Materials and Methods

The setup included a modified patient-couch top coupled with a high-precision robotic arm for positioning. An acrylic phantom was created to hold 4 6-well cell-culture plates at 2 different positions along the Bragg curve in a reproducible manner. The proton treatment plan consisted of 1 large field encompassing all 4 plates with a monoenergetic 76.8-MeV posterior beam. For robust delivery, a mini pyramid filter was used to broaden the Bragg peak (BP) in the depth direction. Both a Markus ionization chamber and EBT3 radiochromic film measurements were used to verify absolute dose.

Results

A treatment plan for the simultaneous irradiation of 2 plates irradiated with high linear energy transfer protons (BP, 7 keV/μm) and 2 plates irradiated with low linear energy transfer protons (entrance, 2.2 keV/μm) was created. Dose uncertainty was larger across the setup for cell plates positioned at the BP because of beam divergence and, subsequently, variable proton-path lengths. Markus chamber measurements resulted in uncertainty values of ±1.8% from the mean dose. Negligible differences were seen in the entrance region (<0.3%).

Conclusion

The proposed proton irradiation setup allows 4 plates to be simultaneously irradiated with 2 different portions (entrance and BP) of a 76.8-MeV beam. Dosimetric uncertainties across the setup are within ±1.8% of the mean dose.

Patient-specific quality assurance for proton depth dose distribution using a multi-layer ionization chamber in a single-ring wobbling method

The use of a multi-layer ionization chamber, Zebra, in patient-specific quality assurance (QA) for proton depth dose distributions in a single-ring wobbling method is investigated. The depth dose distributions measured using Zebra are compared with those calculated using the treatment planning system (TPS), XiO-M, and measured using an ionization chamber with a motorized water phantom system. Because the TPS only provides point doses, the average doses are calculated using in-house software. The detector size-corrected depth dose distributions are obtained by determining the average of the dose distributions from the TPS over a cylindrical region similar to the size of the Zebra detectors. The calculated depth dose distributions from the cases with a simple compensator shape are in good agreement with those obtained from the TPS without performing volume averaging; however, a 15% difference was shown when compared with those from the cases with a complex compensator shape. Then, the measurements are compared with the detector size-corrected depth dose distributions, showing an improved agreement within 3% for the highly steep dose gradient regions. Although there are some field size limitations, the Zebra system is a useful device for the fast measurement of patient-specific QA for depth dose distributions in wobbled proton beams. However, careful consideration is required for complex dose distribution fields, because the measurements obtained using Zebra cannot be directly compared to the depth dose distributions from the TPS owing to the finite detector size of the Zebra chamber.