A commercial proton eyeline has been developed to treat ocular disease. Radiotherapy of intraocular lesions (e.g., uveal melanoma, age-related macular degeneration) requires sharp dose gradients to avoid critical structures like the macula and optic disc. A high dose rate is needed to limit patient gazing times during delivery of large fractional dose. Dose delivery needs to be accurate and predictable, not in the least because current treatment planning algorithms have limited dose modeling capabilities. The purpose of this paper is to determine the dosimetric properties of a new proton eyeline. These properties are compared to those of existing systems and evaluated in the context of the specific clinical requirements of ocular treatments.The eyeline is part of a high-energy, cyclotron-based proton therapy system. The energy at the entrance of the eyeline is 105 MeV. A range modulator (RM) wheel generates the spread-out Bragg peak, while a variable range shifter system adjusts the range and spreads the beam laterally. The range can be adjusted from 0.5 up to 3.4 g/cm(2); the modulation width can be varied in steps of 0.3 g/cm(2) or less. Maximum field diameter is 2.5 cm. All fields can be delivered with a dose rate of 30 Gy/min or more. The eyeline is calibrated according to the IAEA TRS-398 protocol using a cylindrical ionization chamber. Depth dose distributions and dose/MU are measured with a parallel-plate ionization chamber; lateral profiles with radiochromic film. The dose/MU is modeled as a function of range, modulation width, and instantaneous MU rate with fit parameters determined per option (RM wheel).The distal fall-off of the spread-out Bragg peak is 0.3 g/cm(2), larger than for most existing systems. The lateral penumbra varies between 0.9 and 1.4 mm, except for fully modulated fields that have a larger penumbra at skin. The source-to-axis distance is found to be 169 cm. The dose/MU shows a strong dependence on range (up to 4%/mm). A linear increase in dose/MU as a function of instantaneous MU rate is observed. The dose/MU model describes the measurements with an accuracy of ± 2%. Neutron dose is found to be 146 ± 102 μSv/Gy at the contralateral eye and 19 ± 13 μSv/Gy at the chest.Measurements show the proton eyeline meets the requirements to effectively treat ocular disease.
Purpose : Create a Monte Carlo based proton therapy dose distribution verification system which can be used as a clinical aid in determining the adequacy of proton treatment plans. Method and Materials : The GEANT4 Monte Carlo toolkit is used for all simulations. The absolute dose value per proton of the Bragg peak and its location were benchmark against NIST and other published data to ensure the accuracy of the physics models used in the simulations. For the patient specific dose distribution verification, the full nozzle is not simulated, only the beam spectrum, patient specific snout components, and the patient's CT. The beam spectrum, which for the IBA double scattering machine, depends on the time dependent proton current and range modulator, is calculated based on a spreadsheet which uses water equivalent thicknesses for the nozzle components. This allows for relative fast calculations compared with full nozzle simulation. The speed is needed if this system is to be used in a clinical setting. Results : The beam spectrum has been verified for multiple ranges and SOBP widths for the different options. The MC distal and proximal 90% of the SOBP matches the TPS to within 3% or 3mm for all tested cases. The MC penumbra for different aperture sizes match the TPS at 20%, 50% and 80% within 3% and 3mm, however the 10% value of the penumbra for the MC is wider than the TPS, which has been noted as a limitation of the TPS in literature. Comparisons to measurements have not been made to date and adjustments to model parameters will be made to match measurements. Compensator simulations have not been performed to date. Conclusion : This Monte Carlo dose verification system is fast and accurate; therefore it holds promise as a clinically useful tool.
Purpose: To evaluate the Monitor Unit calculation algorithm in Eclipse proton therapy treatment planning. Method and Materials: The Eclipse (Varian) treatment planning system is commissioned for eight double scattering options, i.e. eight range modulator and second-scatterer combinations. When supplying the commissioning beam data in cGy/MU, Eclipse calculates the Monitor Units delivering the prescribed dose. We compare this calculated output to measurements in water for fields of varying range and modulation width. The output variation with source-to-skin distance as well as with aperture size is investigated. To evaluate the effect of the range compensator and target inhomogeneities on the delivered dose, clinical plans are recalculated in a water phantom — both with and without range compensator — and the dose-per-MU in the normalization point is compared to measurement. Results: Eclipse creates the spread-out Bragg peak by adding the individual pristine peaks according to their weight and position as defined in the user-supplied range-modulator wheel file. The observed output strongly depends on the range modulator wheel layout. By optimizing the file in combination with the user-supplied output for the pristine peaks a good agreement with the measured output is obtained. The agreement is best for small modulation width and worsens continuously with increasing modulation width, especially for the options with a higher range. The Eclipse dose decreases with source-to-skin distance as expected, but the measured decrease in output with aperture diameter is underestimated. Conclusion: The accuracy of the Eclipse output calculation depends strongly on the accuracy of the beam data supplied. Optimizing the beam data to minimize the difference between planned and measured output typically leads to an agreement within ±5%.
Purpose: To compare proton dose distributions generated with double‐scattering to uniform‐scanning for different clinical sites. Method and Materials: The ‘universal nozzle’ developed by IBA incorporates several delivery modes. In double‐scattering (DS) a flattening filter scatters the proton beam into a flat circular profile. In uniform‐scanning (US) two dipole magnets scan the beam into a rectangular profile. US covers larger volumes both laterally and in depth, and has dosimetric characteristics that are different from US. This study deals with cases that can be treated with either US or DS. Eclipse (Varian) treatment planning is commissioned for both delivery modes. Comparison of water‐phantom calculations to measurements validates the treatment‐planning algorithm. We compare the dose for the following sites: prostate (2 cases), head‐and‐neck (4 cases), cranio‐spinal (3 cases). Dose‐volume‐histograms are used to evaluate target coverage and dose to critical structures. Results: The US in‐air penumbra is typically smaller because of less scattering material in the beam path. For a range of 15.0g/cm 2 , modulation of 8.0g/cm 2 , and air gap of 12.0cm, the 80%–20% penumbra at 11.0cm depth is 4.4mm in US and 6.9mm in DS. In addition, the US distal fall‐off is sharper because of reduced energy straggling in the treatment head. For a range of 5.0 (28.0) cm in water the 80%–20% fall‐off is 2.7mm (5.5mm) in US, compared to 4.0mm (6.0mm) in DS. For deep seated tumors (prostate) the sharper in‐air dose distribution in US is washed out by in‐patient scatter, resulting in no significant benefit. For targets at shallow and intermediate depth, located next to a critical structure, the sharper fall‐off in US allows for better target coverage and less dose to the critical structure. Conclusion: The sharper lateral and distal penumbra in uniform scanning are beneficial when the target volume abuts a critical structure. For deep‐seated tumors this advantage diminishes.
Purpose: Over the past 3 years, more than 50 patients with ocular disease have been treated with a prototype commercial eyeline developed by Ion Beam Applications (IBA). It is particularly crucial for the dosimetry of the machine to be accurate, precise, and reproducible because of the small treatment volumes and several surrounding critical structures. A quality assurance (QA) program has been developed in order to regularly check and record the machine's dosimetry over time. In this study we have summarized and evaluated three years of patient and system QA data for the eyeline prototype. Methods: In order to accurately deliver the prescribed dose to the patient's treatment site, an output model was created based on several machine parameters. This output model, along with the requested system parameters, are regularly checked and recorded by a QA program. 10 standard proton fields have been created which have regular QA performed monthly with a water tank and parallel‐plate chamber, and recorded in a database. QA has additionally been performed for each patient‐specific treatment field. This is to verify and evaluate the patient output model as well as the system parameters for range&modulation combinations different than the standard QA fields. Results: 140 beam measurements have been analyzed for range, modulation, and output. The range is typically within tolerance, although there are about 6 cases where the system did not perform as expected. The modulation appears to be very stable, always being within tolerance. Output for the standard QA fields are typically within 2.5% from the baseline value. The patient output measurements agree within 2.5% from the model. Conclusion: Overall the eyeline performs well within the tolerances set, and the patient output measurements agree within 2% of the model.
Purpose: We have developed an empirical algorithm to calculate patient specific monitor unit (MU) for intensity modulated scanning proton beam (IMPT). Method and Materials: The algorithm is based on a well-established formula for proton output calculation. It adds additional data for a broad range of SSD, depth, and off-axis. All beam data are generated using GEANT4 Monte-Carlo (MC) simulation. Pristine percentage depth doses (PDD) for several ranges and off-axis ratio (OAR) at several depths of the pencil proton beam are used. This algorithm adopts the concept of proton head-scatter factor, Hp(r,f), to characterize the proton fluence variation with lateral distances (r) and source-to-detector distance (f). Results: Input beam data for the empirical program, PDD and OAR are examined for the proton energies and ranges suitable for the modulated scanning proton beam. OAR is the dose measured in water at depth d and r is the lateral distance of the pencil-beam. Hp is the energy fluence for the same conditions. The program can import DICOMRT plans from the IMPT treatment planning system. Modulated scanning beam generated SOBP depth dose curves for widths between 9 and 12 cm are evaluated. IMPT plans for clinical sites (e.g., prostate) are evaluated. Conclusion: This algorithm is ideally suitable for calculating patient-specific MU for pencil-beam based treatment planning system for protons.
Purpose: To experimentally investigate target motion impact on proton dosimetry and the spatiotemporal interplay in double scattering (DS) and uniform scanning (US) deliveries using 3D polymer gel dosimeters and a programmable motion platform. Methods: A one beam proton plan with 16cm range and 6cm modulation (total 13 energy layers) in both DS and US modes was generated in the Eclipse treatment planning system (TPS) to be used for gel irradiation in static, periodic and random motion. The periodic motion was a single sinusoidal trace along the SI direction with 2‐cm peak‐to‐peak amplitude and 0.25Hz frequency. The random motion trace was an actual patient prostate trajectory from a treatment fraction. All irradiated gel dosimeters were setup on the platform in an identical fashion. The dose distribution of static gel dosimeter was compared to those of treatment plan. Dose distributions were compared between DS and US in both periodic and prostate motion conditions. 3D gamma analysis (4%, 4mm criterion) was calculated against planned dose distribution for all the gel dosimeters. Results: There was an overall good agreement in dose distributions between stationary DS delivery and TPS plan. 93% of voxels has gamma less than 1. 3D gamma failure maps were significantly different between DS and US periodic motion gels. There were 88% and 91% of voxels has gamma less than 1 for DS and US, respectively. In prostate motion cases, even though about 70% voxels has gamma larger than 1 for both DS and US deliveries, significant gamma failure map difference was still observed. Conclusion: Good dose agreement between TPS plan and stationary gel dosimeter demonstrated the validity of using gel dosimeter study target motion effect and its spatiotemporal interplay with proton irradiation. Isodose and 3D gamma comparisons between DS and US for both periodic and prostate motion confirmed their dosimetric impact.
