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.
To evaluate the benefit of adding CT imaging to the simulation process of clip-based proton therapy of ocular melanomas. For thirty ocular melanoma cases, the clip position in the eye model was determined based on orthogonal radiographs as well as on a CT image set. The geometrical shift of the clips between the standard simulation process and standard simulation process with addition of CT imaging (CT-guided) was determined. The dosimetric impact was evaluated by developing treatment plans based on both the standard-process model and the CT-guided model. In 40% of the studied cases, the difference in clip position between eye models created with and without CT was less than 0.5 mm. This difference was more than 1 mm in 17% of cases. The dosimetric impact of shifts below 1 mm was low because these shifts did not exceed the planning margins. For the four cases with a shift of more than 1 mm a reduction in target coverage (ΔV99%) of -3% to -6% was observed. Changes in macula and optic-disc mean dose of up to 16% and 35% of the prescribed dose were seen when these structures abutted the target. Adding CT imaging to the simulation process is beneficial in select cases where discrepancies between the eye model and ophthalmology measurements occur or where a critical structure is located close to the target and improved localization accuracy is wanted. For the majority of patients, addition of CT imaging does not result in quantifiable changes in dosimetry. Nevertheless, CT imaging is a valuable tool in the quality control of the modeling and treatment-planning process of clip-based eye treatments.
Purpose: To verify treatment plan monitor units from iPlan treatment planning system for Vero Stereotactic Body Radiotherapy (SBRT) treatment using both software-based and (homogeneous and heterogeneous) phantom-based approaches. Methods: Dynamic conformal arcs (DCA) were used for SBRT treatment of oligometastasis patients using Vero linear accelerator. For each plan, Monte Carlo calculated treatment plans MU (prescribed dose to water with 1% variance) is verified first by RadCalc software with 3% difference threshold. Beyond 3% differences, treatment plans were copied onto (homogeneous) Scanditronix phantom for non-lung patients and copied onto (heterogeneous) CIRS phantom for lung patients and the corresponding plan dose was measured using a cc01 ion chamber. The difference between the planed and measured dose was recorded. For the past 3 years, we have treated 180 patients with 315 targets. Out of these patients, 99 targets treatment plan RadCalc calculation exceeded 3% threshold and phantom based measurements were performed with 26 plans using Scanditronix phantom and 73 plans using CIRS phantom. Mean and standard deviation of the dose differences were obtained and presented. Results: For all patient RadCalc calculations, the mean dose difference is 0.76% with a standard deviation of 5.97%. For non-lung patient plan Scanditronix phantom measurements, the mean dose difference is 0.54% with standard deviation of 2.53%; for lung patient plan CIRS phantom measurements, the mean dose difference is −0.04% with a standard deviation of 1.09%; The maximum dose difference is 3.47% for Scanditronix phantom measurements and 3.08% for CIRS phantom measurements. Conclusion: Limitations in secondary MU check software lead to perceived large dose discrepancies for some of the lung patient SBRT treatment plans. Homogeneous and heterogeneous phantoms were used in plan quality assurance for non-lung patients and lung patients, respectively. Phantom based QA showed the relative good agreement between iPlan calculated dose and measured dose.
Purpose: To evaluate if addition of CT imaging to the treatment-planning process of ocular-melanoma proton therapy will improve the accuracy of target localization. Methods: For a total of 30 patients a treatment plan is made according to the standard protocol: 1) the ophthalmologist sutures 3–5 small clips on the eye surface; 2) during simulation, a set of orthogonal images is taken identifying the clip positions; 3) the treatment-planning software (EyePlan) fits a geometric eye model to the imaged clips; 4) in the model the tumor is delineated based on the clip positions and registered fundus photographs; 5) dose from the treatment beam is calculated using 2.5 mm margins. In this study an additional high-resolution CT scan is taken of each treated eye. The geometric eye model is registered to the CT based on the clip positions. Agreement of the anatomy (lens, sclera, optic nerve) is evaluated. The translation required to go from the clip-based to anatomy-based registration is determined as measure of the accuracy of clip position in the radiograph-based model. A second treatment plan is made based on the CT clip positions. Target coverage and dose to critical structure is compared between the radiograph-based and CT-based plan. Dose deviations are evaluated for correlation with visual acuity, clip location, and tumor size. Results: For 19 patients the shift in clip position between radiograph-only and CT-based simulation is ≤1 mm; for 28 patients ≤2 mm. For the remaining two patients it is 3.6 and 5.0 mm. For most patients (18) the effect on the tumor coverage is minimal: reduction in V99% of ≤ 2.5 %. For 3 patients V99% reduces by >7.5%. Conclusion: The addition of CT imaging to standard planning protocols improves target coverage in 10% of patients.
Eye-dedicated proton therapy (PT) facilities are used to treat malignant intraocular lesions, especially uveal melanoma (UM). The first commercial ocular PT beamline from Varian was installed in the Netherlands. In this work, the conceptual design of the new eyeline is presented. In addition, a comprehensive comparison against five PT centers with dedicated ocular beamlines is performed, and the clinical impact of the identified differences is analyzed.The HollandPTC eyeline was characterized. Four centers in Europe and one in the United States joined the study. All centers use a cyclotron for proton beam generation and an eye-dedicated nozzle. Differences among the chosen ocular beamlines were in the design of the nozzle, nominal energy, and energy spectrum. The following parameters were collected for all centers: technical characteristics and a set of distal, proximal, and lateral region measurements. The measurements were performed with detectors available in-house at each institution. The institutions followed the International Atomic Energy Agency (IAEA) Technical Report Series (TRS)-398 Code of Practice for absolute dose measurement, and the IAEA TRS-398 Code of Practice, its modified version or International Commission on Radiation Units and Measurements Report No. 78 for spread-out Bragg peak normalization. Energy spreads of the pristine Bragg peaks were obtained with Monte Carlo simulations using Geant4. Seven tumor-specific case scenarios were simulated to evaluate the clinical impact among centers: small, medium, and large UM, located either anteriorly, at the equator, or posteriorly within the eye. Differences in the depth dose distributions were calculated.A pristine Bragg peak of HollandPTC eyeline corresponded to the constant energy of 75 MeV (maximal range 3.97 g/cm2 in water) with an energy spread of 1.10 MeV. The pristine Bragg peaks for the five participating centers varied from 62.50 to 104.50 MeV with an energy spread variation between 0.10 and 0.70 MeV. Differences in the average distal fall-offs and lateral penumbrae (LPs) (over the complete set of clinically available beam modulations) among all centers were up to 0.25 g/cm2 , and 0.80 mm, respectively. Average distal fall-offs of the HollandPTC eyeline were 0.20 g/cm2 , and LPs were between 1.50 and 2.15 mm from proximal to distal regions, respectively. Treatment time, around 60 s, was comparable among all centers. The virtual source-to-axis distance of 120 cm at HollandPTC was shorter than for the five participating centers (range: 165-350 cm). Simulated depth dose distributions demonstrated the impact of the different beamline characteristics among institutions. The largest difference was observed for a small UM located at the posterior pole, where a proximal dose between two extreme centers was up to 20%.HollandPTC eyeline specifications are in accordance with five other ocular PT beamlines. Similar clinical concepts can be applied to expect the same high local tumor control. Dosimetrical properties among the six institutions induce most likely differences in ocular radiation-related toxicities. This interinstitutional comparison could support further research on ocular post-PT complications. Finally, the findings reported in this study could be used to define dosimetrical guidelines for ocular PT to unify the concepts among institutions.
Purpose: To quantify the dosimetric effect of roll and pitch corrections being performed by two types of robotic tables available at our institution: BrainLabTM 5DOF robotic table installed at VERO (BrainLab&MHI) dedicated SBRT linear accelerator and 6DOF robotic couch by IBA Proton Therapy with QFixTM couch top. Methods: Planning study used a thorax phantom (CIRSTM), scanned at 4DCT protocol; targets (IGTV, PTV) were determined according to the institutional lung site‐specific standards. 12 CT sets were generated with Pitch and Roll angles ranging from −4 to +4 degrees each. 2 table tops were placed onto the scans according to the modality‐specific patient treatment workflows. The pitched/rolled CT sets were fused to the original CT scan and the verification treatment plans were generated (12 photon SBRT plans and 12 proton conventional fractionation lung plans). Then the CT sets were fused again to simulate the effect of patient roll/pitch corrections by the robotic table. DVH sets were evaluated for all cases. Results: The effect of not correcting the phantom position for roll/pitch in photon SBRT cases was reducing the target coverage by 2% as maximum; correcting the positional errors by robotic table varied the target coverage within 0.7%. in case of proton treatment, not correcting the phantom position led to the coverage loss up to 4%, applying the corrections using robotic table reduced the coverage variation to less than 2% for PTV and within 1% for IGTV. Conclusion: correcting the patient position by using robotic tables is highly preferable, despite the small dosimetric changes introduced by the devices.
