Abstract
PurposeEye‐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.Material/MethodsThe 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.ResultsA 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%.ConclusionsHollandPTC 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.
Highlights
A pristine Bragg peak of HollandPTC eyeline corresponded to the constant energy of 75 MeV with an energy spread of 1.10 MeV
The largest difference was observed for a small uveal melanoma (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 proton therapy (PT) beamlines
Summary
A pristine Bragg peak of HollandPTC eyeline corresponded to the constant energy of 75 MeV (maximal range 3.97 g/cm[2] in water) with an energy spread of 1.10 MeV. Reported clinical outcomes show a 5-yr local tumor control rate over 95% with an eye retention rate of 90%.2-10 A dedicated ocular nozzle mounted on a horizontal passive scattering beamline is used for the treatment in most facilities. As the number of proton facilities steadily increases worldwide, so does the number of dedicated proton eye treatment rooms, which has recently reached 20.11,12 Most of them are integrated within a multiroom PT center, connected to a high-energy accelerator generating beams with initial extraction energy up to 250 MeV corresponding to a proton range of more than 30.00 g/ cm[2] in depth. This study aimed to report on the design, physical, and dosimetrical properties of the first ocular PT beamline developed by Varian and compare the results against five existing institutions treating UM for many years. The impact of the dosimetrical differences was evaluated by simulation of seven tumor-specific case scenarios to investigate to what extent these differences affect in-depth dose distributions, and radiation-related toxicity outcomes
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