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Monte Carlo dosimetry modeling of focused kV x‐ray radiotherapy of eye diseases with potential nanoparticle dose enhancement
Author(s) -
Yan Huagang,
Ma Xiangyu,
Sun Weiyuan,
Mendez Stacy,
Stryker Stefan,
StarrBaier Sean,
Delliturri Gianpiero,
Zhu Dengsong,
Nath Ravinder,
Chen Zhe,
Roberts Kenneth,
MacDonald Carolyn A.,
Liu Wu
Publication year - 2018
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.13144
Subject(s) - imaging phantom , dosimetry , monte carlo method , brachytherapy , optics , materials science , full width at half maximum , x ray , beam (structure) , absorbed dose , physics , nuclear medicine , radiation , medical physics , radiation therapy , medicine , mathematics , radiology , statistics
Purpose Eye plaque brachytherapy is the most common approach for intraocular cancer treatment. It is, however, invasive and subject to large setup uncertainty due to the surgical operation. We propose a novel‐focused kV x‐ray technique with potential nanoparticle ( NP ) enhancement and evaluate its application in treating choroidal melanoma and iris melanoma by Monte Carlo ( MC ) dosimetry modeling. Methods A polycapillary x‐ray lens was used to focus 45 kV p x rays to achieve pinpoint accuracy of dose delivery to small tumors near critical structures. In addition to allowing for beam focusing, the use of kV x rays takes advantage of the strong photoelectric absorption of metallic NP s in that energy regime and hence strong radiosensitization. We constructed an MC simulation program that takes into account the x‐ray optic modeling and used GEANT 4 for dosimetric calculation. Extensive phantom measurements using a prototype‐focused x‐ray system were carried out. The MC simulation of simple geometry phantom irradiation was first compared to measurements to verify the x‐ray optic lens modeling in conjunction with the Geant4 dosimetric calculation. To simulate tumor treatment, a geometric eye model and two tumor models were constructed. Dose distributions of the simulated treatments were then calculated. NP radiosensitization was also simulated for two concentrations of 2 nm gold NP (Au NP ) uniformly distributed in the tumor. Results The MC ‐simulated full width at half maximum ( FWHM ) and central‐axis depth dose of the focused kV x‐ray beam match those measured on EBT 3 films within ~10% around the depth of focus of the beam. Dose distributions of the simulated ocular tumor treatments show that focused x‐ray beams can concentrate the high‐dose region in or close to the tumor plus margin. For the simulated posterior choroidal tumor treatment, with sufficient tumor coverage, the doses to the optic disc and fovea are substantially reduced with focused x‐ray therapy compared to eye plaque treatment (3.8 vs 39.8 Gy and 11.1 vs 53.8 Gy, respectively). The eye plaque treatment was calculated using an Eye Physics plaque with I‐125 seeds under TG 43 assumption. For the energy spectrum used in this study, the average simulated dose enhancement ratios ( DER s) are roughly 2.1 and 1.1 for 1.0% and 0.1% Au NP mass concentration in the tumor, respectively. Conclusion Compared to eye plaque brachytherapy, the proposed focused kV x‐ray technique is noninvasive and shows great advantage in sparing healthy critical organs without sacrificing the tumor control. The NP radiation dose enhancement is considerable at our proposed kV range even with a low NP concentration in the tumor, providing better critical structure protection and more flexibility for treatment planning.