Preliminary Dosimetric Study of Proton Minibeam Radiation Therapy for the Treatment of Choroidal Melanoma
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Preliminary Dosimetric Study of Proton Minibeam Radiation Therapy for the Treatment of Choroidal Melanoma Myeongsoo Kim, Sang Soo Kim, Haksoo Kim, Sung Ho Moon and Young Kyung Lim∗ Department of Radiation Oncology, National Cancer Center, Goyang 10408, Korea
Ui-Jung Hwang Department of Radiation Oncology, Chungnam National University Hospital, Daejeon 35015, Korea
Sang Hyoun Choi Department of Radiation Oncology, Korea Cancer Center Hospital, Seoul 01812, Korea (Received 8 January 2020; revised 6 April 2020; accepted 16 April 2020) This study aimed to evaluate dosimetrically the feasibility of saving both the lens and the ciliary body in the treatment of a choroidal melanoma by using spatially fractionated proton minibeams. A multi-slit brass collimator was designed by using Monte Carlo simulation data and was fabricated with a 0.4-mm beam opening and a 1.1-mm center-to-center distance. A phantom was also fabricated for proton beam dosimetry, involving five polymethylmethacrylate (PMMA) plates of 2 mm and 8 mm thicknesses, respectively. The peak-to-valley dose ratio (PVDR) of the proton minibeams was used to validate the feasibility of lens and ciliary body sparing. The typical single-scattered proton beam for treating a patient with a choroidal melanoma was delivered to Gafchromic EBT3 films in the phantom after having passed through the multi-slit: the energy of proton beam was 60 MeV, and its depth dose profile was a full spread-out Bragg peak (SOBP). The average dose at mid-depth of the SOBP was approximately 3 CGE with ±3% of ripple. Dosimetric evaluation showed the PVDR depended on the air gap strongly. It varied from 3.4 to 1.1 in the air gap range from 2 mm to 62 mm. The PVDR values at PMMA depths of 2, 12, 22, and 32 mm were 2.0, 1.5, 1.1, and 1.06, respectively. This result shows that a spatially fractionated proton beam might save the lens and ciliary body seated at a shallow depth in the eye ball when a choroidal melanoma is treated using proton minibeam radiotherapy. Keywords: Choroidal melanoma, Proton therapy, Spatial fractionation, Proton minibeam DOI: 10.3938/jkps.77.447
I. INTRODUCTION A choroidal melanoma is the most common primary malignant intraocular tumor. Annually, 6 cases per million in the USA and 0.25 cases per million in Asia were recorded due to a visual handicap, loss of an eye or vision, and metastatic disease [1]. Proton beam has been used for the treatment of an intraocular tumor, such as choroidal melanoma. It can not only kill the tumor effectively, but save the fovea, optic nerves and optic chiasm due to the steep dose-fall-off behind the tumor, so the patient’s optic vision can be conserved [2]. Nevertheless, inevitable side effects may be caused in the anteriorly positioned normal organs, such as lens, iris, and ciliary body. Recently, spatially fractionated radiation therapy (SFRT) is attracting attention because of the potential to protect normal organs lying on the beam path. ∗ E-mail:
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pISSN:0374-4884/eISSN:1976-8524
In the early era of radiation therapy, low-e
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