Dose characteristics of Au-198 eye brachytherapy applicator: A Monte Carlo study

Highlights

• Ocular melanoma brachytherapy using Au-198 plaque.

• Calculation of Au-198 and CCA plaque dose distributions.

• Simulation of CCA and Au-198 plaque using the Monte Carlo method.

• Calculation of doses in the tumor and organs at risk when using ¹⁹⁸Au and CCA plaque.

Abstract

Purpose

The use of ocular plaques is a promising treatment option for eye melanoma brachytherapy. Although several studies have been done on various ocular plaques, little is known about the dose characterization of ¹⁹⁸Au plaque.

Materials and method

The full mathematical model of the eye phantom, tumor, ¹⁰⁶Ru/¹⁰⁶Rh CCA, and ¹⁹⁸Au plaque were simulated using the Monte Carlo MCNPX code. The dose distribution was measured in the plaque's central axis direction, and a dose profile was also measured at a distance of 2.5 mm from the plaque surface.

Results

The findings showed that ¹⁹⁸Au plaque has superior dosimetric characteristics than CCA plaque for tumors with a thickness of greater than 3.5 mm, while CCA plaque is better for tumors with a thickness of less than 3.5 mm. The dose to the sclera and choroid is higher in the case of CCA plaque, while the dose to the organs at risk (lens and optic nerve) is greater in the case of ¹⁹⁸Au applicator. In the case of ¹⁹⁸Au plaque, however, the dose to sensitive organs was within their permissible dose range.

Conclusion

In the treatment of medium and large tumors, ¹⁹⁸Au plaque is more successful than CCA plaque. It can produce a much more homogeneous lateral dose profile in the target. In the treatment of dome-shaped tumors, ¹⁹⁸Au plaque may be more successful than CCA plaque. As a result, the tumor's shape influences the plaque type selection.

Introduction

Uveal melanoma (iris, choroid, and retina) is the most dangerous intraocular tumor in adults, occurring with a probability of 1 in every 100,000 people per year (Gerbaulet et al., 2002). The advanced stages of uveal melanoma are associated with decreased vision, pain, and even death (Gerbaulet et al., 2002; Murray and Boldt, 2014). Melanoma treatments include enucleation, proton therapy, laser therapy, teletherapy using gamma knife and linear accelerator, and brachytherapy. In the past, enucleation has been the main treatment for malignant choroidal melanoma (Ryan et al., 2005). The highly localized and homogeneous dose of proton therapy makes it suitable for the treatment of tumors close to sensitive structures such as the optic nerve (Murray and Boldt, 2014). Patients with uveal melanoma with a base diameter greater than 24 mm and a height greater than 14 mm are treated by proton therapy. Today, semiconductor lasers with a wavelength of 811 nm penetrate the choroid and part of the retina to treat intraocular tumors (Ryan et al., 2005). The results of long-term research on the treatment of uveal melanoma with a gamma knife show an acceptable level of local tumor control of about 90% (Jager et al., 2011). Linear accelerator 6 MV photon beams are also used to treat melanoma. Four to five fractions of 10–12 Gy are prescribed (Jager et al., 2011). Moore performed the first uveal melanoma brachytherapy using the Radon seed in 1930. Today, brachytherapy of ocular tumors using electron emitter radioisotopes such as ¹⁰⁶Ru is common for the treatment of small-sized tumors while photon emitter radioisotopes such as ¹²⁵I are used for the treatment of medium and large-sized tumors (Itzhak and Emanuel, 2012; Ryan et al., 2005). Barbosa et al. (2014) calculated the dose distribution of CCA and CCB ¹⁰⁶Ru ophthalmic applicator in the choroidal tumor and eye structures using the MCNPX Monte Carlo code. Two eye models, water and a heterogeneous model were considered, and the mean dose values of eye structures were compared. They suggested that the homogeneous water eye model is appropriate. In a study by Melhus and Rivard (2008) MCNP5 code was used for dosimetry of ¹²⁵I, ¹⁰³Pd and ¹³⁷Cs COMS (Collaborative Ocular Melanoma Study) eye plaques. After that, they looked at the impact of absorption on plaque materials. They showed that without taking into account plaque component attenuation, plaques can produce therapeutic doses that are 10%–20% lower than desired. Using the EGS5 Monte Carlo code, Itzhak and his colleague (Itzhak and Emanuel, 2012) conducted dosimetry on two CCA and CCB applicators. The depth dose along the applicator axis was then compared to Sanchez et al. (Sánchez-Reyes et al., 1998), who used the PENELOPE Monte Carlo code to measure the dose of the two applicators. Because of statistical uncertainty, the dose difference for the CCA applicator was less than 10% and due to the different electron transport of the EGS5 code, the dose difference was less than 49% in the case of the CCB applicator.

The ¹⁹⁸Au plaque has been used since 1992 at the British Columbia Cancer Agency (Karvat et al., 2001) and King Khaled Eye Specialist Hospital (Chaudhry et al., 2009) to treat melanoma. While there is a lot of information on the dose distribution of different ocular plaques, there is little data on the dose variation of ¹⁹⁸Au plaque. The high energy of the ¹⁹⁸Au gamma rays leads to a higher dose to the anterior structures of the eye and a lower dose to the sclera and retina compared to the ¹⁰⁶Ru and ¹²⁵I applicators (Karvat et al., 2001); so, this radionuclide can be used to treat large melanoma. Karvat et al. (2001) surveyed seventy-nine consecutive patients with primary choroidal melanoma treated with ¹⁹⁸Au plaque brachytherapy. They reported excellent survival and local control with minimal significant toxicity. They also resulted in comparable outcome to other kinds of ocular plaques like ¹²⁵I and ¹⁰⁶Ru. Chaudhry et al. (2009) evaluated CorneoScleral Necrosis (CSN) in 202 patients with uveal melanoma treated over a 22 years period with ¹⁹⁸Au brachytherapy. Only 15 patients developed CSN symptoms after 1 month to 5 years of care. Tumor thickness greater than 6 mm, ciliary body involvement, and intraocular pressure greater than 21 mmHg are all risk factors for the development of CSN and its treatment in patients with choroidal melanoma who received ¹⁹⁸Au brachytherapy. They indicated that eye globe retention was achieved in 88–92.3 percent of the treated eyes, according to the findings.

Dose calculation in tumor and eye's sensitive organs is particularly important in the treatment of uveal melanoma. For this purpose, the ¹⁹⁸Au plaque was simulated in this study using the MCNPX (Monte Carlo N-Particle eXtended) code, and the dose distribution was calculated and compared with the dose distribution of the CCA ¹⁰⁶Ru plaque simulated in this study as well as 12 mm COMS ¹²⁵I applicator (Melhus and Rivard, 2008) to determine the efficiency of the ¹⁹⁸Au plaque in the treatment of melanoma.