1. Introduction

The most common primary intraocular malignancy in adults is choroidal melanoma, accounting for 5% of all melanomas, originating within the pigmented cells of the choroid. More common in Caucasians, the Collaborative Ocular Melanoma Study (COMS) found the mean age at diagnosis to be 60 years. Generally, they are located posterior to the ciliary body and often found during ophthalmic examination [1, 2].

The plaque is temporarily sutured to the sclera overlying the tumour, left in place for 5–7 days and then surgically removed. Temporary placement of an episcleral plaque containing radioactive material has gained acceptance as a treatment for choroidal melanoma. Compared to enucleation, eye plaque brachytherapy provides better outcomes for uveal melanoma [3]. The choice of the treatment

1. Silastic insert with gold backing

DOI: http://dx.doi.org/10.5772/intechopen.88144

COMS eye plaque [17].

used, respectively.

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2.1 103-Pd source description

2. Materials and methods

2. Silastic insert with water replacing gold backing

Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy

4.Liquid water replacing Silastic and gold (seeds alone in water)

Finally, dose along the plaque's central axis and at the critical points are compared with 125-I (6711 model) and 103-Pd (Theragenics model 200) seeds loaded in

In this work, the Monte Carlo calculations have been performed to calculate the dose distribution around the eye plaque. The Monte Carlo method is a numerical technique using random numbers and probability to solve problems. It performs an effort to model nature through direct simulation for any possible results, by a probability distribution, for any cause that has inherent uncertainty. The name of this method comes from the casino name in Monaco, because of roulette, a simple random number generator. Clinical dose calculations are generally carried out with the patient treated as water equivalent and a dose of 85 Gy prescribed to the tumour apex. In the calculation the effects from the presence of the plaque backing, insert and intraocular media are considered [19]. In this work, the dose distributions were simulated using the MCNP5 Monte Carlo (MC) radiation transport code published by the Los Alamos National Laboratory, and the MCPLIB04 photon cross-section library is based on the ENDF/B-VI data [20]. The 103-Pd photon spectrum used in these simulations was obtained from TG-43U1 table XIII [18]. To calculate absorbed dose and kerma, the particle fluence and cell-heating tallies, F4 and F6, have been

The source used in this study is the palladium-103 source model IR06-103Pd seed which is designed and fabricated in Agricultural, Medical and Industrial Research School (AMIRS). The production of 103-Pd is carried out via the 103Rh(p,n)103Pd reaction which is well suited to low-energy cyclotrons. 103-Rh target was irradiated via cyclotron (IBA-Cyclone30, Belgium) at the AMIRS. The solid targetry system in this cyclotron is made up of a pure copper backing on which the target materials are electrodeposited. The target that undergoes bombardments by the proton beam at the cyclotron production consists of three layers as follows: (I) rhodium layer, (II) copper layer and (III) copper layer without induced proton beam [21]. All irradiation of the electroplated Rh targets were performed at 200 μA beam current. The rhodium targets were prepared by the electrodeposition technique. Thus, the elec-

trodeposition experiments were performed in acidic sulphate media using RhCl3.3H2O, Rh2(SO4)3. The radiochemical processing of the irradiation targets involved (a) dissolution, (b) radiochemical separation of 103-Pd from the Rh target solution and (c) recovery of 103-Pd as the final product from the organic phases [22]. After the chemical separation process, 103-Pd radioactive material is absorbed uniformly in the resin Amberlite IR-93 resin (20–50 mesh) bead to encapsulate

3. Liquid water replacing Silastic, with gold

method is depending on the size and location of the tumour [4, 5]. One of the most common primary intraocular malignancies is the choroidal. Patients with a medium-sized choroidal melanoma (between 2.5 and 10 mm in height and <16 mm basal diameter) are candidates for episcleral plaques [6]. It offers good chances of conserving the eye, often with at least some useful vision [7, 8]. Compared to charged particle radiation, the collimating effects of an eye plaque provides better conformality than possible with protons and essentially zero dose to the brain and orbit behind the plaque. Different types of eye plaque are used for the treatment of intraocular tumours, which are most often round, made of gold, silver, or stainless steel and come in several diameters depending on the tumour size. The Collaborative Ocular Melanoma Study group provided the first standardized methods for administering ocular brachytherapy treatments for uveal melanoma in 1985, by the eye plaques, and for the brachytherapy seeds, the dose was calculated using a point source approximation, and no corrections were made for source anisotropy, the plaque or insert materials of the plaques and also photon backscatter or fluorescence photons from the plaque backing. In the early 1990s, the begging of the investigation of the effect of the plaque backing and material insert (such as Silastic) on dose distribution and the recommended dosimetry protocol for eye plaques was issued by the Task Group (TG) 129 report. This report includes the correction factors/formula for heterogeneous plaque materials (backing and insert material) [9–12]. Common isotopes used in ocular brachytherapy are 125I, 103-Pd and 106-Ru. Iodine125 is currently the most commonly used and well documented in the literature. Some few centers use palladium-103, observing that the low gamma emission of 103-Pd, 20 keV, presents less radiation exposure hazard to personnel [13]. But due to the low energy of the photons from 103-Pd, the effect of backscatter from plaque backing on dose distribution is expected to be significant. Many reports are available concerning the effect of the gold plaque backing on dose rate [14, 15]. They reported a dose enhancement near the seed due to the backscatter photons from the gold backing. Chiu-Tsao et al. [16], Thomson et al. [17] and de la Zerda et al. [12] reported that dose at small distances from the seed was reduced due to the presence of gold backing. In this work, the Monte Carlo technique is used to study dose rate distributions around the COMS gold eye plaques having diameters from 10 to 22 mm and fully loaded with a palladium seed. The seeds were distributed into Silastic insert inside the 10–22 mm diameter COMS eye plaques. As reported by the American Association of Physicists in Medicine (AAPM), TG-43U1 recommendations, before using each new source, the dosimetric characteristics of the source need to be determined to provide reliable data for use in treatment planning calculations [18]. As stated by TG-43U1 guidelines, we have calculated the dosimetric parameters of the IR06-103Pd source. As the internal components of the seed are free to move within the titanium capsule, their location can vary with seed orientations. In our Monte Carlo calculations, three geometric models of the seed (ideal, vertical and diagonal) were also developed. Since the tumour control rate for plaque brachytherapy is high, the most important issue is the side effects in healthy structures (points of interest) in the eye region. Doses at points of interest were calculated for selected plaque positions on the surface of an eye. Further Monte Carlo simulations have been employed to investigate the effect of plaque gold backing and Silastic inserts on dose distributions along the central axis of the eye plaque and at critical points in the eye region. To investigate the effect of the plaque backing and Silastic insert on dose distributions, four different configurations of plaque were simulated for the IR06-palladium seed model, namely:

Modelling, Simulation and Dosimetry of 103-Pd Eye Plaque Brachytherapy DOI: http://dx.doi.org/10.5772/intechopen.88144

1. Silastic insert with gold backing

method is depending on the size and location of the tumour [4, 5]. One of the most

Theory, Application, and Implementation of Monte Carlo Method in Science and Technology

medium-sized choroidal melanoma (between 2.5 and 10 mm in height and <16 mm basal diameter) are candidates for episcleral plaques [6]. It offers good chances of conserving the eye, often with at least some useful vision [7, 8]. Compared to charged particle radiation, the collimating effects of an eye plaque provides better conformality than possible with protons and essentially zero dose to the brain and orbit behind the plaque. Different types of eye plaque are used for the treatment of intraocular tumours, which are most often round, made of gold, silver, or stainless steel and come in several diameters depending on the tumour size. The Collaborative Ocular Melanoma Study group provided the first standardized methods for administering ocular brachytherapy treatments for uveal melanoma in 1985, by the eye plaques, and for the brachytherapy seeds, the dose was calculated using a point source approximation, and no corrections were made for source anisotropy, the plaque or insert materials of the plaques and also photon backscatter or fluorescence photons from the plaque backing. In the early 1990s, the begging of the investigation of the effect of the plaque backing and material insert (such as Silastic) on dose distribution and the recommended dosimetry protocol for eye plaques was issued by the Task Group (TG) 129 report. This report includes the correction factors/formula for heterogeneous plaque materials (backing and insert material) [9–12]. Common isotopes used in ocular brachytherapy are 125I, 103-Pd

common primary intraocular malignancies is the choroidal. Patients with a

and 106-Ru. Iodine125 is currently the most commonly used and well

documented in the literature. Some few centers use palladium-103, observing that the low gamma emission of 103-Pd, 20 keV, presents less radiation exposure hazard to personnel [13]. But due to the low energy of the photons from 103-Pd, the effect of backscatter from plaque backing on dose distribution is expected to be significant. Many reports are available concerning the effect of the gold plaque backing on dose rate [14, 15]. They reported a dose enhancement near the seed due to the backscatter photons from the gold backing. Chiu-Tsao et al. [16], Thomson et al. [17] and de la Zerda et al. [12] reported that dose at small distances from the seed was reduced due to the presence of gold backing. In this work, the Monte Carlo technique is used to study dose rate distributions around the COMS gold eye plaques having diameters from 10 to 22 mm and fully loaded with a palladium seed. The seeds were distributed into Silastic insert inside the 10–22 mm diameter COMS eye plaques. As reported by the American Association of Physicists in Medicine (AAPM), TG-43U1 recommendations, before using each new source, the dosimetric characteristics of the source need to be determined to provide reliable data for use in treatment planning calculations [18]. As stated by TG-43U1 guidelines, we have calculated the dosimetric parameters of the IR06-103Pd source. As the internal components of the seed are free to move within the titanium capsule, their location can vary with seed orientations. In our Monte Carlo

calculations, three geometric models of the seed (ideal, vertical and diagonal) were also developed. Since the tumour control rate for plaque brachytherapy is high, the most important issue is the side effects in healthy structures (points of interest) in the eye region. Doses at points of interest were calculated for selected plaque positions on the surface of an eye. Further Monte Carlo simulations have been employed to investigate the effect of plaque gold backing and Silastic inserts on dose distributions along the central axis of the eye plaque and at critical points in the eye region. To investigate the effect of the plaque backing and Silastic insert on dose distributions, four different configurations of plaque were simulated for

the IR06-palladium seed model, namely:

20

2. Silastic insert with water replacing gold backing

3. Liquid water replacing Silastic, with gold

4.Liquid water replacing Silastic and gold (seeds alone in water)

Finally, dose along the plaque's central axis and at the critical points are compared with 125-I (6711 model) and 103-Pd (Theragenics model 200) seeds loaded in COMS eye plaque [17].
