The BLOKX was created from a pre-existing Siemens Orthopantomograph (Model #0P 10 A) unit, reconstructed with the mechanical ability to move in three-dimensional space about an isocentric point. Figure shows a picture of the BLOKX device. The system operates in the 60–90 kVp range with tube currents between 5 to 12 mA, and beam quality may be modified by 2.8 mm aluminum filtration. Through careful geometric planning, a desirable position may be chosen to deliver maximum dose to the tumor, while limiting exposure to other critical non-target ocular structures. The BLOKX unit's output was measured with a Radcal control unit (Model Number 9010) with an ionization chamber/electrometer (Model Number 9060) for different kVp and mA settings. Reproducibility, accuracy, and the half-value layer were measured for each kVp and mA setting available on the unit. After evaluation, the BLOKX was operated at 75 kVp, 12 mA, and a half-value layer of 2.8 mm of aluminum added filtration, chosen as the settings that would provide an effective energy similar to that of I-125.
Figure 1 Diagrammatic representation of BLOKX rotational technique (left); Siemens Orthopantomograph x-ray unit model #0P 10 A reconstructed with the mechanical ability to move in three-dimensional space about an isocentric point (right). The source to axis distance (more ...)
Monte Carlo code & simulation
For this study, the Monte Carlo N-Particle (MCNP) transport code version 5.0[15
] was used (X-5 Monte Carlo Team, Los Alamos, NM). The code was obtained from the Oak Ridge National Laboratory through the Radiation Safety Information Computational Center[15
]. The code has generalized 3-D geometry capabilities using first- and second-degree surfaces and forth-degree elliptical tori, with extensive cross section libraries. MCNP5 has two models for photon interaction: simple and detailed. The detailed physics treatment includes coherent scattering and fluorescent photons after photoelectric absorption, while the simple treatment ignores both. The detailed model is always used as the default model for photons with energies less than 100 MeV.
The MCNP5 program was run on a Dell model DHM GX260 Optiplex Intel ®
Pentium 4 personal computer system running at 2.4 GHz, with a 40 GB hard drive, and supported by Windows PC with a Lahey compiler. Visual Editor[16
] (Visual Editor Consultants, Richland, WA) and Sabrina[15
] (White Rock Science, Los Alamos, NM) software were used to image the setups. Several MCNP input files were written for the aforementioned treatment system and tumor locations in the eye. The input data included the dimensions and location of the tumor, composition and location of the eye structures and the source/plaque description. Dose rates were determined for critical intraocular structures such as the lens, macula, optic disc, base and apex of the tumor. Each MCNP file was benchmarked by COMS-ROCS treatment planning calculations or measurements.
Five MCNP input files were written to model the right eye, with a tumor placed at five different positions with respect to the macula (Figure ). The modeled eye had inner and outer diameters of 11 and 12 mm, respectively. The parts of the eye that were modeled included the sclera, macula, optic nerve, lens, cornea, aqueous humor and vitreous humor. The whole eye was considered to be made of water equivalent tissue. A dome-shaped tumor with a diameter and apical height of 8 mm and 6 mm, respectively, was modeled.
Spatial location of intra-ocular tumor models, showing 90°(A), 60°(B), 45°(C), 30°(D) and 270°(E) positions.
The selected model tumor was a medium-size dome shaped lesion representative of a stereotypical choroidal melanoma. The basal diameter and apical height of this tumor were 8 mm and 6 mm, respectively. Five stereotypical tumor locations within the eye were used for the project: 30, 45, 60, 90 and 270 degrees from the macula. Doses to critical structures were calculated for the sclera at the base of the tumor, the prescription point at the apex of the tumor, the macula, the optic nerve, and the center of the lens. Critical structures and the center of the dome-shaped tumor were all located in the same axial plane. For this study, all the calculation points for the tumor and critical structures were localized in the X-Y plane, with the inner sclera surface at the center of the tumor base is the origin of a Cartesian (x, y, z) coordinate system, (0, 0, 0) mm. The coordinates for tumor apex were (6, 0, 0) mm. The coordinates for the macula, optic nerve and the lens varied consonant with tumor location. In this study, we used a dome-shaped tumor with a circular base, so that the tumor base dimensions in both the macula direction and the optic disc direction were equivalent in each tumor position. The MCNP *f8 tally was used to determine the energy deposited in five spherical tally cells with 0.5 mm radius. These tally cells were placed at the tumor base, tumor apex, lens, macula, and at the center of the optic disc.
A. BLOKX model parameters
The BLOKX was operated at 75 kVp, 12 mA, and a half-value layer of 2.8 mm of aluminum added filtration. These parameters were utilized to characterize comparable inputs for Monte Carlo simulations. A single fixed field size, 9 × 9 mm2, was chosen to encompass the whole tumor with a 1 mm margin of error. Figure shows the energy spectrum of the x-ray beam modeled in the MCNP setup.
Experimentally derived energy spectrum of BLOKX utilized for Monte Carlo calculations.
Five MCNP input files were written to model absorbed dose measurements made in a right eye phantom irradiated by BLOKX. Each input file modeled a one of the five tumor locations. Two beam directions were investigated in MCNP for the tumor located at 45 degrees with respect to the macula, presented in Figure .
Axial view beam directions for tumor at 45° position; a) Original x-ray beam setup for irradiating the tumor based on TLD measurements, b) Revised x-ray beam direction used to spare direct irradiation of the macula.
The center of the tumor was simulated at isocenter. The x-ray source was modeled at 20 cm from isocenter. The setup is presented in Figure . In addition, an MCNP file was written to model the x-ray's percent depth dose in a plastic eye phantom for a superficial depth of up to 2 cm.
The MCNP *f8 energy deposition tally was used to score the dose rate in five spherical tally cells with a 0.5 mm radius. These five tally cells were located at the base of the tumor, the tumor's apex, at the center of the optic nerve, in the middle of the lens and at the fovea (or macula). Iterative simulation was permitted to run for approximately 24 hours for each setup to assure that the tally passed the requisite ten statistical checks.
B. Absorbed dose determination using episcleral plaque brachytherapy
A 12 mm gold-alloy plaque was modeled as being placed on the scleral surface of the eye adjacent to the tumor. The composition and dimensions of the plaque were determined according to COMS protocol. Each plaque contained eight radioactive I-125 seeds with the same apparent activity of 8 mCi. Each I-125 seed was treated like a point-source. The seeds arrangement is similar to those used in the COMS protocol. All five input files used the same effective energy of 0.0274 MeV and emission yield of 1.523 for the radiation source.
Five MCNP input files were written to model absorbed dose measurements made using a gold-manufactured eye-plaque and plastic eye phantom with the tumor placed at different locations in the eye. The files ran for 30 hours for each tumor location to assure that the tally passed all ten requisite statistical checks.