By: Vi Nhan Nguyen University of Arizona Evaluate and improve tumor dose calculation for eye plaques loaded with radioactive seeds By: Vi Nhan Nguyen University of Arizona

Outlines Background Project’s Objectives Procedures Calculate the dose distributions to the tumor and the eye. Procedures Matlab code to calculate the dose Verification: Monte Carlo simulation Radiochromic Film RESULTS?????????????

BACKGROUND Eye Plaque is used to treat tumors locate on the eye Melanoma is a deadly cancer Pain and Vision Loss Mortality is 31% at 5 year CURRENT TREATMENTS – 2 Options: Option 1 : Enucleation to prevent metastasis Option 2: Radiotherapy: brachytherapy using a radioactive plaque.

THE EYE PLAQUE Clinical Results show that: Irradiation using the Eye Plaques could Reduce the mortality rate to 18% at 5 years Saving vision Spare the eye Designed by COMS (Collaborative Ocular Melanoma Study) Different sizes 8mm-25mm Each eye plaque has between 8 and 24 seeds, depending upon the diameter of the plaque. The seeds are inserted into a silastic mold according to a defined pattern

Background The Treatment Plan: Who are the patients? Select by the opthalmologist Consults with the radiation oncologists Usually Patients with tumors from 2.5 mm to 10 mm in apical height and a basal diameter of 16 mm or less Tumor geometry is determined using ultrasound and then send to a medical physicists The Treatment Plan: 85 Gray -total dose The radioactive plaque will be left in place for 5 to 7 days (A prostate implant will use about 100 seeds which are left in permanently)

Eye Plaque Physics staff does most of the radiation work Sources Select the radiation sources Calculate the dose distributions to the tumor Sources Example iodine 125 (IAI-125A) IsoAid Advantage about 4.5 mm long 0.8 mm in diameter strength of each seed is about 5 U or 4 mCi (which is about 10 times stronger than used for Prostate Seed Implants). ruthenium 106

Background

Project’s Objectives To develop a program using matlab to calculate the dose to the tumor and to these Critical Structures. Sclera Retina Lens Macula and optic nerve

Dose Calculation Dose Calculation Line source Approximation Sum up all the dose distribution of each seed Assume that the eye is homogenous and ignore backscatters. AAPM TG 43 2-D formalism to find the DOSE RATE: AAPM TG 43 – Line Source approximation Sk – seed strength (given) ^ - dose rate constant (given) g(r) – radial dose function F -- anisotropy function G – geometry factor

Dose Calculation g(r) is the radial dose function, and accounts for radial dependence of photon absorption and scatter in the medium along the transverse axis with the geometric factor removed.

Dose Calculation F(r,) is the anisotropy factor normalized at θ = π/2 (transverse axis), with the geometric factor factored out. The anisotropy factor accounts for the angular dependence of photon absorption and scatter in the encapsulation and the medium.

Dose Calculation G(r,) is the geometry factor (cm-2) that accounts for the geometric falloff of the photon fluence with distance from the source and depends on the distribution of radioactive material. For a point source, G(r, θ) = 1/r2, and for uniformly distributed line source, G(r, θ) = (θ2 - θ1)/Lr

Methods Verification: Radiochromic film dosimetry Monte Carlo simulation MCNP5 Measurements: Along the plaque’s central axis Off axis direction at depths of 5 and 10 mm Homogenous and heterogeneity dose correction

Questions?