1. 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

2. 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?????????????

3. 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.

4. 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

6. 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)

7. 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

8. Background

10. 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

11. 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

12. 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.

13. 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.

14. 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

15. 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

16. Questions?