Measured, calculated, and egs_brachy I-125 dose distributions in a gold plaque for retinoblastoma treatment

Abstract

Background: This study measured and calculated dose distributions around a unique gold plaque for whole-eye radiotherapy (to treat retinoblastoma). The applicator consists of a pericorneal ring attached to the four extraocular muscles and four legs, each loaded with I-125 seeds. They are inserted beneath the conjunctiva in-between each pair of muscles and attached anteriorly to the ring. The applicator was designed in such a way that the dose is directed toward the middle of the eye while sparing surrounding tissues.

Purpose: (I) To compare the measured and calculated data obtained by thermoluminescent dosimeters (TLDs) in a solid-water phantom, a Gafchromic film in a solid-water phantom, the treatment planning systems, and Monte Carlo simulations; (II) to use Monte Carlo simulations for the determination of the dose to the organs at risk by taking the gold shielding and the anisotropy into account.

Methods: The dose around the applicator was measured using TLDs and Gafchromic EBT2 film in eye-shaped solid-water phantoms. Dose calculations were performed with the TheraPlan Plus and BrachyVision planning system and Monte Carlo simulations with egs_brachy code. A computer-aided design drawing of the applicator was created and used to create the input file for the Monte Carlo simulations.

Results: Monte Carlo calculated dose to the optic nerve is 64.8% of the central dose in the eye, whereas the planned dose is 93.7%. The Monte Carlo lens dose varies from 72.0% to 86.1%, whereas the planned dose varies from 73.0% to 84.3%. Monte Carlo-calculated dose to the bony orbit is 11.3%, whereas the planned dose is as high as 54.7% compared to the dose in the center region of the eye.

Conclusions: The measured and Monte Carlo-simulated dose distributions matched well, whereas planned dose distributions showed discrepancies in some areas of the eye and outside of the eye due to their ignorance of the shielding effects of the plaque.

Keywords: I-125; dosimetry; plaque brachytherapy.