Session Item

Ocular melanomas currently are treated using brachytherapy plaques containing I-125 or Pd-103 seeds. Because the geometry of these seeds are fixed, it is difficult to achieve an optimal dose distribution, which contributes to a significant number of radiation associated ocular complications. This treatment also requires two minor surgeries- the first to suture the plaque onto the sclera and the second to remove the plaque after 5-12 days. Because of this, the surgeon receives a significant dose to the hands of about 2-6 mSv per procedure.

We propose an alternative approach for treating ocular melanomas by using middle energy HDR brachytherapy sources such as Yb-169 and Se-75, along with a gold shielded applicator that is designed to permit the use of an afterloader device with a treatment time of 10 minutes. Because this device uses middle energy photons, it is possible to modulate the dose distributions to optimize dose to the tumor while minimizing the absorbed dose to healthy tissues. In addition, using an afterloader will eliminate dose to the surgeon's hands during applicator placement.

The proposed ring source is an assembly of discrete sources that are delivered together into an applicator that forms a circular ring configuration. This array of sources is contained in a spring-like structure, which maintains containment and provides flexibility. The gold applicator has a conical collimator opening, tangent to the outside of the source tube, which will act to alter and focus the dose distribution. Figure 1 shows the design of the applicator and the collimation. The prescribed dose rate for HDR eye plaque brachytherapy is 3 Gy/min for 10 minutes at a depth of 5 mm. Using MCNP6, we simulated the ring source containing Yb-169 and Se-75 with different diameter applicators and varying collimator angles to reach various depths within the tumor.

The MCNP6 simulation results show that a Yb-169 or Se-75 HDR source array placed in the proposed gold applicator can reduce the absorbed dose to healthy tissue while simultaneously delivering the prescription dose to the target. The dose to critical structures in the eye is about 30% higher when using Se-75 compared to Yb-169 however it is still within acceptable limits. Figure 2 shows the dose distributions of a 12 and 16 mm applicator delivering the prescription dose to a 5 mm depth in the eye for Yb-169 and Se-75 compared to an I-125 COMS plaque. Simulations were also performed using different collimators to deliver the dose to 3.5-8.0 mm depths in the eye to cover tumors 10-15 mm in diameter.  

A novel eye plaque design is proposed using middle energy HDR brachytherapy sources with gold shielding. This source assembly is able to deliver a dose of 3 Gy/min to a depth of 5 mm in the eye in 10 minutes. More conformal dose distributions can be achieved from the conical opening and gold shielding. This design also permits the use of an automated afterloader, which eliminates dose to the surgeon’s hands.