A comprehensive dosimetric comparison between ¹³¹Cs and ¹²⁵I brachytherapy sources for COMS eye plaque implant

Purpose: To verify the dosimetric characteristics of ¹³¹Cs source in the Collaborative Ocular Melanoma Study (COMS) eye plaque brachytherapy, to compare ¹³¹Cs with ¹²⁵I in a sample implant, and to examine the accuracy of treatment planning system in dose calculation. Methods and Materials: Monte Carlo (MC) technique was used to generate three-dimensional dose distributions of a 16-mm COMS eye plaque loaded with ¹³¹Cs and ¹²⁵I brachytherapy sources separately. A spherical eyeball, 24.6mm in diameter, and an ellipsoidal tumor, 6mm in height and 12mm in diameter, were used to evaluate the doses delivered. The simulations were carried out both with and without the gold and gold alloy plaque. A water-equivalent seed carrier was used instead of the silastic insert designed for the traditional COMS eye plaque. The 13 sources involved were also individually simulated to evaluate the intersource effect. In addition, a treatment planning system was used to calculate the doses at the central axis for comparison with MC data. Results: The gold plaque had significantly reduced the dose in the tumor volume; at the prescription point of this study, that is, 6mm from the edge of inner sclera, the gold plaque reduced the dose by about 7% for both types of ¹³¹Cs and ¹²⁵I sources, but the gold alloy plaque reduced the dose only by 4% for both types of sources. The intersource effect reduced the dose by 2% for both types of sources. At the same prescription dose, the treatment with the gold plaque applicator tended to create more hot regions for either type of sources than were seen with the homogeneous water phantom. The doses of TPS agree with the MC. Conclusion: The ¹³¹Cs source is comparable to the ¹²⁵I source in the eye plaque brachytherapy. The TPS can provide accurate dose calculations for eye plaque implants with either type of sources.