Purpose: The COMS trial recommends prescribing a standardized dose of 85 Gy at a prescription point of 5.0 mm depth for tumors with an apex depth of less than 5.0 mm. This work investigates how the change in prescription point to the apex of the tumor affects tumor coverage and dose to ocular structures.
Methods: Nine anonymized patients with various eye plaque sizes and tumor apex depths were analyzed. Two treatment plans were created for each patient in BrachyVision using TG-43 dose formalism. The tumor was contoured on CT using dimensions defined by an ophthalmologist on a fundus image. One plan normalized prescription dose to the tumor apex and the other to the standardized prescription point. The calculated dose for each plan was compared for tumor coverage, and the resulting doses to the tumor and optic disc were analyzed.
Results: On average, the mean dose to the tumor when prescribing to the tumor apex was reduced by 42.57 Gy compared to the standardized treatment method. A minimum reduction of 17.98 Gy was observed for a tumor apex of 4.5 mm treated with an 18 mm plaque, and a maximum reduction of 231.50 Gy occurred for a tumor apex of 2.5 mm treated with a 10 mm plaque. The reduction in the mean dose to the optic disc ranged from 3.31 Gy to 28.2 Gy, with an average of 8.15 Gy.
Conclusion: Prescribing 85 Gy to the tumor apex rather than to a standardized prescription point provides full tumor coverage when considering dose as calculated by the TG-43 model. Normalizing to the tumor apex resulted in reduced dose to ocular structures, such as the optic disc, leading to potentially better treatment outcomes for radiation therapy of ocular melanomas. In future work, we will expand our patient selection and evaluate for heterogeneities.