2277 - Dose Rate Dependence of Rbe of I-125 in Eye Plaque Treatment

Purpose/Objective(s): I125 eye plaque is a common treatment technique for various sizes of uveal melanoma. In a major tertiary referral center and academic teaching hospital, the busy OR schedule may pose limitations on the availability of OR times for the ocular oncologist to perform both eye plaque implant and the removal within a 3-5 day time frame. Pending OR availability, it could results in a shortened treatment duration (3 days) vs a longer (5-7 day) treatment time. This may contribute to delayed scheduling of treatment for the patient with an active growing tumor. Since dose rate is inversely proportional to implant duration, the increase in dose rate may result in different tumor control and complication rates to different eye structures due to the highly inhomogeneous dose distribution in an eye plaque implant. This study aims to investigate the effects of change in implant duration on the RBE of tumor and the effectiveness of tumor cell destruction.

Materials/Methods: 14 eye plaque patients were treated to 85 Gy to the tumor apex with implant duration of 70 to 125 hrs. Tumor dimensions ranged from 3-17.3 mm in length (median 11 mm), width 2-16.8 (median 9.85 mm), and the apex height varied from 2.0-10.8 mm (median 3.2 mm). A base expansion of 2 mm was used for all cases except for one for which 0.5 mm expansion was used. The dose rate varied from 0.68 to 1.22 Gy/h (median 0.75 Gy/h). Number of seeds used varied from 7 to 40 seeds. In treatment planning with the Plaque Simulator, optimization is performed on an equatorial plane that contains all the eye structures. Once the plan is optimized, dose calculation is carried out for a given dose to the tumor apex for a given implant duration. We calculated the RBE of I-125 along the tumor axis of all patients. We also calculate the RBE of I-125 for three different implant durations, 168, 96 and 48 hours to investigate the dose rate change on the RBE. On the other hand, similar LQ modeling parameters for the different eye structures for I-125 brachytherapy are needed to allow determination of normal tissue complication probabilities (NTCP) for the various eye structures in eye plaque treatments.

Results: Table 1 is an example of the calculations of the RBE of I-125 along the tumor axis. 0 mm is the tumor base. RBE increases as the depth increases (dose rate decreases).

Conclusion: We have shown that the dose rate effect on the RBE of I-125 in eye plaque treatment. The lower dose rate may be accomplished by increasing the implant duration for the same tumor depth and seed activity.