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SU-E-T-459: Radiobiological Evaluation of Implant Duration and Radionuclide Selection for COMS Eye Plaque Brachytherapy Using an Objective Function.

PURPOSE
The biologically effective dose (BED) of temporary brachytherapy treatments is a function of both chosen radionuclide (R) and implant duration (T). This study endeavored to evaluate BED delivered to the tumor volume and surrounding ocular structures as a function of plaque position (P), prescription dose, R, and T.


METHODS
Plaque-heterogeneity- corrected dose distributions were generated with MCNP5 for the range of currently-available COMS plaques using low-energy radionuclides. These physical dose distributions were imported into the Pinnacle3 TPS using the TG-43 hybrid technique and used to generate DVHs for a T=7d implant within a reference eye geometry at eight standard treatment positions. The Dale equation was employed to create biologically effective dose volume histograms (BEDVHs), allowing for BED volumetric analysis of all ROIs. Isobiologically-effective prescription doses were calculated for T=5-0.01d, with BEDVHs subsequently generated for all ROIs using correspondingly reduced prescription doses. Objective functions were created to evaluate the BEDVHs as a function of R and T.


RESULTS
Reducing T from 7 to 0.01d for a 10mm plaque produced an average BED benefit of 26%, 20%, and 17% for 103 Pd, 125 I, and 131 Cs, respectively, for all P; 16mm and 22mm plaque results were more position-dependent. 103 Pd produced a 16-35% BED benefit over 125 I, whereas 131 Cs produced a 3-7% BED detriment, independent of P, T, and plaque size. Additionally, corresponding OAR physical doses were lowest using 103 Pd in all circumstances.


CONCLUSIONS
Shorter implant durations may correlate with more favorable outcomes vs. 7d implants for small and medium lesions. T may be safely reduced if the prescription dose is appropriately diminished. 103 Pd offers a substantial 16- 35% radiobiological benefit over 125 I and 131 Cs irrespective of P, T, and tumor size. The objective functions used in this study can be applied to temporary or permanent brachytherapy implants for a variety of disease sites.