SU-E-T-459: Radiobiological Evaluation of Implant Duration and Radionuclide Selection for COMS Eye Plaque...

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.

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.

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.

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.