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Radiobiological Evaluation of Implant Duration and Radionuclide Selection for COMS Eye Plaque Brachytherapy Using An Objective Function

N Gagne

N Gagne*, M Rivard, Tufts Medical Center, Boston, MA

SU-E-T-459 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

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 ¹³Pd, ¹²5I, and ¹³¹Cs, respectively, for all P; 16mm and 22mm plaque results were more position-dependent. ¹³Pd produced a 16-35% BED benefit over ¹²5I, whereas ¹³¹Cs produced a 3-7% BED detriment, independent of P, T, and plaque size. Additionally, corresponding OAR physical doses were lowest using ¹³Pd 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. ¹³Pd offers a substantial 16-35% radiobiological benefit over ¹²5I and ¹³¹Cs 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.

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