BEDVH-A method for evaluating biologically effective dose volume histograms: application to eye plaque brachytherapy implants.

Purpose: A method is introduced to examine the influence of implant duration T, radionuclide, and radiobiological parameters on the biologically effective dose (BED) throughout the entire volume of regions of interest for episcleral brachytherapy using available radionuclides. This method is employed to evaluate a particular eye plaque brachytherapy implant in a radiobiological context.

Methods: A reference eye geometry and 16 mm COMS eye plaque loaded with (103)Pd, (125)I, or (131)Cs sources were examined with dose distributions accounting for plaque heterogeneities. For a standardized 7 day implant, doses to 90% of the tumor volume ( (TUMOR)D(90)) and 10% of the organ at risk volumes ( (OAR)D(10)) were calculated. The BED equation from Dale and Jones and published α/β and μ parameters were incorporated with dose volume histograms (DVHs) for various T values such as T = 7 days (i.e.,  (TUMOR) (7)BED(10) and  (OAR) (7)BED(10)). By calculating BED throughout the volumes, biologically effective dose volume histograms (BEDVHs) were developed for tumor and OARs. Influence of T, radionuclide choice, and radiobiological parameters on  (TUMOR)BEDVH and  (OAR)BEDVH were examined. The nominal dose was scaled for shorter implants to achieve biological equivalence.

Results:  (TUMOR)D(90) values were 102, 112, and 110 Gy for (103)Pd, (125)I, and (131)Cs, respectively. Corresponding  (TUMOR) (7)BED(10) values were 124, 140, and 138 Gy, respectively. As T decreased from 7 to 0.01 days, the isobiologically effective prescription dose decreased by a factor of three. As expected,  (TUMOR) (7)BEDVH did not significantly change as a function of radionuclide half-life but varied by 10% due to radionuclide dose distribution. Variations in reported radiobiological parameters caused  (TUMOR) (7)BED(10) to deviate by up to 46%. Over the range of (OAR)α/β values,  (OAR) (7)BED(10) varied by up to 41%, 3.1%, and 1.4% for the lens, optic nerve, and lacrimal gland, respectively.

Conclusions: BEDVH permits evaluation of the relative biological effectiveness for brachytherapy implants. For eye plaques,  (TUMOR)BEDVH and  (OAR)BEDVH were sensitive to implant duration, which may be manipulated to affect outcomes.