A Dose To Bed Conversion Method For Real Time Bed Based Optimization In Brachytherapy Treatment Planning

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Cancer patients are often treated with multiple treatment modalities for the same lesion, external beam and brachytherapy combination as an example in prostate cancer radiotherapy. Due to dose rate differences, the biological effective dose (BED), instead of total dose, from the different modalities is used for treatment plan evaluation. In brachytherapy, BED based optimization would be preferred in planning. Currently, commercial treatment planning systems in brachytherapy do not provide dose to BED conversion. BED based optimization is thus not available. A fast dose - BED conversion method is introduced for real time BED based optimization in brachytherapy planning. Effective treatment time (Teff) is a function of initial dose rate which varies spatially. BED is a function of dose rate and Teff. A mathematical formula cannot be derived for dose - BED conversion. Numerical approach is used in the conversion, which takes tens of minutes to convert dose to BED for a plan. For plans of one isotope seeds, the dose - BED correspondence is calculated before hand for certain α and β values for cancer and normal tissues respectively. The correspondences are fitted with polynomial equations. During planning, these fitted correspondence equations are used to convert dose to BED. For hybrid seeds which contain multiple isotopes, average approximation from typical plans is applied in establishing the correspondence since dose - BED for hybrid seeds is not a one to one correspondence. Dose - BED conversion equations were obtained by fitting the numerically calculated BED values for prostate cancer, rectum, bladder and other normal tissues for I-125, Pd-103 and hybrid seeds that contain 50/50 I-125 and Pd-103 isotopes. About 50 minutes were needed to generate the BED data using the numerical approach for single isotope seeds. For hybrid seeds, it took about 10 hours. After the data were fitted and the correspondence equations were obtained, the conversion only takes less than a second, which makes the BED based optimization possible. The differences between the numerically derived BED values and the ones by the fitted equations for single isotope plans were within 1% in PTV within normal BED range and less than 1 Gy in normal tissues. The errors introduced by the average approximation and data fitting for hybrid seeds were less than 1% in a typical brachytherapy plan for prostate cancer, and 0.7 Gy in normal tissues. Using the equations obtained by fitting the BED data calculated by numerical approach is a fast and accurate method for dose - BED conversion, which makes BED based optimization possible in brachytherapy treatment planning.