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Optimization of Permanent Seed Implant Dosimetry Incorporating Tissue Heterogeneity

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S Mashouf

S Mashouf1*, E Lechtman1, A Ravi2, B Keller2, J Pignol1,3, (1)University of Toronto, Dept. of Medical Biophysics, Toronto, ON, (2) Sunnybrook Odette Cancer Centre, Dept. of Medical Physics, Toronto, ON, (3) Sunnybrook Odette Cancer Centre, Dept. of Radiation Oncology, Toronto, ON

SU-E-T-566 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Seed brachytherapy is currently used for adjuvant radiotherapy of early stage prostate and breast cancer patients. In this technique low dose rate (LDR) radioactive seeds are permanently implanted across the treatment volume to deliver a prescribed amount of dose. The current standard for calculation of dose surrounding the brachytherapy seeds is based on American Association of Physicist in Medicine Task Group No. 43 (TG43 formalism) which generates the dose in homogeneous water medium. For low energy sources and especially in breast, the absorbed dose is highly influenced by heterogeneities. Our work focuses on the development of a dose calculation algorithm similar to TG43 applicable in heterogeneous media.

Methods: An Inhomogeneity Correction Factor (ICF) is introduced as the ratio of absorbed dose in tissue to that in water medium. ICF is a function of tissue properties and independent of source structure. The absorbed dose in tissue can then be calculated by multiplying the dose as calculated by the TG43 formalism times ICF. We used Monte Carlo simulations in a heterogeneous model of breast to benchmark the improvements achieved in the calculation of dose using the ICF methodology.

Results: The dose distributions obtained through applying ICF to TG43 protocol agreed very well with those of Monte Carlo simulations. Over all and across the whole calculation domain, the mean relative error was reduced from 40.8% to 12.7% when ICF correction factor is applied to the TG43 protocol.

Conclusion: We have developed a new analytical dose calculation method which enables personalized dose calculations in heterogeneous media. The methodology offers several advantages including the use of standard TG43 formalism, fast calculation time and extraction of the ICF parameters directly from Hounsfield Units.

Funding Support, Disclosures, and Conflict of Interest: Funding: NSERC

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