Radioactive seed implant brachytherapy is a common radiotherapy treatment method for prostate cancer. In current clinical practice, a seed consists of a single isotope, such as ¹²⁵I or ¹⁰³Pd. A seed containing a mixture of two isotopes has been proposed for prostate cancer treatment. This study investigates a method for defining a prescription dose for new seed compositions based on matching the biological equivalent dose (BED) of a reference plan.

Ten prostate cancer cases previously treated using single isotope seeds (5 using ¹²⁵I seeds and 5 using ¹⁰³Pd seeds) were selected for this study. Verification of the method was done by calculating prescription doses for ¹⁰³Pd and ¹²⁵I seeds. A prescription dose for a 50/50 hybrid seed was calculated. Number and location of seeds remained invariant within each case. The BED distributions for hybrid and single isotope seed plans were generated and matched to the BED distribution generated off of the optimized plans.

For the ¹²⁵I isotopes, the dose necessary to cover 90% of the prostate with a BED of 110 Gy is 145 Gy. For the same BED coverage, the dose for ¹⁰³Pd and 50/50 hybrid seed is 120 Gy and 137 Gy respectively.

A method is introduced for obtaining prescription doses for new brachytherapy sources. The method was verified by obtaining doses for ¹²⁵I and ¹⁰³Pd isotopes which match clinical prescription doses. The method developed is robust enough to calculate prescription doses in any region of interest, for any seed type, and for any isotope as long as the BED coverage remains invariant with respect to the treatment plan.

Numerical calculations were performed to derive analytical conversions of total dose to BED for 50/50, 75/25 and 25/75 hybrid seeds. These analytical conversions are faster than the original numerical methods employed allowing for real-time BED optimization for hybrid seeds.

Varying seed distribution was seen not to influence the analytical conversions. It was observed that when total dose remained invariant while individual isotope contributions varied, the value of BED varied. The BED variance was seen to be the smaller at larger BED values (∼2% at 100 Gy).

Using the conversions derived in this paper, BED based optimization for hybrid seeds are now performable. However, these conversions should only be used in high dose regions due to high uncertainty in the low regime.