A permanent breast seed implant as partial breast radiation therapy for early-stage patients: A comparison of palladium-103 and iodine-125 isotopes based on radiation safety considerations

Purpose: A permanent breast seed implant (PBSI) technique has been developed as a new form of partial adjuvant radiation therapy for early-stage breast cancer. This study compares iodine-125 (125I) and palladium-103 (103Pd) isotopes by examining the exposure and effective dose (ED) to a patient's partner.Methods and Materials: A low-energy survey meter was used to measure exposure rates as a function of bolus thickness placed over 103Pd or 125I seeds. A general mathematical expression for the initial exposure rate at 1 m (xo,1m) from the skin surface as a function of the implant size, R, and the distance between the skin surface and the implant, d, was derived. Also, a second general equation is proposed to calculate the ED to the patient's partner.Results: The initial exposure rate at 1 meter and the ED are calculated as follows: xo,1m = (3α)/2R3·β3 [e-β(2R+d)(βR + 1) + e-β·d(βR - 1)], and ED = aRb · [e-c(2R+d) · (cR + 1) + e-cd -bar (cR - 1)]. For 125I, the parameters are: α = 0.154409, β = 0.388460, a = 197, b = -0.95, and c = 0.38846. For 103Pd, they are: α = 0.06877, β = 0.421098, a = 18.6, b -0.78, and c = 0.421098. For implant diameters varying from 2 to 6 cm and skin-to-implant distances varying from 0.7 to 4 cm, the ED is consistently below 2.6 mSv using the 103Pd isotope, but more than 5 mSv in many instances and possibly up to 20 mSv using 125I.Conclusions: PBSI using 103Pd seeds appears safe because the patient's partner ED is consistently below 5 mSv. The125I isotope is not recommended for PBSI