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 ((125)I) and palladium-103 ((103)Pd) 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 (103)Pd or (125)I seeds. A general mathematical expression for the initial exposure rate at 1 m (x(o,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: x(o,1m) = 3alpha2R(3) ; ;beta(3) [e(-beta(2R+d))(betaR + 1) + e(-betad)(betaR - 1)], and ED = aR(b) [e(-c(2R+d)) (cR + 1) + e(-cd) (cR - 1)]. For (125)I, the parameters are: alpha = 0.154409, beta = 0.388460, a = 197, b = -0.95, and c = 0.38846. For (103)Pd, they are: alpha = 0.06877, beta = 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 (103)Pd isotope, but more than 5 mSv in many instances and possibly up to 20 mSv using (125)I.
Conclusions: PBSI using (103)Pd seeds appears safe because the patient's partner ED is consistently below 5 mSv. The(125)I isotope is not recommended for PBSI.