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 (I-125) and palladium-103 (Pd-103) 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 Pd-103 or I-125 seeds. A general mathematical expression for the initial exposure rate at 1 m (x(0,1m)) 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 I meter and the ED are calculated as follows: 3 alpha/2R(3)center dot beta(3) [e(-beta(2R + d))(beta R + 1) + e(-beta.d)(beta R - 1)], and ED = aR(b) center dot [e (-c(2R+d)) center dot (cR + 1) + e(-cd) center dot (cR - 1)]. For I-125, the parameters are: alpha = 0.154409, beta = 0.388460, a 197, b = -0.95, and c = 0.38846. For Pd-103, 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 Pd-103 isotope, but more than 5 mSv in many instances and possibly up to 20 mSv using I-125. Conclusions: PBSI using Pd-103 seeds appears safe because the patient's partner ED is consistently below 5 mSv. The (125)i isotope is not recommended for PBSI. (c) 2005 Elsevier Inc.