Dose inhomogeneity in interstitial implants using 125I seeds

Radioactive 125 I seeds have been used increasingly in intraoperative permanent interstitial implants of certain tumors. Dosimetry calculations of dose distribution in the implants have been based on the assumption of isotropic radiation emission from the 125 I seeds. This paper presents experimental results which show that the radiation fluence around single 125 I seeds is highly anisotropic. Dosimetry calculations based on the measured radiation distribution pattern indicate unusual dosage inhomogeneity within the implant volume. The clinical significance of this effect and approaches to circumvent this potential problem are discussed.