Daghighian F, Humm J L, Macapinlac H A, Zhang J, Izzo J, Finn R, Kemeny N, Larson S M
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
J Nucl Med. 1996 Apr;37(4 Suppl):29S-32S.
The radiotoxicity of 125I is highly sensitive to the site of decay relative to nuclear DNA. This paper describes a new approach, based upon pharmacokinetic clearance of radioactivity from the tumor, with which to quantify the fraction of [125I]IUdR incorporated within the DNA of tumor cells.
Patients were injected with [125I]IUdR through the hepatic artery. Iodine-131-IUdR was used as a tracer for imaging and quantitation. Both conventional and DNA-level dosimetry were performed.
We calculated that if 15% of the tumor cells were in S phase at the time of injection, there would be 250 decays of 125I in the DNA per tumor cell after an infusion of 5 mCi [125I]IUdR. According to in vitro data based on 5 x 10(8) cells per g tumor, 99% of these cells in S phase would be killed.
The estimate of cell inactivation is strongly dependent on the number of cells per gram and the fraction of cells in S phase at the time of injection, which indicates that repeat injections would be necessary to achieve a therapeutic effect.
