Biodistribution and Dosimetry of an Aqueous Solution Containing Sodium 3-( 125 I)iodo-4-hydroxybenzenesulfonate (IotrexTM) for Brachytherapy of Resected Malignant Brain Tumors

IotrexTM is an aqueous radiotherapy solution containing sodium 3-(125I)iodo-4-hydroxybenzenesulfonate (l25I-HBS), which is used as the radiation source for the brachytherapy of resected of brain tumor cavity margins with the GliaSite® catheter. Duringroutine clinical use of this brachytherapy applicator and radiation source, ∼0.1% of the afterloaded Iotrex will diffuse through the GliaSite balloon. Our purpose was to assess the radiation doses to normal organs under routine clinical use of the GliaSite.

Methods

Five groups of rats receivedintracerebral injections of a¹³¹I-HBS solution (¹³¹ I used as a surrogate for ¹²⁵I in the synthesis of¹²⁵I-HBS) with one group sacrificed at 15 minutes, 30 minutes, 1 hour, 2 hours and 4 hours post-administration. Urine was collected and activity retention in numerous organs was measured. The biodistribution data were used to estimate radiation doses to normal organs of the Reference Adult Male and Female phantoms.

Results

Radioactivity was rapidly and completely cleared from the brain (98% cleared by 2 hours) and total body (urinary clearance; 93% @ 2 hours). No organ retained >0.7% of the radioactivity at 4 hours. For 100% loss of the radiotherapy solution from the balloon catheter (device failure), all organs would receive less than 100 mGy (10 rad), except the bladder wall (2800 mGy, 280 rad), uterus (130 mGy, 13 rad) and distal colon (270 mGy, 27 rad). Under normal conditions, all organ doses are 1000-fold lower (<3 mGy or 0.3 rad).

Conclusions

Under routine clinical conditions, the radiation doses to normal organs are inconsequential. Should the maximum clinical load of Iotrex (16.7 GBq of¹²⁵I) be released intracerebrally, the radiation doses to all organs would be below the thresholds for deterministic effects.