Radiation dosimetry models of intralumbar injection of 14 radiopharmaceuticals

Objectives For radiopharmaceuticals introduced into the CSF via the lumbar spine, It is important to derive radiation dosimetry to predict and correlate with potential toxicities and efficacies . This study uses common dosimetry models of CSF flow to compare radiation dosimetry of 14 radiopharmaceuticals.

Methods 14 radiopharmaceuticals from 12 different radionuclides (F-18, Ga-66, Ga-67, Ga-68, Y-90, In-111, Tl-201, I-123, I-124, I-125, I-131and Sm-153) were studied. Biokinetic models were constructed from transit times between the CSF compartments in convexity, upper and lower spinal cord. Monte Carlo simulations were carried out using MCNP4. Organ doses were from anthropomorphic models. Depth doses were constructed from 20-30 concentric layers from the CSF compartments outwards to 0.44, 3.0 and 4.3 cm for the CSF.

Results Y-90 chloride delivers the highest radiation absorbed dose to red marrow at 12 cGy/mCi, F-18 FDG delivers the lowest at 0.18 cGy/mCi. Up to 8 fold increase in the radiation dose is noted with stasis. Without CSF flow obstruction, the highest dose to the CSF is found with Y-90 radiopharmaceuticals. Except for Y-90, 10% isodose ranges are less than 0.3 cm in the spinal cord and all radiopharmaceuticals have 10% isodose range less than 0.3 cm in the convexity.

Conclusions It is possible to predict toxicity and efficacy after intrathecal applications of radionuclides. Depth dosimetry allows refined estimate of radiation to nerve roots bath by the CSF. Although patient-specific dosimetry needs verification, current models allow initial designs of intralumbar radionuclide therapies.