Monte Carlo calculation of dose to spinal structures from intrathecally administered Yittrium-90

Abstract

The treatment of cerebrospinal fluid (CSF) malignancies by intrathecal administration of Iodine-131 radiolabeled monoclonal antibodies has been previously studied. Based on the assumption that more healthy tissue will be spared when a pure beta-emitter is employed, Yittrium-90 has been proposed as a preferable radioimmunoconjugate to I-131. It is believed that the shorter range of beta particles will offer better localization of the dose to metastases found within the subarachnoid space. However, such assumptions have not been accurately verified and validated. The purpose of this study is to quantitatively evaluate the dose distribution to the CSF space and its surrounding spinal structures following intrathecal administration of Y-90 based upon Monte Carlo modeling of the Y-90 source coupled with a 3-D digital phantom of a region of the spinal cord. A 3-D digital phantom of a section of the T-spine was constructed from the Visible Human Project series of images. The original color photographs of the female cadaver were used to produce an 8-bit gray-scale series of images. This series of images was further processed so that only certain anatomical features of interest were represented. These features of interest include the spinal cord, central canal, subarachnoid space, pia mater, arachnoid, dura mater, vertebral bone, and intervertebral disc. Monte Carlo N-Particle (MCNP) version 4C was used to model the Y-90 radiation distribution. The size of the cells modeled with MCNP was made to coincide with the voxel size of the 3-D phantom. A set of images of just the CSF compartment was convolved with the radiation distribution obtained from the MCNP calculations to determine the dose distribution within the subarachnoid space and surrounding tissues. A fast Fourier transform (FFT) was utilized to transform the two sets of data into frequency space where they were multiplied together. The inverse Fourier transform was performed on their product in frequency space to obtain the convolution. The cumulated activity of Y-90 in the CSF compartment was calculated based on previously-published I-131 pharmacokinetic biodistribution data to obtain the overall dose due to the presence of Y-90 in the CSF. Dose calculations show that a significant dose is delivered to the subarachnoid space compared to the surrounding structures including vital tissues such as the spinal cord and bone marrow. Specifically, the average doses to tissues in Gy per mCi of initial activity are calculated to be: subarachnoid space = 1.4; central canal = 0.9; spinal cord = 0.6; pia mater = 0.9; arachnoid = 0.6; dura mater = 0.3. The vertebral bone (including marrow) and disc receive <0.02 and <0.005 Gy/mCi respectively. Monte Carlo dose distribution calculations confirm that Y-90 delivers localized radiation dose, which is confined largely to the subarachnoid space as compared to the surrounding vital tissues. Based upon our dose distribution calculations, Y-90 is well suited for the treatment of CSF malignancies