GliaSite RTS Procedure / Dosimetry & Treatment Planning

Dosimetry

The dosimetric characteristics of the GliaSite Radiation Therapy System (RTS) have been researched and published by Jeffery F. Williamson, PhD and James F. Dempsey, PhD of the Mallinckrodt Institute of Radiology at the Washington University School of Medicine. Initially, a simple analytic dosimetric model based on results from Monte Carlo photon-transport (MCPT) simulations was used to determine the dosimetric characteristics of the spherical balloon geometry filled with a photon-emitting radionuclide solution (Dempsey, et al. Int J Radiat Oncol Biol Phys. 1998;42:421-429). Subsequently, absolute low dose-rate radiochromic film dosimetry measurements validated the accuracy of an extensive MCPT benchmark simulation (Monroe, et al. Med Phys. 2001;28:73-85).

The system is designed to provide a high spatial localization of the prescribed radiation dose and a favorable therapeutic ratio afforded by the continuous administration of radiation. The dosimetric strengths of the system—spherically uniform conformal therapy with no target-volume underdosing and no normal tissue hot spots—make the GliaSite RTS a valuable option for delivering internal radiation therapy.

The illustration shows the manner in which the radiation is delivered to the tissue surrounding a tumor resection site. The isodose profile demonstrates a rapid drop in radiation doses as distance from the surface of the balloon increases. A full course of radiation therapy is delivered to the tissue within 1 cm from the edge of the tumor resection cavity. Outside this 1 cm ring of tissue, the radiation dose rapidly diminishes such that dose to healthy brain tissue is minimized.

Treatment Planning

Based on the results of the MCPT simulations, Cytyc Surgical Products has developed a series of treatment planning tables in a reference manual for hospital use. Once the radiation oncologist has determined the desired radiation dose and treatment depth, the appropriate table in the treatment planning book is referenced, specifying the net activity of Iotrex^® Radiotherapy Solution required, as well as the dwell time and corresponding dose rate. A sample table is shown below.