Dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation.
There is no simple way to know precisely the radiation dose delivered to tumors and normal liver when SIR-Spheres® Y-90 resin microspheres are implanted because yttrium-90 only emits pure beta radiation with limited penetration range in tissue. The deposited dose is highly dependent on the distribution of microspheres and this cannot be known with any great precision other than by microscopic examination of tissue after implantation has occurred. Such analyses do confirm that tumors receive a lethal dose of radiation while the average dose to normal tissue is well below harmful levels. This approach, however, is clearly not appropriate for treatment planning purposes.
In general, 1 GBq (27 mCi) of yttrium-90/kg of tissue provides the equivalent of 50 Gy of radiation dose. However, because of the non-uniform distribution of the dose between the tumor and the normal liver tissue, a proportionally larger amount of radiation will be delivered to the tumor tissue.
The method used in clinical practice to calculate the patient-specific activity with SIR-Spheres Y-90 resin microspheres is based on extensive clinical experience, both within and outside clinical studies. Please refer to the package insert or speak to your local Sirtex representative for further information on the appropriate method of dose calculation.