AbstractID: 8756 Title: Dosimetry Of In-Situ Activated Dysprosium Microspheres: A
Monte-Carlo Study.
The dose distribution and average dose delivered to a given lesion resulting from the activation of stable Dysprosium (Dy)
microspheres by an 18 MV photon beam is investigated by Monte-Carlo computer simulation. The simulation allows the Dy
microspheres to diffuse randomly within the lesion with equal probability of occupying any given position. Different depths,
lesion volumes and volume concentrations of microspheres are studied. The linac lower collimator jaws are assumed completely
closed to shield the target volume from the photon dose. Using a single AP field with 0 x 0 cm2 field size (closed jaws), a photon
dose rate of 600 MU/min and 80 cm SSD for 10 minutes, an average dose exceeding 1 Gy can be delivered to spherical lesions
of 0.5 cm and higher diameter. The center of each lesion is assumed at 5 cm depth in water and the corresponding volume
concentration set to 30%. Due to the limited range of the beta particle emitted by Dy, the variation of the average dose with
lesion size exhibits saturation for volumes larger than 1 cm in diameter. Also, because of the thermal flux attenuation, the same
irradiation conditions yield a 50% reduction in the average dose when the center of the lesion seats at 10 cm depth. In
comparison to commercially available products based on radioactive Yttrium-90, the use of non-radioactive stable Dy
microspheres has the potential to greatly reduce toxicity to organs such as the liver and the spleen.
Supported in part by a grant from the Alberta Heritage Foundation for Medical Research