AbstractID: 7281 Title: Development and dosimetry of an ⁹⁰Y plaque for
irradiation of sarcomas of the spine
We have initiated a clinical study of the treatment of the spine by IMRT or proton
radiation therapy and surgery, combined intraoperatively with radiation to the dura by
means of an 90Y foil in a semi-cylindrical polycarbonate plaque. The maximum beta
energy of 2.23 MeV allows irradiation of the dura with significant dose sparing to the
cord 2 mm below. MRI scans were used to determine the radius, arc width and length of
the plaque. An 89Y foil was cut to the dimensions of the plaque. The foil was irradiated in
a neutron flux for 1.25 hours. The calculated intrinsic activity at the irradiation time was
74.32 millicuries of 90Y. The plaque was calibrated and characterized with radiochromic
film. A calibration curve was determined by exposing film in an electron beam degraded
to an energy of 2.0 MeV. Films were then irradiated on the surface and along the axis of a
cylindrical spinal cord phantom. These films were used to ascertain the surface dose,
surface isodose distribution and %DD. The measured surface dose at the time of
treatment was 58.8 cGy/min. Surface profiles showed a dose of 50% at 1.3 mm and a
dose of 95% at 4.5 mm from the edge of the foil. The %DD measured at 2mm and 4mm
was 29% and 9% respectively. In addition the %DD data correlated well with a
mathematical model and published data from a 90Sr ophthalmic applicator.