A NEW MINIATURE X-RAY DEVICE FOR INTERSTITIAL RADIOSURGERY - DOSIMETRY

A miniature, battery operated 40 kV x-ray device has been developed fo r the interstitial treatment of small tumors (<3 cm diam) in humans. X rays are emitted from the tip of a 10 cm long, 3 mm diameter probe th at is stereotactically inserted into the tumor. The beam, characterize d by half-value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective ener gy of 20 keV at a depth of 10 mm in water. The absolute output from th e device was measured using a parallel plate ionization chamber, modif ied with a platinum aperture. The dose rate in water determined from t hese chamber measurements was found to be nominally 150 cGy/min at a d istance of 10 mm for a beam current of 40 mu A and voltage of 40 kV. T he dose in water falls off approximately as the third power of the dis tance. To date, 14 patients have been treated with this device in a ph ase I clinical trial. (C) 1996 American Association of Physicists in M edicine.