A miniature, battery operated 40 kV x-ray device has been developed for 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 that is stereotactically inserted into the tumor. The beam, characterized by half-value layer (HVL), spectrum analysis, and isodose contours, behaves essentially as a point isotropic source with an effective energy of 20 keV at a depth of 10 mm in water. The absolute output from the device was measured using a parallel plate ionization chamber, modified with a platinum aperture. The dose rate in water determined from these chamber measurements was found to be nominally 150 cGy/min at a distance of 10 mm for a beam current of 40 microA and voltage of 40 kV. The dose in water falls off approximately as the third power of the distance. To date, 14 patients have been treated with this device in a phase I clinical trial.

Several groups are developing ultra-miniature x-ray machines for clinical use in radiation therapy. Current systems are for interstitial radiosurgery and for intravascular insertion for irradiation to prevent re-stenosis. Typical generating voltages are low, in the 20 to 40 kV range. It is well established that the biological effectiveness of such low-energy photons is large compared with higher-energy gamma rays, because of the dominance of photoelectric absorption at low energies. We have used microdosimetric analyses to estimate RBEs for such devices, both at low doses and clinically relevant doses, relative to radiations from 60Co, 192Ir, 125I and 90Sr/90Y. The RBEs at clinically relevant doses and dose rates for these low-energy x-ray sources are considerably above unity, both relative to 60Co and to 192Ir photons, and also relative to 125I and 90Sr/90Y brachytherapy sources. As a function of depth, the overall effect of the change in dose and the change in beam spectrum results in beams whose biologically weighted dose (dose x RBE) decreases with depth somewhat more slowly than does the physical dose. The estimated clinically relevant RBEs are sufficiently large that they should be taken into account during the treatment design stage.

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The Photon Radiosurgery System is a miniature x-ray device developed for the treatment of small intracranial neoplasms. The x-rays are generated at the tip of a 10-cm-long, 3-mm-diam probe with a nearly isotropic distribution. Results from measurements of the two-dimensional dose distribution around the x-ray source are presented using two methods: (1) dose measurement with an ionization chamber and a water phantom system and (2) dose measurement with radiochromic film and a solid water phantom. The shape of the two angular dose distributions in the axial plane agree with each other to with approximately 10% and the dose at 10 mm from the source, orthogonal to the probe axis, was about 20% lower than at the same distance along the axis. The relative dose difference of 20% corresponds to a change in distance from the source of +/- 0.3 mm at 10 mm. It is shown that the anisotropy of radiation distribution in the axial plane can be improved to approximately 10% by adjusting the electron beam with a 12% reduction in the overall radiation output.

Because this device requires relatively few supporting resources, has sharp dosimetric properties, and seems to be safe, it may be useful as a clinical tool for interstitial stereotactic radiosurgery.

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A method of one-beam laser speckle interferometry was applied to the measurement of transient thermal strain fields in an aluminum plate heated at a small region. A ruby pulse laser minimized the effect of thermal air turbulence on speckle correlation. High quality isothetic fringe patterns were obtained. The result compares well with available existing data.

No abstract