CLINICAL RELATIVE BIOLOGICAL EFFECTIVENESS OF LOW-ENERGY X RAYS EMITTED BY MINIATURE X-RAY DEVICES

Clinical Relative Biological Effectiveness Of Low-Energy X Rays Emitted By Miniature X-Ray Devices

David J. Brenner and Cheng-Shiun Leu (with John F. Beatty, Radiation Oncology Department, Mass General Hospital, and Ruth E. Shefer, Newton Scientific Inc., Cambridge, Mass)

Several groups are developing ultra-miniature x-ray machines for clinical use in radiation therapy. One system, currently undergoing clinical trial for interstitial radiosurgery, transports electrons along a long thin target tube which can be inserted along the track left by a biopsy cannula, such that the x-ray production target is adjacent to or inside a brain tumor. A different approach involves miniaturizing the entire x-ray generating device, which can then be inserted intra-vascularly, with the aim of allowing local irradiation after coronary angioplasty, to prevent or delay restenosis. Both types of devices involve low x-ray generating voltages, both out of practicality and to optimize dose distributions over the target region. Typical generating voltages are 40 kV for the interstitial radiosurgery system, and 20 kV for the intra-vascular device.

It is, however, now well established that the relative biological effectiveness (RBE) of photons increases with decreasing photon energy. This observation is both well established experimentally, and well understood mechanistically. We have used microdosimetric analyses to estimate RBE's for such devices, both at low doses and clinically relevant doses, relative to radiations from ⁶⁰Co, ¹⁹²Ir, ¹²⁵I and ⁹⁰Sr/⁹⁰Y.

Both from experimental results, and the current calculations, it is clear that the RBE's at clinically relevant doses and dose rates for these low-energy x-ray sources are considerably greater than 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 ´ RBE) decreases with depth somewhat more slowly than does the physical dose.

The estimated clinically-relevant RBE's are sufficiently large that they should be taken into account during the treatment design stage. Assuming this to be the case, the increased RBE may be considered an advantage for these new miniature machines, in that it decreases the dose and thus the exposure time necessary to yield the same biological effect.

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