Calibrations of electronic brachytherapy sources, traceable to national measurement standards: A number of miniature x-ray generators have been developed for interstitial and surface irradiation lesions, as a substitute for radionuclide-based brachytherapy sources. The advantages of such sources, which operate at energies up to about 50 kV, are that they can be turned off when not in use, the absorbed-dose distribution can be varied by varying the potential, and they require little in the way of shielding and radiation-protection measures so the physician/technologist can remain with the patient. As in traditional brachytherapy, the source is placed within or in contact with the treatment volume, so healthy tissue is largely spared harmful doses. These systems are finding expanding clinical use, including neurosurgery and prophylactic irradiation of the tumor bed following lumpectomy.
A new laboratory has been established to provide calibrations of these sources. The air-kerma rate will be directly realized through use of a primary-standard free-air chamber. A high-purity germanium (HPGe) x-ray spectrometer will be used to monitor the energy spectrum of the beam in real time, as well as provide necessary input data for Monte Carlo calculations of correction factors for the free-air chamber. To account for spatial anisotropy of emissions, the free-air chamber and spectrometer will be rotated about the long axis of the x-ray tube during measurements. The response of well-ionization chambers to the x-ray sources will be studied in preparation for the development of a measurement assurance program for the dissemination of the new NIST standard to Accredited Dosimetry Calibration Laboratories (ADCLs), which are accredited by the American Association of Physicists in Medicine (AAPM) and calibrate radiation-measuring instruments for use in therapy clinics.
Calibrations traceable to national measurement standards for these electronic brachytherapy sources will assure accuracy, consistency, and confidence in the dosimetry for this relatively new radiation-therapy modality, and support development of treatment protocols for improved outcomes as has been done with the traditional modalities.