The properties of the ultramicrocylindrical ionization chamber for small field used in stereotactic radiosurgery.

Accurate dosimetry of small-field photon beams tends to be difficult to perform due to the presence of lateral electronic disequilibrium and steep dose gradients. In stereotactic radiosurgery (SRS), small fields of 6-30 mm in diameter are used. Generally thermoluminescence dosimetry chips, Farmer, Thimble ion chamber, and film dosimetry are not adequate to measure dose in SRS beams. These techniques generally do not provide the required precision due to their energy dependence and/or poor resolution. It is necessary to construct a small, accurate detector with high spatial resolution for the small fields used in SRS. The ultramicrocylindrical ionization chamber (UCIC) with a gold wall of 2.2 mm in diameter and 4.0 mm in length has dual sensitive volumes of air (8.0 mm3) and borosilicate (2.6 mm3) cavity. Reproducibility, linearity, and radiation damage with respect to absorbed dose, beam profile of small beam, and independence of dose rate of the UCIC are tested by the dose measurements in high energy photon (5, 15 MV) and electron (9 MeV) beams. The UCIC with a unique supporting system in the polystyrene phantom is demonstrated to be a suitable detector for the dose measurements in a small beam size.

Measurement of radiation absorbed dose in endovascular Ho-166 brachytherapy using a balloon angio-catheter.

The purpose of this study was to estimate the absorbed dose distribution of Ho-166 endovascular beta irradiation using an angio-catheter. The liquid form of Ho-166 was produced at the Korea Atomic Energy Research Institute (KAERI) by an (n,gamma) reaction. Ho-166 has a half-life of 26.8 h and emits a high-energy beta particle with a maximum energy of 1.85 MeV. GafChromic film was used for the estimation of the absorbed dose of beta particles. A Co-60 teletherapy source and a 6 MV photon beam from a linear accelerator were used to generate dose-optical density calibration curves. The exposed films were read using a videodensitometer. With a modified micrometer, the film was positioned accurately on the surface of the balloon in water. The balloon was filled with Ho-166 solution to a pressure of 4 atm. Several film exposures were made with varying irradiation times and activities. The radiation absorbed dose rates were 1.02, 0.51 and 0.35 Gy x min(-1) x GBq(-1) x ml(-1) at the balloon surface, 0.5 and 1 mm from the balloon surface, respectively. The absorbed dose distribution revealed that Ho-166 is a good source for endovascular irradiation as the beta range is very short, avoiding unnecessary irradiation of normal tissue. A clinically applicable irradiation and duration of exposure were achievable utilizing our system.

Improvement of X-ray beam quality for treating cancer using double focus electric field strings.

Accurate knowledge of the distribution and amount of contamination electrons arising from the gantry head at the surface and in the first few centimeters of tissue is essential for the clinical practice of radiation oncology. These electrons tend to increase the surface dose and deteriorate the buildup in the radiation field compared with a pure photon field. In this study, the relative quantity and reduction of contamination electrons in a therapeutic radiation photon beam (15 MV) was investigated. The contamination electrons can be separated out by a special device. This device, consisting of a double-focus electric field (8 x 10(5) V/m) made by a large number of strings 2 x 10(-4) m in diameter, removes contamination electrons and positrons without affecting the photon beam. It is located under the tray holder. In clinical practice, the device can decrease the relative surface charge and relative surface dose due to contamination electrons in the photon beam used in radiation therapy.