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 per form due to the presence of lateral electronic disequilibrium and steep dos e gradients. In stereotactic radiosurgery (SRS), small fields of 6-30 mm in diameter are used. Generally thermoluminescence dosimetry chips, Farmer, T himble 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 s mall fields used in SRS. The ultramicrocylindrical ionization chamber (UCIC ) with a gold wall of 2.3 mm in diameter and 4.0 mm in length has dual sens itive volumes of air (8.0 mm (3)) and borosilicate (2.6 mm(3)) cavity. Repr oducibility, linearity, and radiation damage with mm respect to absorbed do se, beam profile of small beam, and independence of dose rate of the UCIC a re tested by the dose measurements in high energy photon (5. 15 MV) and ele ctron (9 MeV) beams. The UCIC with a unique supporting system in the polyst yrene phantom is demonstrated to be a suitable detector for the dose measur ements in a small beam size. (C) 2001 American Association of Physicists in Medicine. [DOI: 10.1118/1.1350437].