The response of prototype plane‐parallel ionization chambers in small megavoltage x‐ray fields

Abstract

Accurate small-field dosimetry has become important with the use of multiple small fields in modern radiotherapy treatments such as IMRT and stereotactic radiosurgery. In this study, we investigate the response of a set of prototype plane-parallel ionization chambers, based upon the Exradin T11 chamber, with active volume diameters of 2, 4, 10, and 20 mm, exposed to 6 MV stereotactic radiotherapy x-ray fields. Our goal was to assess their usefulness for accurate small x-ray field dose measurements. The relative ionization response was measured in circular fields (0.5 to 4 cm diameter) as compared to a 10 x 10 cm2 reference field. A large discrepancy (approximately 40%) was found between the relative response in the smallest plane-parallel chamber and other small volume dosimeters (radiochromic film, micro-metal-oxide-semiconductor field-effect transistor and diode) used for comparison. Monte Carlo BEAMnrc simulations were used to simulate the experimental setup in order to investigate the cause of the under-response and to calculate appropriate correction factors that could be applied to experimental measurements. It was found that in small fields, the air cavity of these custom-made research chambers perturbed the secondary electron fluence profile significantly, resulting in decreased fluence within the active volume, which in turn produces a chamber under-response. It is demonstrated that a large correction to the p(fl) correction factor would be required to improve dosimetric accuracy in small fields, and that these factors could be derived using Monte Carlo simulations.

Glass beads and Ge-doped optical fibres as thermoluminescence dosimeters for small field photon dosimetry

“…The largest observed discrepancy is for the optical fibre in a 1 × 1 cm 2 field, which could be due to the volume averaging effect, as the fibres were cut to a length of 5 mm. It is indicated that at 1 × 1 cm 2 and for smaller field sizes there will be a large variation in detector response and these results fit within the range of values for various available radiation detectors reported in the literature (McNiven et al 2006, Das et al 2008, Francescon et al 2011a, 2011b, PTW 2011. For example a discrepancy of 15% between the results from a microLion and Semiflex 0.125 at 1 × 1 cm 2 field is reported in the PTW application guide for small field dosimetry.…”

Quality in radiation oncology

“…For example, in the November 2006 issue of Medical Physics, there are seven papers devoted to this type of research. [38][39][40][41][42][43][44] There are no papers in that issue devoted to modern industrial approaches to quality. Research sessions at the annual American Association of Physicists in Medicine ͑AAPM͒ and American Society for Therapeutic Radiology and Oncology ͑ASTRO͒ meetings that are specific to techniques of quality and error reduction need to be instituted.…”

Measurement of superficial dose from a static tomotherapy beam

“…A separate calibration factor was determined for each SDD, angle of incidence, and field size, since parallel-plate chamber overresponse and detector volume effects are dependent on those measurement conditions. 18,19 Calibration factors were obtained by cross calibrating the Attix parallel-plate chamber with a calibrated Exradin cylindrical chamber ͑model A1SL, Standard Imaging, Middleton, WI͒ on the TomoTherapy Hi-FIG. 1.…”