Choice of Radiation Detector in Dosimetry of Stereotactic Radiosurgery–Radiotherapy

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Dosimetry of the small radiation fields used in radiosurgery is often difficult because of finite detector size and loss of lateral electronic equilibrium. However, small-field dosimetry is critical in radiosurgery where a relatively high dose in a single fraction is often delivered. Radiosurgery dosimetry varies significantly in small fields depending upon the choice of detector. Small-volume (0.015 cm3–0.125 cm3) cylindrical ion chambers, parallel-plate ion chamber, films (Kodak and CEA), thermoluminescent dosimeter (TLD), diamond detector, and Monte Carlo simulation were used to study the small-field (≤ 4 cm) dosimetry of a radiosurgery unit, which consists of Radionics hardware with cone sizes of 12.5 mm to 40 mm with a 6-MV linear accelerator. Results indicate that, in general, ion chambers and TLDs are not suitable for beam profiles. The diamond detector provides better resolution when faced parallel, rather than the perpendicular, to the beam axis. The dose profiles measured with laser-film densitometer are found to be moderately sensitive to the aperture sizes (200 μm–470 μm). The profiles generated using film and diamond detectors are nearly identical. The cone factors measured using film, diamond detectors, and TLD varies (+ 5%) depending upon the uncertainty of the measurements; however, the measured cone factors differ significantly (≤ 10%) among various ion chambers. The Pinpoint (0.015 cm3) ion chamber provides reliable data, which differs from other detectors. Monte Carlo data are in between all other measured data. The large variation in SRS dosimetry suggests that there should be a national protocol to review the dosimetry in small fields.

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