(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: When field sizes get small, plane-parallel chambers typically become too large for measurements of depth dose curves. Therefore, the suitability of numerously available small field detectors and their limitations are of interest.
Methods: Depth dose curves measured with different detectors were compared with curves obtained with EBT3 film at 6 MV and field sizes down to 0.6×0.6 cm². 14 detectors were included: Plane-parallel, Farmer type and cylindrical ionization chambers of different sizes, shielded and unshielded diodes and a microDiamond. Effective points of measurements for the ionization chambers were derived. The role of volume averaging due to a finite detector size in a diverging beam was estimated from profiles. Measurements were taken at both polarities and with an additional lead foil reducing electron contamination in order to quantify the contribution of those effects to the detector signal.
Results: No measurements with any detector matched the curves obtained with film. Down to 4×4 cm² field size, a plane-parallel chamber (iba PPC05) was suitable. Below that field size, the microDiamond and the smallest studied ionization chambers (iba CC01, CC003) agreed best with the film down to the smallest studied field sizes both near the surface and at large depths. The necessary shift between reference point and effective point of measurement for small ionization chambers decreases with miniaturization and is always smaller than 0.6 times the cavity radius. Polarity effects in the build-up region increased for smaller chambers, e.g., 12% difference at 3 mm depth (CC003). Volume effects were negligible for diode detectors.
Conclusion: No ideal detector was identified for measurements of depth dose curves including the build-up region of small fields. Among the studied detectors, the microDiamond and small ionization chambers agreed best with the film. Polarity effects for small chambers and detector positioning need to be considered.
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