The effective point of measurement for depth-dose measurements in small MV photon beams with different detectors

The effective point of measurement (EPOM) of cylindrical ionization chambers differs from their geometric center. The exact shift depends on chamber construction details, above all the chamber size, and to some degree on the field-size and beam quality. It generally decreases as the chamber dimensions get smaller. In this work, effective points of measurement in small photon fields of a range of cylindrical chambers of different sizes are investigated, including small chambers that have not been studied previously.

In this investigation, effective points of measurement for different ionization chambers (Farmer type, scanning chambers, micro-ionization chambers) and solid state detectors were determined by measuring depth-ionization curves in a 6 MV beam in field sizes between 2 × 2 cm² and 10 × 10 cm² and comparing those curves with curves measured with plane-parallel chambers.

It was possible to average the results to one shift per detector, as the results were sufficiently independent of the studied field sizes. For cylindrical ion chambers, shifts of the EPOM were determined to be between 0.49 and 0.30 times the inner chamber radius from the reference point.

We experimentally confirmed the previously reported decrease of the EPOM shift with decreasing detector size. Highly accurate data for a large range of detectors, including new very small ones, were determined. Thus, small chambers noticeably differ from the 0.5-times to 0.6-times the inner chamber radius recommendations in current dosimetry protocols. The detector-individual EPOMs need to be considered for measurements of depth-dose curves.