5- 9 Monte Carlo simulations have been carried out to compare the dose in water and the response of different detectors. Dosimetry protocols typically suggest a shift between 0.5 2 and 0.6 times 3, 4 the cavity radius r of cylindrical ionization chambers to obtain the EPOM for measurements in photon beams.ĭifferent methodologies to obtain EPOM values have been described in the literature. 1 Otherwise, the measured curve would be shifted against the true dose distribution. There, its reading corresponds to the dose in the medium at the initial point. This can be accounted for with a displacement correction factor or by shifting the chamber position from the central axis to a point - the effective point of measurement (EPOM) - where the detector is further immersed in water. When an air-filled ionization chamber is placed in water, some of the medium in the chamber cavity is replaced with air. The detector-individual EPOMs need to be considered for measurements of depth-dose curves. Thus, small chambers noticeably differ from the 0.5-times to 0.6-times the inner chamber radius recommendations in current dosimetry protocols. Highly accurate data for a large range of detectors, including new very small ones, were determined. We experimentally confirmed the previously reported decrease of the EPOM shift with decreasing detector size. 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. It was possible to average the results to one shift per detector, as the results were sufficiently independent of the studied field sizes. 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 cm 2 and 10 × 10 cm 2 and comparing those curves with curves measured with plane-parallel chambers. 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. It generally decreases as the chamber dimensions get smaller. The exact shift depends on chamber construction details, above all the chamber size, and to some degree on the field-size and beam quality. Other English adjective-noun pairs are related in this way, too: e.g., hale as in “hale and hearty” and health (but hale, except in that expression, is now mostly replaced by “healthy”).The effective point of measurement (EPOM) of cylindrical ionization chambers differs from their geometric center. Obsolete forms include heighth and highth, and it is still common to hear people pronounce it that way.) wi de They follow a common English pattern that involves a vowel change (often to a shorter vowel) and the addition of th. Length, width, height, and depth are nouns are derived from the adjectives long, wide, high, and deep. And, as in two dimensions, terms like “length,” “width,” and “height” won’t feel natural or be clear for some shapes, like a tennis ball. When height would be unclear-for example if the figure is not “level” -people cannot know what is meant by width, depth, or height without labels, although length is generally still assumed to refer to the longest measurement on the figure.
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