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

Sonja Wegener,Otto A Sauer

doi:10.1002/mp.13788

Abstract

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 6MV beam in field sizes between 2×2cm2 and 10×10cm2 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.

Highlights

When an air-filled ionization chamber is placed in water, some of the medium in the chamber cavity is replaced with
This can be accounted for with a displacement correction 0.5r seems to be a valid choice for a typical Farmer type ionfactor 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
Depth dose curves obtained with different detectors were recorded and shifted to provide the best agreement with a reference curve measured with a plane-parallel chamber, which was assumed to yield the correct curve

Summary

Introduction

When an air-filled ionization chamber is placed in water, some of the medium in the chamber cavity is replaced with. The literature suggests that the position of the EPOM strongly varies among different chamber types.[6,7,10] While air. This can be accounted for with a displacement correction 0.5r seems to be a valid choice for a typical Farmer type ionfactor 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. Dosimetry protocols typically suggest a shift between 0.52 and 0.6 times[3,4] the cavity radius r of cylindrical ionization chambers to obtain the EPOM for measurements in photon beams. EPOM shift per chamber over all beam qualities and field sizes can be applied in practice.[6,10]

Methods

Results

Discussion

Conclusion