Effective Point of Measurement of a Thimble Ionization Chamber In Kilovoltage X-Ray Beams
Abstract
Purpose
To experimentally determine the effective point of measurement (EPOM) shift of an Exradin A1SL ion chamber in kilovoltage x-ray beams (40-250 kVp) using water tank measurements.
Methods
Water tank depth-dose scans were performed using an Exradin A1SL thimble ionization chamber and a PTW microdiamond detector, with the latter serving as the reference detector. Measurements were conducted at a source-to-surface distance of 100 cm in accredited M-series kilovoltage x-ray beams spanning half-value-layers from 0.75 to 18.49 mm of aluminum. Depths from 2 cm to 10 cm in water were sampled at 4 mm intervals using a motorized stage. EPOM shifts were calculated relative to the chamber’s cavity radius using a chi-square minimization to determine a best-fit shift per beam quality.
Results
The best-fit shift showed a statistically significant dependence on beam quality. The EPOM shift is relative to the central electrode. Low energy beams (< 100 kVp) required a shift away from the x-ray source, ranging from 0.8 to 0.2 times the cavity radius, with the magnitude of the shift decreasing towards zero with increasing beam hardness.
Conclusion
The measured EPOM trend as a function of beam quality is approximately in agreement with prior Monte-Carlo-based studies involving different ion chambers in this energy range. For medium-energy x-ray beams of approximately 100 kVp and greater, the geometric center of the A1SL chamber effectively serves as the EPOM, consistent with AAPM TG-61 guidance. In contrast, low-energy beams require an upstream shift that depends on beam quality to accurately align chamber readings with absorbed dose in water without the presence of the dosimeter.