Assessment of ion recombination correction and polarity effects for specific ionization chambers in flattening-filter-free photon beams

Guadalupe Martin-Martin; Pedro Borja Aguilar; Benigno Barbés; Eduardo Guibelalde

doi:10.1016/j.ejmp.2019.07.018

Assessment of ion recombination correction and polarity effects for specific ionization chambers in flattening-filter-free photon beams

Phys Med. 2019 Nov:67:176-184. doi: 10.1016/j.ejmp.2019.07.018. Epub 2019 Nov 15.

Authors

Guadalupe Martin-Martin¹, Pedro Borja Aguilar², Benigno Barbés², Eduardo Guibelalde³

Affiliations

¹ Medical Physics and Radiation Protection Service, Hospital Universitario de Fuenlabrada, C/ Camino del Molino 2, 28492 Fuenlabrada, Madrid, Spain. Electronic address: guadalupe.martin@salud.madrid.org.
² Clínica Universidad de Navarra, Department of Radiation Physics, Avenida Pío XII, 31080 Pamplona, Navarra, Spain.
³ Medical Physics Group, Department of Radiology, University Complutense of Madrid, 28040 Madrid, Spain.

PMID: 31734555
DOI: 10.1016/j.ejmp.2019.07.018

Abstract

Purpose: To investigate ion recombination correction and polarity effects in four ion chamber models in flattening-filter-free (FFF) beams to (1) evaluate their suitability for reference dosimetry; (2) assess the accuracy of the two-voltage technique (TVA) against the Bruggmoser formalism; and (3) examine the influence of the accelerator type on the recombination correction.

Methods: Jaffé plots were created for a variety of microchambers, small-volume and Farmer-type chambers to obtain k_S, the recombination correction factor, using two different types of accelerators. These values were plotted against dose-per-pulse and Jaffé plots for opposite polarities were created to determine which chambers meet the AAPM TG-51 addendum recombination and polarity specifications.

Results: Nearly all small-volume chambers exhibited reference-class behavior with respect to ion recombination and polarity effects. The microchambers exhibited anomalous recombination and polarity effects, precluding their use for reference dosimetry in FFF beams. For the reference-class chambers, agreement between TVA-determined k_S values and Jaffé and Bruggmoser formalisms-determined k_S values was within 0.1%. No significant differences were found between the k_S values obtained with the two different accelerators used in this work.

Conclusions: This study stresses the need to characterize ion recombination correction and polarity effects for small-volume chambers and microchambers on an individual chamber basis and with the more rigorous criteria of the AAPM TG-51 addendum. Furthermore, the study demonstrated the suitability of the TVA method for chambers that exhibit reference-class behavior in FFF beams. Finally, this work has shown that the recombination correction does not depend on the type of accelerator but on its dose-per-pulse.

Keywords: Bruggmoser formalism; Flattening-filter-free beams; Recombination correction; Small-volume chambers.

MeSH terms

Particle Accelerators
Photons*
Radiometry / instrumentation*