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