The Italian affair: the employment of parallel-plate ionization chambers for dose measurements in high dose-per-pulse IORT electron beams.

PURPOSE

At the high dose-per-pulse rates used by some intraoperative radiotherapy (IORT) units, the employment of ionization chambers for dose measurements needs an appropriate correction (k(sat)) for ion recombination. Through a revision of the existing literature, the authors compared different methods for the determination of the recombination correction factor and their impact on clinical dosimetry.

METHODS

A dosimetric characterization of IORT electron beams from a Linac Hitesys Novac7 (Aprilia-Latina, Italy) was performed. Dose-to-water (D(w)) values were measured with dose-per-pulse independent chemical dosimeters (operated by the Italian Primary Standard Dosimetry Laboratory, ENEA) and compared to doses obtained by two different parallel-plate ionization chamber models (Markus and Advanced Markus, PTW, Freiburg, Germany). For dose measurements using ionization chambers, the authors applied two different methods for the determination of the ion recombination correction factor (k(sat)), as suggested in previous articles. The first method is based on the experimental estimation of the free-electron fraction values p; the second one is based on the "nonstandard" two-voltage analysis including the free-electron component.

RESULTS

For a Markus type chamber, there is a good agreement between the results on k(sat) and those reported in the literature for both methods, while for the Advanced Markus chamber, no data are available for comparison.

CONCLUSIONS

Comparing values of D(w) obtained by dose-per-pulse independent dosimeters and by ionization chambers measurements corrected using the two different approaches, the authors can conclude that the k(sat) factor determination is very critical and that only an experimental protocol using ionization chamber intercalibration can be considered reliable.