Estimation of free electron fraction value p for a parallel-plate ionization chamber in high dose per pulse IORT electron beams

Purpose: to verify the extent of equivalence of different Markus ionization chambers and to validate the estimation of free electron fraction value p for parallel ionization chambers in very high dose-per-pulse high-energy electron beams from an intra-operative radiotherapy (IORT) dedicated linear accelerator as proposed in literature. Materials and Methods: dose-to-water values were measured with chemical dosimeters (operated by the Italian Primary Standard Dosimetry Laboratory) and with a parallel-plate ionization chambers (Markus, PTW) in IORT electron beams from a Linac Hitesys Novac7 (Aprilia-Latina, Italy) installed at University "Federico II" of Naples. We determined the ion recombination correction factor (ksat) as a function of dose per pulse for the ionization chamber through the estimation of free electron fraction value (p). Results: The estimate of p from experimental data was p=0.336 ± 0.013 .This result was in good agreement with the value found by Di Martino et al. for the same ionization chamber model. Conclusions: This study confirms that parallel ionization chamber can be employed for IORT absorbed dose-to-water measurements and in principle could allow to avoid the use of the more complex chemical dosimetry.