Background and Aims

Ionization chambers (IC) represent the standard for performing the commissioning of medical linacs. Nevertheless, their use in the UHDR range is not currently possible, due to the amount of charge produced by each pulse.

For dose-per-pulse (dpp) above 0.5 cGy/p, the approach implemented by international protocols for modelling ion recombination failed, because the free electron fraction p contribution must be considered. We modify the approach of Di Martino (2005) in order to obtain p by means of ionometric measurements only.

Methods

According to the proposed model:

where

q^col is the charge collected by IC;

V is the voltage applied to IC;

q^gen is the charge generated by the pulse;

A and λ are parameters depending on the IC..

By varying the voltage applied V, such function can be determined versus the unknown parameters (q^gen, A and λ ); then, such parameters can be determined by means of the non-linear regression method.

Once all parameters are known, p is calculated and and then k_sat is determined as:

being α = A / V .

Results

The method has been adopted for estimating k_sat for the Adv. Markus, both with the beam produced by ElectronFlash and by LIAC HWL.The fit provided an agreement better than 1% within IORT range and better than 5% with 0.6 Gy/p.

Conclusions

Once k_sat is known, ionometric measurements at the larger distance might become the central element of a reliable Quality Assurance program for any Flash linac.