gms | German Medical Science

Background: Ionisation chambers are widely used for reference dosimetry in radiotherapeutic proton beams. Recent codes of practice for dosimetry in proton beams assume that perturbation factors for ionisation chambers are unity. There have, however, been some indications in the literature, both from experimental and theoretical studies, that perturbations may be as significant as for high-energy photon and electron beam dosimetry.

Materials and methods: In the present work the wall perturbation of two cylindrical ionisation chamber types in proton beams due to secondary electrons is calculated by Monte Carlo simulations using EGSnrc. A cavity theory model is developed as well. A parameterisation of the wall perturbation factor is proposed enabling it to be easily calculated for any cylindrical ionisation chamber. This work presents a refinement of earlier performed and published simulations and modelling. For a Farmer type ion chamber, the central electrode perturbation due to an aluminium central electrode has also been calculated using EGSnrc.

Results: The Monte Carlo simulations and the cavity theory model for the wall perturbation agree well and result in wall perturbation factors that are dependent on the proton energy, chamber geometry, wall thickness and wall material. The largest corrections at the highest therapeutic proton energies are of the level of 1%. The central electrode correction for the Farmer is found to be about 0.2%, similar in size as the value used in dosimetry protocols for high-energy electron beams.

Conclusion: Wall and central electrode perturbation corrections for cylindrical ionisation chambers in proton beams are substantial enough to be considered in radiotherapy.