Ion recombination for ionization chamber dosimetry in a helical tomotherapy unit

Ion recombination for ionization chambers in pulsed high-energy photon beams is a well-studied phenomenon. Despite this, the correction for ion recombination is often determined inaccurately due to the inappropriate combination of using a high polarizing voltage and the simple two-voltage method. An additional complication arises in new treatment modalities such as IMRT and tomotherapy, where the dosimetry of a superposition of many constituting fields becomes more relevant than of single static fields. For these treatment modalities, the irradiation of the ion chamber geometry can be instantaneously inhomogeneous and time dependent.

This article presents a study of ion recombination in ionization chambers used for dosimetry in a helical tomotherapy beam. Models are presented for studying the recombination correction factors in a continuous beam, in pulsed large and small fields, and in helical fields. Measurements using Exradin A1SL, NE2571, and NE2611 type chambers and Monte Carlo simulations using PENELOPE are performed in support of these models.

Initial recombination and charge multiplication are found to be the same in 60Co and in the pulsed high-energy photon beam for the chambers and operating voltages used in this study. Applying the two-voltage technique for the A1SL chamber at its recommended operating voltage of 300 V leads to an overestimation of the recombination. Operating at a voltage of 100 V yields larger but more accurate values for the recombination correction. The recombination correction measured for this chamber in the TomoTherapy HiArt unit is lower than the 1% applied in the routine dosimetry for this treatment unit. For a helical dose delivery with a small slice width, lateral electron scatter in the cavity makes that the recombination is smaller than for an open beam delivering the same total dose. In a Farmer type chamber, a helical delivery with a 1 cm slice field results in a time and spatially integrated volume recombination of 55% of that with a 2.5 cm slice field. The relative recombination corrections for different slice widths and different field offsets with respect to the chamber center obtained from the developed models are in good agreement with experimental data.

Because of the presence of charge multiplication, it is more accurate to determine the recombination correction at lower operating voltages than are often applied using the two-voltage method. Models and experiments for partial irradiation conditions of the ion chamber show that resulting recombination corrections are reduced compared to those for an open field. A model for helical dose deliveries results in recombination corrections that get lower with smaller slice widths. This model could be adapted to any IMRT delivery where the ion chamber is instantaneously partial and/or inhomogeneously irradiated, and could provide a practical procedure to calculate the recombination for complex deliveries for which it is difficult to be measured.