SU-E-T-415: An Ionization Chamber Array with High Spatial Resolution for External Beam Radiotherapy

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Abstract

Purpose:

To characterize an ionization chamber array technology with high spatial resolution and high charge collection efficiency for external beam radiotherapy.

Methods:

The prototype under test is a linear array of air vented ionization chambers developed by IBA Dosimetry, consisting of 80 pixels with 3.5mm spatial resolution and 4mm3 sensitive volume. The detector was characterized in a plastic phantom with 6 0 Co radiation and MV X-rays from an ELEKTA Agility LINAC (with flattened and unflattened beam qualities). Bias voltage was varied in order to evaluate charge collection efficiency. A commercial array of ionization chambers (MatriXX Evolution, IBA Dosimetry) and an amorphous silicon flat panel in direct conversion configuration were used as references.

Results:

Repeatability (0.4%) and stability under continuous gamma irradiation (0.3%) are very good, in spite of low active volume and sensitivity (∼200pC/Gy). Charge collection efficiency is higher than 99% already at 150V with ∼2mGy dose per pulse, leading to a ±1.1% sensitivity change with dose per pulse in the range 0.09-2mGy (covering all flattened and unflattened applications). Measured dose profiles are in agreement with MatriXX for fields larger than 2×2cm2, in which case the linear array offers a much better characterization of the penumbra region. Down to 1×1cm2, measured profiles are in very good agreement with the flat panel.

Conclusion:

The array represents a valuable tool for the characterization of treatment fields in which high spatial resolution is required, together with the dosimetric performance of air vented ionization chambers. Such a technology would be particularly valuable in association with advanced treatment modalities such as rotational radiotherapy, stereotactic treatments (even with unflattened beam qualities) and proton therapy, due to the insensitivity of the chambers on dose per pulse. In the future, a two dimensional prototype based on this technology will be developed and tested.

This research project has been supported by a Marie Curie Early Initial Training Network Fellowship of the European Community's Seventh Framework Programme under contract number (PITN-GA-2011-289198-

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