AN INVESTIGATION INTO THE DOSIMETRY OF A 9-FIELD TOMOTHERAPY IRRADIATION USING BANG-GEL DOSIMETRY

BANG-gel dosimetry offers the potential for measuring the dose deliver ed by a radiotherapy treatment technique, in three dimensions, with hi gh spatial resolution and good accuracy. The ability to measure compre hensively a 3D dose distribution is a major advantage of the gel dosim eter over conventional planar and point-based dosimeter devices, parti cularly when applied to the verification of complex dose distributions characteristic of intensity-modulated radiotherapy (IMRT). In this pa per an in-house manufactured BANG-gel dosimeter was applied to study t he dose distributions of two irradiation experiments for which the dis tributions were known: (i) a dosimetrically simple parallel-opposed ir radiation, and (ii) a more complex nine-field 'static tomotherapy' int ensity-modulated irradiation delivered with the Nomos MIMiC. The unifo rm distribution in (i) allowed a study of the magnetic resonance (MR) imaging parameters to achieve an optimal trade-off between noise and i mage resolution (optimum image resolution for the Siemens 1.5T Vision system was determined to be approximate to 0.8 mm(2) with a slice thic kness of 2 mm). The spatial uniformity of gel sensitivity to radiation was found to depend strongly on the presence of oxygen, which must be eliminated for the gel dosimeter to be of use. The gel dosimeter was found to agree well with predicted dose distributions and accurately m easured the steep penumbral fall-off of dose, even after many days, pr oving its potential for the verification of IMRT distributions. In the nine-field IMRT delivery (ii) the predicted dose was computed by both an in-house 'component-delivery' dose algorithm and the Peacock plann ing-system dose algorithm. Good agreement was found between the two al gorithms despite the latter's omission of the change in penumbral char acteristics with aperture-size during delivery, lack of inhomogeneity correction and approximate modelling of leaf leakage. These effects we re found to be small for the problem studied. The predicted distributi on agreed well with the gel-measured distribution at medium and high d oses (50-90% isodose lines) although differences of up to 10% were obs erved at lower doses (30% isodose line). The gel dosimeter was found t o have the potential to verify IMRT distributions but required conside rable care to achieve accurate results. Attention was required to achi eve uniformity of gel sensitivity (to prevent oxygen contamination), a nd in the calibration process.