Verification of dynamic radiotherapy: the potential for 3D dosimetry under respiratory-like motion using polymer gel.

Following the implementation of advanced treatment procedures in radiotherapy, there is a need for dynamic dose verification in 3D. Gel dosimetry could potentially be used for such measurements. However, recently published data show that certain types of gels have a dose rate and fractionation dependence. The aim of this study was to investigate the feasibility of using a polymer gel dosimeter for dose verification of dynamic radiotherapy. To investigate the influence of dose rate dependence during respiratory-like motion in and out of the beam, a respiration robot together with two types of gel systems (normoxic methacrylic acid gel (nMAG) and normoxic polyacrylamide gel (nPAG)) were used. Reference measurements were obtained using a linear diode array (LDA). Expected results, if there was no influence of the dose rate variation, were calculated by convolving the static irradiated gel data with the motion function controlling the robot. To investigate the fractionation dependence, the gels were irradiated using gated and ungated deliveries. Magnetic resonance imaging was used to evaluate the absorbed dose response of the gel. The measured gel data coincided well with the LDA data. Also, the calculated data agreed well with the measured dynamic gel data, i.e. no dose rate dependence due to motion was observed. The difference in the R2 response for the gels receiving ungated and gated, i.e. fractionated, deliveries was less than 1% for the nPAG and 4% for the nMAG, for absorbed doses up to 2 Gy. The maximum difference was 1.2% for the nPAG and 9% for the nMAG, which occurred at the highest given dose (4 Gy). The investigated gels were found to be feasible detectors for dose measurements under respiratory-like motion. For dose verification of dynamic RT involving gated delivery, e.g. breathing-adapted radiotherapy, relative absorbed dose evaluation should be used in order to minimize the effects of fractionated irradiation.

What margins should be added to the clinical target volume in radiotherapy treatment planning for lung cancer?

BACKGROUND: The planning target volume in radiotherapy treatment planning takes into account both movements of the clinical target volume (CTV) and set-up deviations. MATERIALS AND METHODS: A group of patients who received radiotherapy for lung cancer were studied. In order to measure the CTV movements due to respiration and other internal organ motions, fluoroscopy was performed for 20 patients. To study the accuracy and reproducibility of patient and beam set-up, 553 electronic portal images from 20 patients were evaluated. Discrepancies between planned and actual field positions were measured and the systematic and random errors were identified. The combined effect of these geometrical variations was evaluated. RESULTS: The average CTV movement with quiet respiration was about 2.4 mm in the medio-lateral and dorso-ventral directions. Movement in the cranio-caudal direction was on average 3.9 mm with a range of 0-12 mm. The systematic set-up errors were on average 2.0 mm in the transversal plane and 3.0 mm in the cranio-caudal direction. The random errors can be described by their standard deviations of 3.2 and 2.6 mm. In this study, the combined effect of the two parameters (CTV movement and set-up deviations) varied between 7.5 and 10.3 mm in different anatomical directions. CONCLUSIONS: In our daily clinical routine, we use a margin of 11 mm in the transversal plane and 15 mm cranially and caudally, also taking into account other unquantified variations and uncertainties.

Which depth dose data should be used for dose planning when wedge filters are used to modify the photon beam?

The use of beam data for open photon fields when calculating absorbed dose distributions for beams with wedge filters has been studied. The depth doses for beams with wedge filters are changed through beam hardening and the dose maximum can be shifted; both these changes result in errors in the final dose calculations of several per cent if open beam data are used. The errors are larger for 6 MV than for 18 MV x-rays. The depth of measurement for determining the wedge factor and the influence of other beam modifying devices are discussed. It is recommended that the reference depth be used instead of the dose maximum for these kinds of measurements since the influence of contaminating electrons in the beam will then be avoided and the wedge factor will be correct at a clinically relevant depth.