Abstract
Three-dimensional (3D) gel dosimetry has a unique role to play in safeguarding conformal radiotherapy treatments as the technique can cover the full treatment chain and provides the radiation oncologist with the integrated dose distribution in 3D. It can also be applied to benchmark new treatment strategies such as image guided and tracking radiotherapy techniques. A major obstacle that has hindered the wider dissemination of gel dosimetry in radiotherapy centres is a lack of confidence in the reliability of the measured dose distribution. Uncertainties in 3D dosimeters are attributed to both dosimeter properties and scanning performance. In polymer gel dosimetry with MRI readout, discrepancies in dose response of large polymer gel dosimeters versus small calibration phantoms have been reported which can lead to significant inaccuracies in the dose maps. The sources of error in polymer gel dosimetry with MRI readout are well understood and it has been demonstrated that with a carefully designed scanning protocol, the overall uncertainty in absolute dose that can currently be obtained falls within 5% on an individual voxel basis, for a minimum voxel size of 5 mm3. However, several research groups have chosen to use polymer gel dosimetry in a relative manner by normalizing the dose distribution towards an internal reference dose within the gel dosimeter phantom. 3D dosimetry with optical scanning has also been mostly applied in a relative way, although in principle absolute calibration is possible. As the optical absorption in 3D dosimeters is less dependent on temperature it can be expected that the achievable accuracy is higher with optical CT. The precision in optical scanning of 3D dosimeters depends to a large extend on the performance of the detector. 3D dosimetry with X-ray CT readout is a low contrast imaging modality for polymer gel dosimetry. Sources of error in x-ray CT polymer gel dosimetry (XCT) are currently under investigation and include inherent limitations in dosimeter homogeneity, imaging performance, and errors induced through post-acquisition processing. This overview highlights a number of aspects relating to uncertainties in polymer gel dosimetry.
Highlights
1.1. 3D dose verification with integrating dosimeters 3D radiation dosimetry provides a unique ability to display dose distributions occurring in clinical radiotherapy in three dimensions (3D) in humanoid shaped phantoms [1,2,3]
Evaluating the uncertainty of 3D dosimetry The evaluation methods that have been applied by many 3D dosimetry research groups to give evidence of the ‘potential’ of 3D dosimeters are often based on a percentage number of voxels that pass gamma criteria between a measured dose distribution and a treatment planning system (TPS) calculated dose distribution
Given the uncertainties with respect to the calibration, we argue that a more stringent approach using other dosimeters and treatment planning should be applied to benchmark 3D radiation dosimetry than a gamma evaluation against a single treatment plan
Summary
1.1. 3D dose verification with integrating dosimeters 3D radiation dosimetry provides a unique ability to display dose distributions occurring in clinical radiotherapy in three dimensions (3D) in humanoid shaped phantoms [1,2,3]. When utilizing polymer gels with MRI readout, significant differences in dose-R2 response have been found between different 3D dosimeters that were poured in small calibration vials as compared to larger phantom recipients [4, 5] This finding severely compromises the use of an externally derived calibration curve to convert the measured R2 maps to dose maps. Evaluating the uncertainty of 3D dosimetry The evaluation methods that have been applied by many 3D dosimetry research groups to give evidence of the ‘potential’ of 3D dosimeters are often based on a percentage number of voxels that pass gamma criteria between a measured dose distribution and a treatment planning system (TPS) calculated dose distribution This value of pass rates obviously depends on the number of slices and the size (field-of-view) of the dose maps taken into consideration. Given the uncertainties with respect to the calibration, we argue that a more stringent approach using other dosimeters and treatment planning should be applied to benchmark 3D radiation dosimetry than a gamma evaluation against a single treatment plan
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