Investigation of X-ray CT dose in normoxic polyacrylamide gel dosimetry

Abstract

Polymer gel dosimetry is the most promising method for three dimensional measurement of complex radiation therapy dose distributions. X-ray CT imaging is an attractive modality for performing polymer gel dosimetry due to its robustness, ease of use, and clinical accessibility. An outstanding issue in x-ray CT polymer gel dosimetry is the possibility of inducing gel polymerization through dose delivered by CT imaging and hence potentially altering the recorded dose distribution during the read-out procedure. To address this issue we have undertaken a study to quantify the effects of CT dose on polymer gel dosimeters. We utilize ion chamber and film measurements to quantify the dose imparted to polymer gels under a range of gel CT imaging protocols and in both small calibration vials and full (e.g. 16 cm diameter) gel phantoms. We further utilize Raman spectroscopy to quantify the response of normoxic polyacrylamide gels to CT dose. Results indicate that normoxic polyacrylamide gels exhibit weaker responses to dose imparted via a CT scanner (i.e. keV energy photons) as opposed to doses imparted by a linac (MeV energy photons). Furthermore, CT doses for volumetric gel imaging protocols (120 kVp, 200 mAs, 10 mm slice thickness) are in the range of 0.0343 Gy/slice whereas single slice imaging protocols (140 kVp, 200 mAs, 10 mm slice thickness) can range from 0.0445 Gy/slice for large (20 cm diameter phantoms) to 0.0231 Gy/slice for gel calibration vials. The non-uniformity of CT dose across gel phantoms is also addressed. In conclusion, CT dose in polymer gel dosimeters is not a concern for standard gel image protocols (e.g. 16 image averages) but could potentially become noticeable for imaging protocols which utilize higher numbers of image averages.