Document Type : Original Paper
M.Sc. Student of Medical Physics, Medical Physics Dept., Tehran University of Medical Sciences, Tehran, Iran
Associate Professor, Medical Physics Dept., Tehran University of Medical Sciences, and Radiotherapy Physics Department of Cancer Institute, Tehran, Iran
Assistant Professor, Radiotherapy Physics Dept., Cancer Institute, Tehran, Iran
Radiotherapy Physicist, Radiotherapy Physics Dept., Cancer Institute, Tehran, Iran
Associated Professor, Medical Physics Dept., Tehran University of Medical Sciences, Tehran, Iran
Introduction: Multiple fields and presence of heterogeneities create complex dose distributions that need three dimensional dosimetry. In this work, we investigated MR-based MAGIC gel dosimetry as a three-dimensional dosimetry technique to measure the delivered dose to bladder and rectum in prostate radiation therapy.
Materials and Methods: A heterogeneous slab phantom including bones was made. Paired cubes in the phantom representing bladder and prostate and a cylindrical container representing rectum were filled with MAGIC gel and placed in the anthropomorphic pelvic phantom. The phantom was irradiated with four beams as planned using a treatment planning system (TPS). Magnetic resonance transverse relaxation rate images were acquired and turned into dose distribution maps using a calibration curve. This calibration curve was obtained by linear fitting to R2 values of 4 test tubes against their given known doses. Image processing and data analysis were performed in MATLAB7 software. The gel dosimeter was validated using an ionization chamber. Dose maps and dose volume histograms (DVHs) were compared with dose distributions and DVHs of the TPS.
Results: Mean “distance-to-agreement” and mean “dose difference” were 2.98 mm and 6.2%, respectively, in the comparison of profiles obtained from ionization chamber and gel dosimetry. Mean relative difference of DVHs between gel dosimetry and TPS data were 3.04%, 10.4% and 11.7%, for prostate, bladder and rectum, respectively.
Discussion and Conclusions: Gel dosimetry is a good method for three dimensional dosimetry although it has a low precision in high dose gradient regions. This method can be used for evaluation of complicated dose distribution accuracy in 3D conformal radiotherapy, especially in presence of heterogeneities.