[ dosimetric evaluation of organs at risk in prostate radiation therapy using a MAGIC gel dosimeter]

ABSTRACT

Multiple fields and presence of hetrerogeneities create complex dose distributions that need three dimentisonal dosimetry. In this work, we investigated MR-based MAGIC gel dosimetry as a three-dimentional dosimetry technique to measure the delivered dose to bladder and rectum in prostate radiation therapy. 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 preformed in MATLAB 7 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. Mean "distance-to-agreement" and mean "does difference" were 2.98 mm and 6.2%, respectively, in the comparison of profiles obtained from ionization chamber and gel dostimetry. Mean relative difference of DHVs between gel dosimetry and TPs data were 3.04%, 10.4% and 11.7%,for prostate, bladder and rectum, respectively. Gel dosimetry is a good method for three dimensional dosimetry although it has a low precision in high close gradient regions. This method can be used for evaluation of complicated dose distribution accuracy in 3D conformal radiotherapy, especially in presence of heterogeneities