In vivo dosimetry in 60 Co teletherapy using electron paramagnetic resonance in L-alanine

The aim of this study was to evaluate the accuracy of doses absorbed during teletherapy with 60Co beams by comparing them with doses calculated by the radiotherapy treatment planning system. The doses were measured in vivo using electron paramagnetic resonance (EPR) in L-alanine. This method of dosimetry is based on the detection of free radicals generated by ionizing radiation in the dosimetric material which is polycrystalline L-alanine. The concentration of free radicals is proportional to the absorbed dose. The EPR technique allows for determining relative concentrations of the radicals and for calibration of the EPR signal intensity vs. the absorbed dose. The measurement is non-destructive and therefore allows for dosimetry of a single fraction dose, as well as the total dose of radiotherapy using the same detector. The detectors are small polyethylene bags (16 mm x 16 mm x 1.6 mm) filled with crystalline L-alanine powder. Clinical research was performed on a group of patients undergoing radical and palliative treatment at the Department of Oncology and Radiotherapy of the Medical University of Gdansk. The entry dose was measured for 72 fields irradiated by 60Co photons localized within the head and neck, the chest and the pelvis. The accuracy of the EPR dosimetry was about 3% (one standard deviation) for doses above 2 Gy. The results of in vivo dosimetry were compared with doses calculated by radiotherapy treatment planning systems. The average deviation of the measured doses from doses obtained during planning was 0.21%, with a data scatter of 3.5% (standard deviation for a single measurement). A detailed procedure of the developed dosimetric method is presented.The results of in vivo dosimetry show good correlation between the prescribed and the actually delivered doses. The 0.21% average difference can be accounted as satisfactory in routine radiotherapy treatments. The sources of four measurement errors exceeding 10% were investigated and explained. (author)