299. In vivo measurements of entrance doses in 60Co teletherapy using epr/alanine dosimetry

Abstract

The results of in vivo dosimetry in radiotherapy using alanine detectors are presented. This dosimetric method is based on detection of free radicals gene-rated by ionizing radiation in polycrystalline Lalanine. The concentration of free radicals is proportional to the absorbed dose. Electron Paramagnetic Resonance (EPR) technique allows to determine relative concentrations of the radicals. The EPR measurements are non-destructive and therefore the same detector can be used for dosimetry of a single fraction and the total dose delivered in a radiotherapy treatmenl. Clinical research was performed on a group of patients undergoing palliative and radical treatments in Department of Oncology and Radiotherapy, Medical University of Gdansk. The entrance doses were measured for 72 fields irradiated by 60Co photon beam. Those fields were localized in head and neck, chest, and pelvis regions. The detectors had a form of small polyethylene bags (16 mm, 16 mm, 1.6 mm) filled with a polycrystalline L-alanine powder. Accuracy of the measurements was analyzed and discussed. The calculated 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 procedures. The average deviation of the measured doses from those obtained from the planning was 0.2%, with the data scatter of 3.5% (standard deviation for a single measurement). The sources of measurement errors were discussed. The largest differences exceeding three standard deviations between the measured and planned doses were carefully investigated – their sources were identified and explained.