Dozymetria EPR in vivo w brachyterapii
In vivo dosimetry was performed in six gynaecological brachytherapy patients to verify concordance between the planned and the delivered doses. The dose measurements were performed using lithium formate as an EPR dosimeter. The dosimeters in the form of 0.5×1.7 cm cylindrical capsules were placed in a PCV applicator inserted into the rectum of 6 patients treated with 1.38 GBq, 137Cs sources for up to 50% of the total treatment time. The use of the applicator significantly improved radiographic visualization of the dosimetrs. The EPR signals in the dosimeters were measured with Bruker EMX spectrometer and their amplitudes were used to determine doses absorbed in the dosimeters and then the doses in tissue at the dosimeters’ location. The dosimetric signal was stable within 2% during 7 weeks after irradiations. A temperature correction factor of +0.22%/K was found to account for temperature differences between in vivo measurements and calibration of the dosimeters. The results showed differences between planned and in vivo measured doses in the range of (-33%, +170%). The observed discrepancies cannot be attributed either to uncertainties of the EPR measurements (which were below 9%), or to an imprecise conversion of the doses in dosimeters into doses in tissue. Therefore, the differences reflect the real variations between the planned and actual doses, resulting mainly from movements of the dosimeters and brachytherapy applicators inside the patients body during the period between the simulation procedure and the treatment. In the light of the presented results, supported also by other studies, such displacements seem to be unavoidable, leading to a discrepancy between the planned and actual doses in brachytherapy of gynecological malignancies.