EPR/alanine dosimetry applied to Leksell gamma knife perfexion

Abstract

Introduction Specificities of the Leksell Gamma Knife (LGK) forbid using reference conditions recommended by international radiotherapy beam calibration protocols. In this framework, the Laboratoire National Henri Becquerel and the Pitie-Salpetriere Hospital decided to check the beams calibration and the dose delivered (”end-to-end” test) by the LGK Perfexion. Moreover, this study aims to compare delivered dose by the two latest software versions of the Gammaplan Treatment Planning System (TPS). Materials The dosimetric method chosen is the Electron Paramagnetic Resonance (EPR) of alanine combined with Monte Carlo simulations. Dose rate measurements have required the phantom spherical ABS Elekta and for the test ‘‘end-to-end” an anthropomorphic head phantom. Methods EPR measurements associated with Monte Carlo simulations allowed the LNHB to develop a method to compare the results to the dose rates measured by ionization chamber. The ‘‘end-to-end” test was done following the patient circuit: CT-scan processing, Treatment planification with 16 mmand 16 mm+8 mmcollimations. The measured values were compared with those delivered by the TPS. Results An average difference of 0.8% between dose rates measured by alanine and ionization chamber was obtained. For the test ‘‘endto- end”, a mean difference of −1.7% (version 9 TPS) and −0.9% (version 10) were obtained. Conclusion Results show a good agreement between beam calibration and alanine measurements brought back to the TRS 398 reference conditions. Results of ‘‘end-to-end” tests gave a good agreement between measured values calculated. However, this study highlighted the impact of the heterogeneities that are not taken into account for TPS dose calculations.