engleski

Analysis of the application of different clinical CT imaging protocols as factors in the calculation of absorbed dose distributions using the Elekta Monaco radiotherapy planning system

Introduction The Monte Carlo based Elekta Monaco treatment planning system (TPS) is used to calculate absorbed dose distributions for intensity modulated radiotherapy (IMRT). The application of IMRT technique requires high accuracy of dose calculation and according to international guidelines a maximum error of up to 3% is recommended1. To calculate dose distributions, the TPS uses patient/phantom image data sets obtained from CT data translated into voxel geometry where relative electron density (RED) is assigned to the associated Hounsfield number (HU) of each voxel using a calibration curve. Therefore, the application of different imaging protocols (different Utube and FOV) can affect the accuracy of the dose calculation2. Materials and methods A study was conducted to investigate the effect of CT imaging protocols and their associated HU-RED curves on the accuracy of the calculation of absorbed dose distributions using the Elekta Monaco 5.10.02 TPS. Siemens Somatom Open CT simulator and CIRS Thorax semi-anthropomorphic phantom were also used. Two different CT protocols were applied for the acquisition of the phantom data sets using different voltages (Utube= 80, 100, 120, 140kVp) with standard (FOV=438mm) and extended field of view (eFOV=650mm)3. Simple 15x15cm2 field, 3DCRT and IMRT plan were calculted using the 6MV X-ray beam on 8 phantom data sets creating 24 absorbed dose distributions in total. Dose distributions obtained using 120 kVp voltage and standard FOV were defined as reference (using the optimal protocol) and were compared with the others. The analysis was performed using the gamma index to calculate differences between the 3 reference and 21 corresponding absorbed dose distributions comparing dose and distance to agreement differences with 95% confidence level. Results Comparison of reference dose distributions with the others shows that for simple fields, more than 95% of the analysed points have a dose difference of less than 2% for all fields and all compared protocols irrespective of the Utube and FOV/eFOV used. For the 3DCRT plan, the analysis shows different behaviour when compared to protocols with standard FOV applied (95% of the analysed points have a dose difference of less than 3%) versus those using eFOV that exhibit larger differences (95% of the analysed points have a dose difference of less than 4%). IMRT plan analysis also shows different behaviour when compared to protocols with standard FOV applied (95% of points analysed have a dose difference of less than 1%) versus those using eFOV (95% of points have a dose difference of less than 7%). Conclusion Evaluation of the differences in dose calculated by the Elekta Monaco system on a phantom imaged using different clinical CT protocols shows an impact of using different tube voltages and high dependence on whether standard or eFOV are applied4. The use of different CT protocols can lead to an increase in the differences in calculated absorbed dose distributions with increasing complexity of the radiotherapy planning techniques used. In order to preserve the accuracy of the clinical dose distribution calculations, it is important to use HU-RED curves that exactly match the CT protocols used for patient data set acquisitioning.

treatment planning ; radiotherapy

nije evidentirano