Using the iterative kV CBCT reconstruction on the Varian Halcyon linear accelerator for radiation therapy planning for pelvis patients

Purpose: The ability to utilise a linear accelerators kV cone beam CT, for treatment planning and dosimetry calculation, has a number of potential uses including adaptive planning and daily dose analysis. This work validates the use of the iterative reconstruction of the Varian Halcyon cone beam CT (iCBCT) as datasets for radiation therapy planning of pelvis patients. Methods: A CT to electron density (ED) curve was created in the Varian Eclipse treatment planning system (TPS) using scans of a CIRS ED phantom under a variety of scattering conditions. A pelvis phantom was imaged using a diagnostic CT scanner and Halcyon iCBCT. Ten pelvis patient plans were copied onto each dataset and compared using a global 3D gamma analysis. Each patient being treated on Halcyon has a daily iCBCT, and each plan was also recalculated on their respective day 1 iCBCT dataset. Results: 3D Gamma analysis results for the patients planned on the pelvis phantom were analysed using a 1%/1 mm and 2%/2 mm, 10% low dose threshold criteria. The average pass rate was 99.4% +/- 0.2%. The same metrics were used to analyse the patient plans recalculated on day 1 iCBCTs. The average result for the 1%/1 mm gamma analysis was 94.4% +/- 6.1%, and 98.6% +/- 2.0% for 2%/2 mm. A comparison of typical patient volumes showed average mean dose volume differences of 0.0% +/- 1.4%, 0.0% +/- 0.9% and 1.0% +/- 4.0% for the PTV, Bladder and Rectum volumes respectively. Conclusions: Halcyon iCBCT datasets can be used for dose calculation in radiation therapy treatment planning for pelvis patients.