Use ofin vivotransit portal images to detect gross inter-fraction patient geometry changes on an O-ring type linear accelerator for pelvis and head/neck patients.

The purpose of this work was to investigate the use of the Varian Portal Dosimetry application in conjunction within vivomegavoltage portal images on a Varian Halcyon O-ring type linear accelerator as anin vivodosimetry constancy (IVDc) tool for pelvis and head/neck patients receiving VMAT treatments. Sensitivity testing was conducted on phantoms with varying thicknesses (0.2 cm-1.0 cm) using static and modulated fields. A cohort of 96 portal dose images across eight patients was then compared with PTV metrics derived from daily CBCT image based treatment plan re-calculations to determine whether the IVDc tool could detect gross inter-fraction anatomical changes. A final cohort of 315 portal dose images across 22 patients was then assessed to demonstrate the application of IVDc tool. The IVDc tool, using 2%/2 mm criteria, detected all phantom thickness changes of 1.0 cm, some phantom thickness changes of 0.5 cm, and no changes of 0.2 cm. For the cohort of 96 results, a IVDc passing criteria of 95% (2%, 2 mm) was able to identify all cases that had PTV metric changes of 2% or more. Using the IVDc tool on the cohort of 315 results, and the IVDc passing criteria of 95%, resulted in 74 IVDc failures. A simple, easy to implement, methodology has been presented that is capable of detecting gross inter-fraction changes in patient geometry on the Varian Halcyon O-ring linac linear accelerator.

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%/1mm and 2%/2mm, 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%/1mm gamma analysis was 94.4% ± 6.1%, and 98.6% ± 2.0% for 2%/2mm. 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.