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Using Varian Portal Dosimetry to Detect Inter-Fraction Patient Geometry Changes On a Varian Halcyon Linear Accelerator

T Aland*, T Jarema , ICON Group, QLD, Australia


(Sunday, 7/29/2018) 4:00 PM - 4:30 PM

Room: Exhibit Hall | Forum 7

Purpose: The purpose of the present work was to validate the use of the Varian Portal Dosimetry software as an in-vivo dosimetry constancy tool (IVDc) capable of detecting inter-day variations in patient geometry. This is important given Varian Halcyon does not have an ODI and has a 28cm FOV MV CBCT.

Methods: Initial, phantom based, testing was performed using slab and thorax phantoms to quantify the detection sensitivity of the system under a variety of different treatment techniques, including static, IMRT, and VMAT fields.Once determined, this validation was then extended to patients undergoing prostate, rectum, head and neck, and brain treatments. In conjunction with IVDc, these patients also underwent a weekly ruler based patient separation check- either physically measured for pelvis, or measured on MV CBCT for head and neck / brain patients. These two sets of results were then correlated.

Results: Initial testing showed that when 1cm bolus material was added to phantoms of thickness between 20cm – 30cm, this was clearly detected by IVDc using a 2%/2mm gamma evaluation for static, IMRT, or VMAT fields.Results for pelvis based patients showed a weak correlation between weekly measured separation data and IVDc results. Some false positives were noted, which could be a results of gas movement. Results for head and neck patients showed a strong correlation with MV CBCT measured separation data and slowly degraded as treatment progressed (correlating to an expected reduction in patient separation).

Conclusion: IVDc on Halcyon has been shown to be an effective and accurate surrogate for historical ODI patient separation checks that is able to detect separation changes as little as 1cm in patients. The strongest correlation was found in head and neck patients.


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