(Tuesday, 7/16/2019) 1:15 PM - 1:45 PM
Room: Exhibit Hall | Forum 3
Purpose: The decision criteria of when to replan a head and neck patient is not well-defined, and is generally based on a qualitative assessment of the patients daily setup images. In-vivo EPID images collected during treatment provide additional time-series data of how the patient changes over the course of treatment. We retrospectively analyzed head and neck patients treated with IMRT and VMAT and assessed how the in-vivo EPID images evolved over the course of treatment, with the goal of finding a quantitative threshold to determine when replanning should be performed.
Methods: We analyzed two groups of head and neck patients: those who underwent replanning, and those who did not. The in-vivo EPID images were automatically collected for each fraction of delivery on the Varian Halcyon. The EPID images of each fraction were compared to the EPID images of the first fraction to detect trends over the course of the treatment, the mean relative difference (MRD) between pixels of EPID images taken on different days of treatment was used to quantify differences. To estimate the correlation between MRD and dose parameters, head and neck weight loss was simulated with a phantom and analyzed with MRD metrics and ion chamber measurements.
Results: Phantom measurements simulating weight loss showed a correlation between changes in the MRD and the PTV mean. For patient treatments that did not require any replanning the MRD was less than 5% over the entire course of treatment. For plans that underwent replanning the mean difference in the EPID images was greater than 5% for one or more fractions prior to replanning.
Conclusion: A physicians decision to initiate replanning coincided with the EPID MRD difference exceeding 5%. In the future, we hope to use this tool to prospectively analyze head and neck patients and to indicate the need for replanning.
Funding Support, Disclosures, and Conflict of Interest: UCSD Center of Precision Medicine Grant
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