Poster R14 - Identification of cases that may need replanning due to significant anatomical changes during head & neck radiotherapy in replanned and non-replanned patients using a center-to-surface distance metric

Introduction: The purpose of this study is to create a metric which will identify cases that may need to be replanned during the course of their treatment based on changes in their external anatomy. In this study, we retrospectively identified 89 head and neck patients, some of whom required replanning during Image Modulated Radiation Therapy (IMRT) treatment. To quantify changes in patient anatomy, we employed Matlab to analyze patient CT images.

Materials and

Methods:

Methods: A cohort of head and neck cancer patients who received radiation therapy were identified retrospectively. These patients were treated with 60 Gy in 30 fractions using IMRT and daily image guidance. Two groups of patients were identified: 57 patients who had their treatment replanned before it was completed and 30 patients who did not receive a replan. Patient CT images were quantified for changes in anatomy using Matlab. We calculated the distance in pixels between the center of the patient to the external contour at 72 different angles. We repeated this process for the planning CT and all the daily CBCTs. These values were then converted into mm using the image resolution. Subsequently, the difference in center-to-surface distance between the planning CT and daily CBCTs was calculated.

Results, Conclusions, and Discussions:

Results: Patients in the lower quartile of the data were identified for both groups. For patients with a replan, this included 5 patients whose change in center-to-surface distance from the planning CT was larger than 2% (range: -7.2% to -2%). 4 patients had a difference between -1% to -2%, 11 between -1% to 0%, and 9 patients had an increase between 0% - 2.8%. We found that 3 of the 5 patients had this 2% change in less than 15 fractions and 2 in more than 26 fractions. Visually, any changes in center-to-surface distance larger than 1% seem to be significant. For patients without a replan, this included 15 patients whose change in center-to-surface distance from the planning CT was larger than 2% (range: -7.2% to -2%). 11 patients had a difference between -1% to -2%, 16 between -1% to 0%, and 17 patients had an increase between 0% to - 2.8%. We found that 6 of the 17 patients had this 2% change in less than 16 fractions, 8 in less than 25 fractions, and 7 in more than 25 fractions. Visually, any changes in center-to-surface distance larger than 1% seem to be significant.

Conclusion: Out of 59 patients who received a replan, 15 had a 2% or larger change in center-to-surface distance during their treatment compared to the planning CT. Out of 30 patients who did not receive a replan, 5 had a 2% or larger change in center-to-surface distance. A higher percentage of patients who were replanned had large anatomical changes than those that did not receive a replan. However, the delivered dose to those patients needs to be estimated using their daily CBCTs so that a dosimetric comparison with the treatment plan can indicate the dosimetric and potentially the clinical impact of those changes. Some of the patients that were not replanned showed a high change in our metric during treatment. This suggests that some of those patients may have benefitted from a replan but did not receive one. Future work will include comparison of the dosimetric values to investigate this.

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