(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To compare the dose to the parotids in head and neck adapted plans versus completing the course un-adapted, and to correlate dose changes with anatomical changes in this region.
Methods: For this study 15 patients with oropharynx cancer that received adaptive radiotherapy were included. Most patients received 30Gy in 15 fractions to the low-risk region with a sequential boost to 70Gy in 20 fractions to the gross disease. The decision to replan was made based on weekly MR assessment of nodal targets and plans were adapted between fraction 5 and 30. The dosimetric data of the planning target volume (PTV) and parotids were compared between the adapted course and original course plans. Anatomical distances and overlap between the PTV and the parotids were computed and compared with the dosimetric changes.
Results: The gross disease PTV coverage for both the original and adapted plan remained the same. The parotid volume changed by on average -10.6% [-36%-11.6%], and the dose to the parotids changed by -3.7% [-22.2%-6.3%] in the adapted plan. The largest correlation (-0.32, slope -0.17) between anatomy and dose was observed in the parotid volume change. The degree of overlap between parotids and PTV also contributes as the PTV shrinks away from the parotid, the parotid mean dose reduces. There was also a positive correlation (0.31, slope 0.09) the fraction at which the plan was adapted and the dose of the parotids. This indicates that the earlier the plan was adapted due to shrinking PTV volumes, the more dose difference was seen in the parotids.
Conclusion: This study showed that adapting the plan was effective in decreasing the dose to the parotids. The change in the volume of the parotid and in the overlap of the parotids with the PTV had the most impact on the parotid dose change.
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