Title

Daily cbct reveals shoulder movement and its dosimetric effects during head and neck IMRT.

Document Type

Conference Proceeding

Publication Date

2017

Publication Title

Med Phys

Abstract

Purpose: 1) to utilize daily cone-beam CT (CBCT) to assess shoulder position variations during head and neck (H&N) IMRT; 2) to quantify the dose deviation from the plan in the shoulder level. Methods: 15 H&pN patients planned with 30-35 fractions (2 Gy/fraction) of IMRT were retrospectively studied. CBCT with visible shoulders was acquired each day. A deformable image registration (DIR) between each CBCT and the planning CT (pCT) were carried out to evaluate daily voxel-wise shifts. The average shift of landmarks in superior/inferior direction was used to quantify the shoulder movement. Resampled pCT from the DIR was used to calculate the dose-of-theday (DoD), which was mapped back to the pCT space and accumulated using an in-house developed energy-mass mapping algorithm. The deviation of the DoD/total cumulative dose from the plan was analyzed respectively in a region-of-interest in the shoulder level. Results: Among the cohort, the shoulder movement range is (-10.0 mm∼17.6 mm) while (-6.2 mm∼30.9 mm) and the average daily movement is 3.4 mm ± 4.5 mm and 8.1 mm ± 7.4 mm for left and right shoulder, respectively. The maximum difference between the mean DoD and the mean planned dose is 0.07 Gy or 7.5% with an average of 0.01 Gy ± 0.02 Gy or 1.0% ± 3.0%. The maximum difference between the total cumulative dose and the planned dose is 0.05 Gy or 6.1% with an average of 0.01 Gy ± 0.02 Gy or 1.6% ± 2.8%. The Pearson's correlation coefficient between the average shoulder shift and the total dose difference was 0.48. Conclusion: Daily CBCT shows that a single-day shoulder movement reached 30.9 mm during H&N IMRT. The average landmark movement was in inferior direction (positive numbers). Daily mean dose in the ROI increased by a maximum of 7.5%. Caution has to be taken when a structure is close to the shoulder level especially when the number of fractions is small.

Volume

44

Issue

6

First Page

3052

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