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Daily CBCT Reveals Shoulder Movement and Its Dosimetric Effects During Head and Neck IMRT

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C Liu*, F Siddiqui , I Chetty , W Mao, Henry Ford Health System, Detroit, MI


MO-L-GePD-J(B)-5 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Joint Imaging-Therapy ePoster Lounge - B

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&N patients planned with 30-35 fractions (2Gy/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-the-day (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 (-10.0mm~17.6mm) while (-6.2mm~30.9mm) and the average daily movement is 3.4mm±4.5mm and 8.1mm±7.4mm 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 0.01Gy±0.02Gy 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 0.01Gy±0.02Gy 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.

Funding Support, Disclosures, and Conflict of Interest: Sponsored research grants from: 1. Varian Medical Systems, Palo Alto, CA 2. Philips HealthCare, Best, Netherlands.

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