Daily cbct reveals shoulder movement and its dosimetric effects during head and neck IMRT.
Liu C, Siddiqui F, Chetty I, and Mao W. Daily cbct reveals shoulder movement and its dosimetric effects during head and neck IMRT. Med Phys 2017; 44(6):3052.
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.