Purpose: To guide surface-guided radiotherapy (SGRT) setup for breast DIBH patients and determine if the simulation CT surface reference should be used or treatment surface reference is needed through a retrospective assessment of 28 patients’ daily setup data of both SGRT and 2DkV in 25-fraction DIBH treatments.
Methods: Twenty-eight left-sided breast cancer patients treated with DIBH in 25 fractions were studied retrospectively. These patients had locally advanced breast cancer and no previous RT. Volumetric-modulated arc therapy (VMAT) technique was applied, targeting both the breast and local lymph nodes, and both daily AlignRT surface imaging and 2DkV were prescribed to ensure the accuracy. AlignRT was first applied to match the body contours of free-breathing and DIBH CTs, and then 2DkV was used to align the anterior bones at DIBH for final adjustment and then capture a new on-site surface reference for DIBH gating. Therefore, setup differences between the 2 imaging modalities were recorded and the vertical shifts were compared for all patients and all fractions. It was hypothesized that patients who had no previous RT experience may need time to adapt and achieve a consistent DIBH, and therefore, they were likely to go through changes before settling down with consistency. To test this, the average of vertical shifts in first 5 fractions was compared to that of the remaining 20 fractions.
Results: Among 28 patients, 14 patients’ vertical setup shifts support the hypothesis: 8 patients have smaller mean shifts in the first 5 fractions than remaining fractions by at least 50% while 4 patients’ data show the opposite. The remaining 14 patients’ data show no significant difference.
Conclusion: This study has demonstrated that 50% patients experience substantial difference between the first 5 fractions and remaining 20 fractions. This suggests that SGRT reference surface may need to update during DIBH treatment.