Purpose: To investigate the heart position variability in deep-inspiration breath-hold (DIBH) radiation therapy (RT) for breast cancer when 3D surface imaging would be used for monitoring the BH depth during treatment delivery. For this purpose, surface setup data were compared with heart setup data.
Materials and methods: Twenty patients treated with DIBH-RT after breast-conserving surgery were included. Retrospectively, heart registrations were performed for cone-beam computed tomography (CBCT) to planning CT. Further, breast-surface registrations were performed for a surface, captured concurrently with CBCT, to planning CT. The resulting setup errors were compared with linear regression analysis. Furthermore, geometric uncertainties of the heart (systematic [Σ] and random [σ]) were estimated relative to the surface registration. Based on these uncertainties planning organ at risk volume (PRV) margins for the heart were calculated: 1.3Σ-0.5σ.
Results: Moderate correlation between surface and heart setup errors was found: R(2)=0.64, 0.37, 0.53 in left-right (LR), cranio-caudal (CC), and in anterior-posterior (AP) direction, respectively. When surface imaging would be used for monitoring, the geometric uncertainties of the heart (cm) are [Σ=0.14, σ=0.14]; [Σ=0.66, σ=0.38]; [Σ=0.27, σ=0.19] in LR; CC; AP. This results in PRV margins of 0.11; 0.67; 0.25 cm in LR; CC; AP.
Conclusion: When DIBH-RT after breast-conserving surgery is guided by the breast-surface position then PRV margins should be used to take into account the heart-position variability relative to the breast-surface.
Keywords: Breast cancer; Breath hold; Cone-beam computed tomography (CBCT); Heart; Image-guided radiotherapy (IGRT); Surface imaging.
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