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Technical Note: Issues related to external marker block placement for deep inspiration breath hold breast radiotherapy

Purpose: It has been suggested that the Real‐time Position Management (RPM) marker block should be placed directly on the breast or sternum to verify deep inspiration breath hold (DIBH) level for breast radiotherapy. We explore three potential issues with this practice: (a) surface dose effect of placing the marker block in the primary beam; (b) effect of marker block tilt on the accuracy of the RPM system; and (c) correlation between marker block positions on the patient surface and internal chest wall position. Methods: (a) The surface dose under the two‐, four‐, and six‐dot marker blocks was measured at incident angles of 0° and 30°; (b) the motion amplitude detected when using the two‐ and six‐dot marker blocks was recorded for block tilts from 0° to 60° about the RPM camera line of sight; (c) the correlation between median displacement of the chest wall and median displacement of the surface contour between breath holds was investigated for superior, middle, and inferior block positions using contours extracted from portal images of eight left‐sided breast cancer patients. Results: (a) The marker blocks increased the surface dose for a 6 MV direct field by 48.2–52.2% of Dmax; (b) at lateral tilts greater than 10°, the two‐dot marker block overestimated the motion amplitude; however, the six‐dot marker block amplitude remained accurate up to 60°; (c) the whole, superior, and middle surface positions were strongly correlated with chest wall displacement (R2 = 0.83; R2 = 0.90; R2 = 0.83), whereas the inferior position was moderately correlated (R2 = 0.36). Conclusions: The RPM marker block can be placed on the breast for DIBH treatments; however, caution should be used regarding surface dose effects. The two‐dot marker block should not be used for block tilts beyond 20°. Marker block placement at a middle or superior position on the breast results in the strongest correlation with chest wall position.