Conroy Leigh, Guebert Alexandra, Smith Wendy L
Department of Medical Physics, Tom Baker Cancer Centre, 1331 29 Street NW, Calgary, AB, Canada, T2N 4N2.
Department of Physics & Astronomy, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4.
Med Phys. 2017 Jan;44(1):37-42. doi: 10.1002/mp.12005. Epub 2017 Jan 3.
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.
(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.
(a) The marker blocks increased the surface dose for a 6 MV direct field by 48.2-52.2% of D ; (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 (R = 0.83; R = 0.90; R = 0.83), whereas the inferior position was moderately correlated (R = 0.36).
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.
有人建议将实时位置管理（RPM）标记块直接放置在乳房或胸骨上，以验证乳腺癌放疗时的深吸气屏气（DIBH）水平。我们探讨了这种做法存在的三个潜在问题：（a）将标记块置于原射线束中对表面剂量的影响；（b）标记块倾斜对RPM系统准确性的影响；（c）患者体表标记块位置与胸壁内部位置之间的相关性。
（a）在0°和30°入射角下测量两点、四点和六点标记块下的表面剂量；（b）记录使用两点和六点标记块时，标记块围绕RPM摄像机视线从0°到60°倾斜时检测到的运动幅度；（c）使用从8例左侧乳腺癌患者的射野图像中提取的轮廓，研究屏气期间胸壁中位位移与体表轮廓中位位移之间的相关性，标记块分别位于上、中、下位置。
（a）对于6 MV直接射野，标记块使表面剂量增加了D的48.2% - 52.2%；（b）在侧向倾斜大于10°时，两点标记块高估了运动幅度；然而，六点标记块的幅度在60°以内仍保持准确；（c）整个体表、上体表和中体表位置与胸壁位移密切相关（R = 0.83；R = 0.90；R = 0.83），而下体表位置相关性中等（R = 0.36）。
RPM标记块可放置在乳房上用于DIBH治疗；然而，应注意表面剂量效应。两点标记块倾斜超过20°时不应使用。标记块放置在乳房的中或上位置与胸壁位置的相关性最强。
