Low D A, Zhu X R, Harms W B, Purdy J A
Mallinckrodt Institute of Radiology, St. Louis, Missouri, USA.
Med Phys. 1995 Sep;22(9):1395-403. doi: 10.1118/1.597621.
A method has been developed to measure a photon penumbra-generating kernel using dosimetry equipment available in most radiation therapy departments. The kernel is used in a convolution-adapted ratio-TAR algorithm in our three-dimensional treatment planning system. The kernel is assumed to be invariant with respect to off-axis position, axially symmetric, and is divided into short- and long-range components, with a different measurement technique for each. The data required to obtain the short-range component are measured by scanning across a split-field geometry incident on a water phantom. The derivative of the measured profile is proportional to one-dimensional projections across the kernel. Because the kernel is axially symmetric, only one profile measurement is required for each depth. A CT reconstruction technique is used to extract the radial dependence of the kernel from the strip integrals. Electronic noise in the acquisition system yields significant uncertainties in the kernel shape for distances beyond 3 cm. The long-range portion of the kernel is obtained by examining tissue-air ratios (TARs). The derivative of the TAR at the center of a circular field is proportional to the kernel value at the distance corresponding to the radius of the field. The kernel measurement method was tested by comparing measured and calculated square-field profiles at a variety of depths. Agreement was within 1% within the field boundary and 3% outside the field boundary for all depths.
已经开发出一种方法，可使用大多数放射治疗科室都有的剂量测定设备来测量产生光子半影的核。该核用于我们三维治疗计划系统中的卷积自适应比值组织空气比（ratio-TAR）算法。假定该核对离轴位置不变、具有轴对称性，并分为短程和长程分量，且每种分量采用不同的测量技术。获取短程分量所需的数据是通过扫描入射到水体模上的分裂野几何形状来测量的。测量轮廓的导数与核上的一维投影成比例。由于核是轴对称的，所以每个深度只需要进行一次轮廓测量。使用CT重建技术从条带积分中提取核的径向依赖性。采集系统中的电子噪声会在距离超过3 cm时给核形状带来显著的不确定性。核的长程部分是通过检查组织空气比（TARs）来获得的。圆形野中心处TAR的导数与对应于野半径距离处的核值成比例。通过比较在各种深度下测量的和计算的方野轮廓，对核测量方法进行了测试。在所有深度下，野边界内的一致性在1%以内，野边界外的一致性在3%以内。
