2H06-The Physics of Wedge
Chen-Shou Chui
Memorial Sloan-Kettering Cancer Center, New York City, New York.
Wedge is a device commonly used in radiation therapy to shape the dose distribution from external photon beams. It is available on all major manufacturers of radiation therapy machines.
The most basic form of wedge is the physical wedge, made of metals such as lead or stainless steel. On Varian and Siemens machines, the physical wedge is mounted outside the machine head below the collimating jaws, called the ‘external’ wedge. Standard wedge angles are 15°, 30°, 45°, and 60°. On Elekta machines, a single wedge of 60° is mounted inside the machine head, called the ‘internal’ wedge.
Dose calculation for wedged field commonly uses empirical-based methods. With this method, dose is first calculated for the open field; then modified by empirically obtained profiles which account for the wedge shape. These profiles are measured for each wedge angle at a set of selected depths during machine commissioning. The wedge factor, defined as the ratio of the dose at a depth on the central axis of the wedged field to that of the corresponding open field, increases with depth and field size. This increase is primarily due to beam hardening through the metal wedge, and secondarily due to extra scatter from the wedge itself.
For Elekta machines, a single internal 60° wedge is available. Other wedge angles less than 60° can be obtained by combining the 60° wedge field and the open field with proper weights. This is called the ‘universal’ wedge. The respective weights of the wedged and the open fields are determined by the desired wedge angle.
The physical wedge has some inherent undesirable features. Its depth-dose characteristics differ from that of the open field due to beam hardening. Its field size is limited by the size and weight of the wedge. And for external wedges, it’s cumbersome to load and unload. A new form of wedge is the ‘dynamic’ or ‘virtual’ wedge. With this method, the wedge shape is generated not by a physical wedge, but rather by moving one jaw while the beam is on. With proper design of the jaw motion, any arbitrary wedge angle can be produced. The resultant wedged beam is clean, more flexible in terms of field size and wedge angle, and does not require manual loading/unloading.
In some applications, it is desirable to have the wedge oriented in a direction other than the X- or the Y-axis. This can be achieved by combining the wedged fields along the X- and the Y-axis with proper weights, determined by the desired wedge orientation. This is called the ‘omni’ wedge.
The idea of ‘omni’ wedge can be extended further to produce a wedged field of any angle along any orientation. This is accomplished by combining the open field, the wedged fields along the X- and the Y-axis, all with proper weights. This is called the ‘super omni’ wedge. This method greatly simplifies the problem of optimization of wedged fields.
2H06-楔形板的物理学
Chen-Shou Chui
Memorial Sloan-Kettering Cancer Center, New York City, New York.
楔形板是放射治疗中通常用于改变外照射射野剂量分布形状的装置。放射治疗机的主要生产厂商都提供这种装置。
楔形板的基本形式是物理楔形板，由铅或不锈钢材料制成。在瓦里安和西门子的加速器上，物理楔形板置于机头下方准直器口末端，称做外置楔形板。标准楔形板角度是15°，30°，45°，和60°。在医科达加速器上，一块60°的楔形板被置于机头里面，叫做内置楔形板。
楔形野的剂量计算通常使用经验的方法。使用这种方法，首先是对平野所致剂量分布；然后按经验要求的楔形剂量分布和楔形角对平野剂量分布进行修改。对某种楔角的楔形板的剂量分布，应在一组深度下在加速器验收时进行测量。楔形因素定义为某一深度处的楔形野的中心轴深度剂量与平野深度剂量之比，它随深度和射野大小的增加而增加。这种增加主要是由于射线通过金属楔形板后硬化，其次是由于楔形板本身的额外散射。
医科达加速器，提供一块内置的60°楔形板。其它小于60°的楔形板角度可以通过适当地整合60°楔形板野和开野的权重来获得。这被称为“通用”楔形板。楔形板野和开野的各自的权重由所需的楔形板角度决定。
物理楔形板有一些固有的缺点。由于射线的硬化，它的深度-剂量特征与开野有所不同。它的射野尺寸受楔形板尺寸和重量的限制。而且对于外置楔形板，装卸比较麻烦。一种新的楔形板方式称为“动态”或“虚拟”楔形板，这种楔形板产生的楔形分布不是由物理楔形板而是由出光时一侧铅门的移动产生。通过铅门运动的适当设计，可以产生任意楔形角度。产生的楔形射线能量更纯，射野尺寸和楔形板角度更灵活，且不需要手工装卸。
在一些应用中，楔形方向需要顺着一个比X方向或是Y方向更好的方向。这可以通过整合由所需楔形方向决定的适当权重的X方向和Y方向的楔形野实现。这被称为“全向”楔形板。
“全向”楔形板的概念可以延伸产生出任意角度任意方向的楔形野。它是通过整合适当权重的开野、X方向和Y方向的楔形野来完成的。这被称为“超全向”楔形板。这种方法大大简化了楔形野优化的问题。