Biggs P, Capalucci J, Russell M
Department of Radiation Medicine, Massachusetts General Hospital, Boston 02114.
Med Phys. 1991 Jul-Aug;18(4):753-8. doi: 10.1118/1.596669.
The penumbra from a 10-MV x-ray beam has been measured using various field-shaping blocks located in the normal blocking tray position and compared to the penumbra of the adjustable photon jaws of the machine. The results showed that the penumbra was substantially degraded by these blocks only for field sizes greater than 15 x 15 cm2 and at the depth of maximum dose, dmax. At depths of 11 cm or greater, there was a significant difference only for the same range of field sizes in the 80% to 50% dose region. There was no significant difference at these depths for either the 95%-50% or the 90%-50% dose regions. No significant difference was observed between the use of lead and tungsten blocks and also between lead blocks with a straight edge and those with either an 0.5-mm or a 1-mm step in the face. Since the width of the 95% dose relative to the 50% dose is of greatest interest clinically, straight-edged blocks are as effective as divergent blocks in most situations. These results imply that the design and complexity of a multileaf collimator can be greatly simplified from a double focusing device to one that is single focusing in the plane orthogonal to the leaves.
使用位于常规阻挡托盘位置的各种射野成形块测量了10兆伏X射线束的半值层，并与机器可调节光子准直器的半值层进行了比较。结果表明，仅在射野尺寸大于15×15平方厘米且在最大剂量深度dmax时，这些射野成形块才会使半值层显著变差。在11厘米或更深的深度，仅在80%至50%剂量区域内相同射野尺寸范围内存在显著差异。在这些深度，95%-50%或90%-50%剂量区域均无显著差异。使用铅块和钨块之间以及直边铅块与表面有0.5毫米或1毫米台阶的铅块之间均未观察到显著差异。由于临床上最关注的是95%剂量相对于50%剂量的宽度，在大多数情况下，直边射野成形块与发散射野成形块的效果相同。这些结果表明，多叶准直器的设计和复杂性可以从双聚焦装置大大简化为在与叶片正交的平面内单聚焦的装置。
