Roback D M, Johnson J M, Khan F M, Engeler G P, McGuire W A
Department of Radiation Oncology, University of Minnesota, Minneapolis 55455, USA.
Int J Radiat Oncol Biol Phys. 1997 Mar 15;37(5):1187-92. doi: 10.1016/s0360-3016(97)00108-9.
The spine can be treated with an electron beam when its maximum posterior depth is within the therapeutic range of electrons. Electron fields treated at extended source-to-surface distances (SSDs), however, have larger penumbras and narrower therapeutic isodose widths relative to those at the standard SSD of 100 cm. We investigated the use of tertiary collimation close to the patient surface for these fields to sharpen the penumbra, minimizing dose to normal tissue and maximizing target coverage.
Using film dosimetry in a polystyrene phantom, we measured the dose distribution for electron fields at extended SSD under varying collimation conditions. Beam penumbra and therapeutic width as a function of depth, SSD, applicator insert size, and tertiary collimator opening were determined. We also measured the dose distributions in the junction region for various gaps between x-ray fields and an electron field as used for craniospinal irradiation.
Measurements show that tertiary collimation close to the skin surface reduces penumbra width (lateral distance between the 90 and 20% isodose lines) by 56% and increases therapeutic isodose width (lateral width of the 90% isodose curve) by 25% at a depth of dmax relative to standard collimation. These numbers change to 23 and 13%, respectively, at an average depth of the spine. When lateral brain and posterior spine fields are used to irradiate the entire craniospinal axis, tertiary collimation aids in reducing the volume of the hot spot in the junction region by as much as 10% without compromising target coverage.
Tertiary collimation for extended SSD electron fields is preferable to standard collimation in order to minimize dose to normal tissue and increase target coverage. This technique can be applied to both spinal and craniospinal irradiation. Support structures for the tertiary blocking are needed because the weight of the lead is usually too great for placement on the skin.
当脊柱的最大后深度在电子束的治疗范围内时，可用电子束进行治疗。然而，相对于标准源皮距（SSD）100 cm时的电子野，在延长源皮距下治疗的电子野具有更大的半值层和更窄的治疗等剂量宽度。我们研究了在靠近患者体表处使用三级准直器来锐化半值层，使正常组织剂量最小化并使靶区覆盖最大化。
在聚苯乙烯模体中使用胶片剂量测定法，我们测量了在不同准直条件下延长SSD时电子野的剂量分布。确定了半值层和治疗宽度随深度、SSD、施源器插入尺寸和三级准直器开口的变化情况。我们还测量了用于全脑全脊髓照射的X射线野和电子野之间各种间隙在交界区域的剂量分布。
测量结果表明，相对于标准准直，靠近皮肤表面的三级准直在dmax深度处可使半值层宽度（90%和20%等剂量线之间的横向距离）减小56%，并使治疗等剂量宽度（90%等剂量曲线的横向宽度）增加25%。在脊柱的平均深度处，这些数值分别变为23%和13%。当使用侧脑和后脊柱野照射整个全脑全脊髓轴时，三级准直有助于在不影响靶区覆盖的情况下将交界区域的热点体积减少多达10%。
对于延长SSD的电子野，三级准直优于标准准直，以便使正常组织剂量最小化并增加靶区覆盖。该技术可应用于脊柱和全脑全脊髓照射。由于铅的重量通常太大，无法放置在皮肤上，因此需要三级阻挡的支撑结构。
