School of Applied Science, RMIT University, GPO Box 2476, Melbourne, Victoria 3000, Australia.
Phys Med Biol. 2010 Feb 7;55(3):873-81. doi: 10.1088/0031-9155/55/3/021. Epub 2010 Jan 14.
Despite the highly localized doses that may be delivered via stereotactic radiotherapy, a small dose is nonetheless delivered to out-of-field regions, which may cause detriment to the patient. In this work, a systematic set of dose measurements have been undertaken up to a distance of 45 cm from the isocentre, for stereotactic fields shaped by a BrainLAB mini-multileaf collimator (MMLC) mounted on a Varian 600C linear accelerator. A range of treatment parameters were varied so as to determine the factors of greatest influence and establish relationships with dose. The commercial treatment planning software (TPS) miscalculates the dose to out-of-field regions. Measured dose decreases consistently out to 45 cm, whereas the TPS decreases out to 10-15 cm, at which point the predicted dose is constant. At the 5-10 cm off-axis distance (OAD), measurements indicate doses of about 5-10% of the dose at the isocentre, 1% at 15 cm OAD and 0.1% at 45 cm OAD. There are several observed trends. Greater MMLC field sizes (with static jaw) result in higher out-of-field dose, as do shallower depths. The source-to-surface distance does not greatly influence peripheral dose. However, the results given in this work do indicate that simple treatment arrangements, such as preferable collimator rotation, would in certain cases reduce out-of-field dose by an order of magnitude. Peripheral dose raises questions of treatment optimization, particularly in cases where patients have a long life expectancy in which secondary effects may become manifest, such as in the treatment of paediatric patients or those with a non-malignant primary. For instance, for a 20 Gy hypo-fractionated treatment, dose to out-of-field regions is of the order of cGy-a substantial dose in radiation protection terms.
尽管立体定向放射治疗可能会提供高度局部的剂量，但仍会向场域外区域输送小剂量的辐射，这可能会对患者造成损害。在这项工作中，我们对距离等中心点 45 厘米范围内的立体定向场进行了系统的剂量测量，这些场是由安装在瓦里安 600C 线性加速器上的 BrainLAB 迷你多叶准直器（MMLC）形成的。我们改变了一系列治疗参数，以确定影响最大的因素，并建立与剂量的关系。商业治疗计划软件（TPS）错误地计算了场外区域的剂量。测量结果表明，剂量一直持续下降到 45 厘米，而 TPS 则下降到 10-15 厘米，此时预测剂量保持不变。在离轴距离（OAD）为 5-10 厘米处，测量结果表明，离轴距离为 5-10 厘米处的剂量约为等中心剂量的 5-10%，15 厘米处的剂量为 1%，45 厘米处的剂量为 0.1%。观察到了几个趋势。更大的 MMLC 射野尺寸（采用静态遮线器）会导致更高的场外剂量，较浅的深度也是如此。源皮距对周围剂量的影响不大。然而，本工作中的结果确实表明，在某些情况下，简单的治疗安排，如优选的准直器旋转，可以将场外剂量降低一个数量级。周边剂量引发了治疗优化的问题，特别是在患者预期寿命较长的情况下，二次效应可能会显现出来，例如在治疗儿科患者或非恶性原发性疾病的患者时。例如，对于 20Gy 的亚分次治疗，场外区域的剂量约为 cGy-这在放射防护方面是一个相当大的剂量。
