Pirotta Martin, Aquilina Dorothy, Bhikha Tilluck, Georg Dietmar
Physics Office, Radiotherapy Department, Sir Paul Boffa Hospital, Floriana, Malta.
Z Med Phys. 2005;15(4):235-46. doi: 10.1078/0939-3889-00280.
The ESTRO formalism for monitor unit (MU) calculations was evaluated and implemented to replace a previous methodology based on dosimetric data measured in a full-scatter phantom. This traditional method relies on data normalised at the depth of dose maximum (Zm), as well as on the utilisation of the BJR 25 table for the conversion of rectangular fields into equivalent square fields. The treatment planning system (TPS) was subsequently updated to reflect the new beam data normalised at a depth ZR of 10 cm. Comparisons were then carried out between the ESTRO formalism, the Clarkson-based dose calculation algorithm on the TPS (with beam data normalised at Zm and ZR), and the traditional "full-scatter" methodology. All methodologies, except for the "full-scatter" methodology, separated head-scatter from phantom-scatter effects and none of the methodologies; except for the ESTRO formalism, utilised wedge depth dose information for calculations. The accuracy of MU calculations was verified against measurements in a homogeneous phantom for square and rectangular open and wedged fields, as well as blocked open and wedged fields, at 5, 10, and 20 cm depths, under fixed SSD and isocentric geometries for 6 and 10 MV. Overall, the ESTRO Formalism showed the most accurate performance, with the root mean square (RMS) error with respect to measurements remaining below 1% even for the most complex beam set-ups investigated. The RMS error for the TPS deteriorated with the introduction of a wedge, with a worse RMS error for the beam data normalised at Zm (4% at 6 MV and 1.6% at 10 MV) than at ZR (1.-9% at 6 MV and 1.1% at 10 MV). The further addition of blocking had only a marginal impact on the accuracy of this methodology. The "full-scatter" methodology showed a loss in accuracy for calculations involving either wedges or blocking, and performed worst for blocked wedged fields (RMS errors of 7.1% at 6 MV and 5% at 10 MV). The origins of these discrepancies were quantified and the shortcomings of these MU calculation methodologies are discussed in the paper.
对用于监测单位（MU）计算的欧洲放射治疗与肿瘤学学会（ESTRO）形式体系进行了评估和实施，以取代先前基于在全散射体模中测量的剂量学数据的方法。这种传统方法依赖于在剂量最大值深度（Zm）处归一化的数据，以及利用BJR 25表将矩形野转换为等效方形野。随后更新了治疗计划系统（TPS），以反映在10 cm深度ZR处归一化的新射束数据。然后对ESTRO形式体系、TPS上基于克拉克森的剂量计算算法（射束数据在Zm和ZR处归一化）以及传统的“全散射”方法进行了比较。除“全散射”方法外，所有方法都将头部散射与体模散射效应分开，并且除ESTRO形式体系外，没有一种方法在计算中使用楔形深度剂量信息。针对在5、10和20 cm深度处，在固定源皮距（SSD）和等中心几何条件下，6和10 MV的方形和矩形开放及楔形野以及遮挡的开放及楔形野，在均匀体模中的测量结果，验证了MU计算的准确性。总体而言，ESTRO形式体系表现出最准确的性能，即使对于所研究的最复杂射束设置，相对于测量的均方根（RMS）误差仍保持在1%以下。引入楔形后，TPS的RMS误差恶化，射束数据在Zm处归一化时的RMS误差（6 MV时为4%，10 MV时为1.6%）比在ZR处归一化时更差（6 MV时为1.9%，10 MV时为1.1%）。进一步增加遮挡对该方法的准确性影响很小。“全散射”方法在涉及楔形或遮挡的计算中显示出准确性损失，并且在遮挡的楔形野中表现最差（6 MV时RMS误差为7.1%，10 MV时为5%）。本文对这些差异的来源进行了量化，并讨论了这些MU计算方法的缺点。
