Carlsson Tedgren Asa, Carlsson Gudrun Alm
Department of Medical and Health Sciences, Radiation Physics, Faculty of Health Sciences, Linköping University, SE-581 85 Linköping, Sweden.
Med Phys. 2009 Jun;36(6):2228-35. doi: 10.1118/1.3121508.
In treatment planning of brachytherapy, absorbed dose is calculated by superposing predetermined distributions of absorbed dose to water in water for the single source according to the irradiation pattern [i.e., placement of the source(s) or dwelling position(s)]. Single-source reference water data are derived from Monte Carlo (MC) simulations and/or experiments. For reasons of positional accuracy, experimental brachytherapy dosimetry is most often performed in plastic phantoms. This work investigates the water equivalence of phantoms made from polystyrene, PMMA, and solid water for 192Ir dosimetry. The EGSnrc MC code is used to simulate radial absorbed dose distributions in cylindrical phantoms of dimensions ranging in size from diameter and height of 20 cm to diameter and height of 40 cm. Water equivalence prevails if the absorbed dose to water in the plastic phantom is the same as the absorbed dose to water in a water phantom at equal distances from the source. It is shown that water equivalence at a specified distance from the source depends not only on the size of the plastic phantom but also on the size of the water phantom used for comparison. Compared to equally sized water phantoms, phantoms of polystyrene are less water equivalent than phantoms of PMMA and solid water but compared to larger water phantoms they are the most water equivalent. Although phantom dimension is the most important single factor influencing the dose distributions around 192Ir sources, the effect of material properties is non-negligible and becomes increasingly important as phantom dimensions increase. The importance of knowing the size of the water phantom whose data underlies treatment planning systems, when using such data as a reference in, e.g., detector evaluation studies, is discussed. To achieve the highest possible accuracy in experimental dosimetry, phantom-specific correction factors should be used.
在近距离放射治疗的治疗计划中，吸收剂量是通过根据照射模式（即源的放置或驻留位置）将单个源在水中的预定吸收剂量分布叠加到水中来计算的。单源参考水数据来自蒙特卡罗（MC）模拟和/或实验。出于位置精度的原因，实验性近距离放射治疗剂量测定通常在塑料模体中进行。这项工作研究了由聚苯乙烯、聚甲基丙烯酸甲酯和固体水制成的模体用于192Ir剂量测定时的水等效性。EGSnrc MC代码用于模拟尺寸范围从直径和高度为20 cm到直径和高度为40 cm的圆柱形模体中的径向吸收剂量分布。如果在距源相等距离处，塑料模体中水的吸收剂量与水模体中水的吸收剂量相同，则存在水等效性。结果表明，在距源指定距离处的水等效性不仅取决于塑料模体的尺寸，还取决于用于比较的水模体的尺寸。与尺寸相同的水模体相比，聚苯乙烯模体的水等效性低于聚甲基丙烯酸甲酯模体和固体水模体，但与较大的水模体相比，它们的水等效性最高。尽管模体尺寸是影响192Ir源周围剂量分布的最重要单一因素，但材料特性的影响不可忽略，并且随着模体尺寸的增加变得越来越重要。讨论了在例如探测器评估研究中使用治疗计划系统所基于的数据作为参考时，了解该数据所依据的水模体尺寸的重要性。为了在实验剂量测定中实现尽可能高的精度，应使用特定于模体的校正因子。
