Desai Dharmin D, Johnson E L, Cordrey Ivan L
Department of Radiation Oncology, CHI Memorial Hospital, Chattanooga, TN, USA.
Department of Radiation Medicine, University of Kentucky Chandler Medical Center, Lexington, KY, USA.
J Appl Clin Med Phys. 2021 Feb;22(2):203-210. doi: 10.1002/acm2.13168. Epub 2021 Jan 25.
The intermediate dose spill for a stereotactic radiosurgery (SRS) plan can be quantified with the metric R50%, defined as the 50% isodose cloud volume (V ) divided by the volume of the planning target volume (PTV). By coupling sound physical principles with the basic definition of R50%, we derive an analytical expression for R50% for a spherical PTV. Our analytical expression depends on three quantities: the surface area of PTV (SA ), the volume of PTV (V ), and the distance of dose drop-off to 50% (Δr). The value of ∆r was obtained from a simple set of cranial phantom plan calculations. We generate values from our analytical expression for R50% (R50% ) and compare the values to clinical R50% values (R50% ) extracted from a previously published SRS data set that spans the V range from 0.15 to 50.1 cm . R50% is smaller than R50% in all cases by an average of 15% ± 7%, and the general trend of R50% vs V is reflected in the same trend of R50% . This comparison suggests that R50% could represent a theoretical lower limit for the clinical SRS data; further investigation is required to confirm this. R50% could provide useful guidance for what might be achievable in SRS planning.
立体定向放射外科（SRS）计划中的中等剂量泄漏可以用R50%这一度量来量化，R50%定义为50%等剂量云体积（V）除以计划靶体积（PTV）的体积。通过将合理的物理原理与R50%的基本定义相结合，我们推导出了球形PTV的R50%的解析表达式。我们的解析表达式取决于三个量：PTV的表面积（SA）、PTV的体积（V）以及剂量下降到50%时的距离（Δr）。Δr的值是通过一组简单的头颅体模计划计算得到的。我们根据R50%的解析表达式生成值（R50% ），并将这些值与从先前发表的SRS数据集中提取的临床R50%值（R50% ）进行比较，该数据集涵盖的V范围为0.15至50.1 cm³。在所有情况下，R50% 均比R50% 小，平均小15%±7%，并且R50% 与V的总体趋势反映在R50% 的相同趋势中。这种比较表明，R50% 可能代表临床SRS数据的理论下限；需要进一步研究来证实这一点。R50% 可为SRS计划中可能实现的目标提供有用的指导。
