Rivard Mark J, Melhus Christopher S, Granero Domingo, Perez-Calatayud Jose, Ballester Facundo
Department of Radiation Oncology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA.
Med Phys. 2009 Jun;36(6):1968-75. doi: 10.1118/1.3121510.
Certain brachytherapy dose distributions, such as those for LDR prostate implants, are readily modeled by treatment planning systems (TPS) that use the superposition principle of individual seed dose distributions to calculate the total dose distribution. However, dose distributions for brachytherapy treatments using high-Z shields or having significant material heterogeneities are not currently well modeled using conventional TPS. The purpose of this study is to establish a new treatment planning technique (Tufts technique) that could be applied in some clinical situations where the conventional approach is not acceptable and dose distributions present cylindrical symmetry. Dose distributions from complex brachytherapy source configurations determined with Monte Carlo methods were used as input data. These source distributions included the 2 and 3 cm diameter Valencia skin applicators from Nucletron, 4-8 cm diameter AccuBoost peripheral breast brachytherapy applicators from Advanced Radiation Therapy, and a 16 mm COMS-based eye plaque using 103Pd, 125I, and 131Cs seeds. Radial dose functions and 2D anisotropy functions were obtained by positioning the coordinate system origin along the dose distribution cylindrical axis of symmetry. Origin:tissue distance and active length were chosen to minimize TPS interpolation errors. Dosimetry parameters were entered into the PINNACLE TPS, and dose distributions were subsequently calculated and compared to the original Monte Carlo-derived dose distributions. The new planning technique was able to reproduce brachytherapy dose distributions for all three applicator types, producing dosimetric agreement typically within 2% when compared with Monte Carlo-derived dose distributions. Agreement between Monte Carlo-derived and planned dose distributions improved as the spatial resolution of the fitted dosimetry parameters improved. For agreement within 5% throughout the clinical volume, spatial resolution of dosimetry parameter data < or = 0.1 cm was required, and the virtual brachytherapy source data set included over 5000 data points. On the other hand, the lack of consideration for applicator heterogeneity effect caused conventional dose overestimates exceeding an order of magnitude in regions of clinical interest. This approach is rationalized by the improved dose estimates. In conclusion, a new technique was developed to incorporate complex Monte Carlo-based brachytherapy dose distributions into conventional TPS. These results are generalizable to other brachytherapy source types and other TPS.
某些近距离放射治疗剂量分布，如低剂量率前列腺植入的剂量分布，很容易通过治疗计划系统（TPS）进行建模，这些系统利用单个籽源剂量分布的叠加原理来计算总剂量分布。然而，使用高原子序数屏蔽或存在显著材料不均匀性的近距离放射治疗的剂量分布，目前使用传统TPS无法很好地建模。本研究的目的是建立一种新的治疗计划技术（塔夫茨技术），该技术可应用于某些传统方法不可接受且剂量分布呈圆柱对称的临床情况。用蒙特卡罗方法确定的复杂近距离放射治疗源配置的剂量分布用作输入数据。这些源分布包括核通公司直径为2厘米和3厘米的巴伦西亚皮肤施源器、先进放射治疗公司直径为4 - 8厘米的AccuBoost外周乳腺近距离放射治疗施源器，以及使用103Pd、125I和131Cs籽源的基于16毫米COMS的眼敷贴器。通过将坐标系原点沿剂量分布圆柱对称轴定位来获得径向剂量函数和二维各向异性函数。选择源到组织的距离和有效长度以最小化TPS插值误差。将剂量学参数输入到PINNACLE TPS中，随后计算剂量分布并与原始蒙特卡罗导出的剂量分布进行比较。新的计划技术能够重现所有三种施源器类型的近距离放射治疗剂量分布，与蒙特卡罗导出的剂量分布相比，剂量学一致性通常在2%以内。随着拟合剂量学参数空间分辨率的提高，蒙特卡罗导出的剂量分布与计划剂量分布之间的一致性得到改善。为了在整个临床体积内达到5%以内的一致性，需要剂量学参数数据的空间分辨率≤0.1厘米，并且虚拟近距离放射治疗源数据集包含超过5000个数据点。另一方面，由于未考虑施源器不均匀性效应，在临床关注区域导致传统剂量高估超过一个数量级。通过改进的剂量估计使该方法合理化。总之，开发了一种新技术，将基于复杂蒙特卡罗的近距离放射治疗剂量分布纳入传统TPS。这些结果可推广到其他近距离放射治疗源类型和其他TPS。
