Awan Shahid B, Meigooni Ali S, Mokhberiosgouei Ramin, Hussain Manzoor
Department of Radiation Medicine, University of Kentucky, Lexington, Kentucky 40536, USA.
Med Phys. 2006 Nov;33(11):4271-9. doi: 10.1118/1.2357024.
The original and updated protocols recommended by Task Group 43 from the American Association of Physicists in Medicine (i.e., TG-43 and TG-43U1, respectively), have been introduced to unify brachytherapy source dosimetry around the world. Both of these protocols are based on experiences with sources less than 1.0 cm in length. TG-43U1 recommends that for 103Pd sources, 2D anisotropy function F(r, theta), should be tabulated at a minimum for radial distances of 0.5, 1.0, 2.0, 3.0, and 5.0 cm. Anisotropy functions defined in these protocols are only valid when the point of calculation does not fall on the active length of the source. However, for elongated brachytherapy sources (active length >1 cm), some of the calculation points with r < 1/2 active length and small theta may fall on the source itself and there is no clear recommendation to handle this situation. In addition, the linear interpolation technique recommended by TG-43U1 is found to be valid for seed types of sources as the difference between F(r, theta) for two consecutive radii is <10%. However, in the present investigations it has been found that values of F(r, 5 degrees) for a 5 cm long RadioCoil 103Pd source at radial distances of 2.5, 3.0, and 4.0 cm were 2.95, 1.74, and 1.19, respectively, with differences up to about a factor of 3. Therefore, the validity of the linear interpolation technique for an elongated brachytherapy source with such a large variation in F(r, theta) needs to be investigated. In this project, application of the TG-43U1 formalism for dose calculation around an elongated RadioCoil 103Pd brachytherapy source has been investigated. In addition, the linear interpolation techniques as described in TG-43U1 for seed type sources have been evaluated for a 5.0 cm long RadioCoil 103Pd brachytherapy source. Application of a polynomial fit to F(r, theta) has also been investigated as an alternate approach to the linear interpolation technique. The results of these investigations indicate that the TG-43U1 formalism can be extended for elongated brachytherapy sources, if the two-dimensional (2D) anisotropy function is tabulated at a minimum for radial distances of 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 cm, L/2, and L/2 +/- 0.2 cm. Moreover, with the addition of recommended radial distances for 2D anisotropy functions, the linear interpolation technique more closely replicates Monte Carlo simulated data than a polynomial fit.
美国医学物理学家协会第43任务组推荐的原始及更新协议（即分别为TG - 43和TG - 43U1）已被引入，以统一全球近距离放射治疗源剂量测定方法。这两个协议均基于长度小于1.0厘米的源的经验。TG - 43U1建议，对于¹⁰³Pd源，二维各向异性函数F(r, θ)应至少在径向距离为0.5、1.0、2.0、3.0和5.0厘米时制成表格。这些协议中定义的各向异性函数仅在计算点不落在源的活性长度上时才有效。然而，对于细长型近距离放射治疗源（活性长度>1厘米），一些r < 1/2活性长度且θ较小的计算点可能落在源本身上，并且对于如何处理这种情况没有明确的建议。此外，发现TG - 43U1推荐的线性插值技术对于种子型源是有效的，因为两个连续半径处的F(r, θ)之差<10%。然而，在目前的研究中发现，对于一个5厘米长的RadioCoil¹⁰³Pd源，在径向距离为2.5、3.0和4.0厘米时，F(r, 5°)的值分别为2.95、1.74和1.19，差异高达约3倍。因此，对于F(r, θ)变化如此大的细长型近距离放射治疗源，线性插值技术的有效性需要进行研究。在这个项目中，研究了TG - 43U1形式主义在细长型RadioCoil¹⁰³Pd近距离放射治疗源周围剂量计算中的应用。此外，对于一个5.0厘米长的RadioCoil¹⁰³Pd近距离放射治疗源，评估了TG - 43U1中描述的种子型源的线性插值技术。还研究了将多项式拟合应用于F(r, θ)作为线性插值技术的替代方法。这些研究结果表明，如果二维各向异性函数至少在径向距离为0.2、0.5、1.0、1.5、2.0、2.5、3.0、3.5、4.0、4.5、5.0厘米、L/2以及L/2 +/- 0.2厘米时制成表格，TG - 43U1形式主义可以扩展到细长型近距离放射治疗源。此外，随着二维各向异性函数推荐径向距离的增加，线性插值技术比多项式拟合更能紧密地复制蒙特卡罗模拟数据。
