Verhaegen F, Das I J, Palmans H
Department of Biomedical Physics, Gent University, Belgium.
Phys Med Biol. 1998 Oct;43(10):2755-68. doi: 10.1088/0031-9155/43/10/006.
Small-field and stereotactic radiosurgery (SRS) dosimetry with radiation detectors, used for clinical practice, have often been questioned due to the lack of lateral electron equilibrium and uncertainty in beam energy. A dosimetry study was performed for a dedicated 6 MV SRS unit, capable of generating circular radiation fields with diameters of 1.25-5 cm at isocentre using the BEAM/EGS4 Monte Carlo code. With this code the accelerator was modelled for radiation fields with a diameter as small as 0.5 cm. The radiation fields and dosimetric characteristics (photon spectra, depth doses, lateral dose profiles and cone factors) in a water phantom were evaluated. The cone factor (St) for a specific cone c at depth d is defined as St(d, c) = D(d, c)/D(d, c(ref)), where c(ref) is the reference cone. To verify the Monte Carlo calculations, measurements were performed with detectors commonly used in SRS such as small-volume ion chambers, a diamond detector, TLDs and films. Results show that beam energies vary with cone diameter. For a 6 MV beam, the mean energies in water at the point of maximum dose for a 0.5 cm cone and a 5 cm cone are 2.05 MeV and 1.65 MeV respectively. The values of St obtained by the simulations are in good agreement with the results of the measurements for most detectors. When the lateral resolution of the detectors is taken into account, the results agree within a few per cent for most fields and detectors. The calculations showed a variation of St with depth in the water. Based on calculated electron spectra in water, the validity of the assumption that measured dose ratios are equal to measured detector readings was verified.
用于临床实践的带有辐射探测器的小射野和立体定向放射外科（SRS）剂量测定法，由于缺乏横向电子平衡和束流能量的不确定性，常常受到质疑。对一台专用的6兆伏SRS装置进行了剂量测定研究，该装置能够使用BEAM/EGS4蒙特卡罗代码在等中心产生直径为1.25 - 5厘米的圆形辐射野。利用此代码对加速器进行建模，以模拟直径小至0.5厘米的辐射野。对水模中的辐射野和剂量学特征（光子能谱、深度剂量、横向剂量分布和射束因子）进行了评估。特定射束c在深度d处的射束因子（St）定义为St(d, c) = D(d, c)/D(d, c(ref))，其中c(ref)是参考射束。为了验证蒙特卡罗计算结果，使用了SRS中常用的探测器进行测量，如小体积电离室、金刚石探测器、热释光剂量计和胶片。结果表明束流能量随射束直径变化。对于6兆伏的射束，0.5厘米射束和5厘米射束在最大剂量点处水中的平均能量分别为2.05兆电子伏和1.65兆电子伏。模拟得到的St值与大多数探测器的测量结果吻合良好。考虑探测器的横向分辨率时，大多数射野和探测器的结果在百分之几的范围内相符。计算结果显示St在水中随深度变化。基于水中计算得到的电子能谱，验证了测量剂量比等于测量探测器读数这一假设的有效性。
