Sánchez-Doblado F, Andreo P, Capote R, Leal A, Perucha M, Arráns R, Núñez L, Mainegra E, Lagares J I, Carrasco E
Radiofísica, Hospital Univ Virgen Macarena, Avda Dr Fedriani s/n, E-4 1009 Sevilla, Spain.
Phys Med Biol. 2003 Jul 21;48(14):2081-99. doi: 10.1088/0031-9155/48/14/304.
Absolute dosimetry with ionization chambers of the narrow photon fields used in stereotactic techniques and IMRT beamlets is constrained by lack of electron equilibrium in the radiation field. It is questionable that stopping-power ratio in dosimetry protocols, obtained for broad photon beams and quasi-electron equilibrium conditions, can be used in the dosimetry of narrow fields while keeping the uncertainty at the same level as for the broad beams used in accelerator calibrations. Monte Carlo simulations have been performed for two 6 MV clinical accelerators (Elekta SL-18 and Siemens Mevatron Primus), equipped with radiosurgery applicators and MLC. Narrow circular and Z-shaped on-axis and off-axis fields, as well as broad IMRT configured beams, have been simulated together with reference 10 x 10 cm2 beams. Phase-space data have been used to generate 3D dose distributions which have been compared satisfactorily with experimental profiles (ion chamber, diodes and film). Photon and electron spectra at various depths in water have been calculated, followed by Spencer-Attix (delta = 10 keV) stopping-power ratio calculations which have been compared to those used in the IAEA TRS-398 code of practice. For water/air and PMMA/air stopping-power ratios, agreements within 0.1% have been obtained for the 10 x 10 cm2 fields. For radiosurgery applicators and narrow MLC beams, the calculated s(w,air) values agree with the reference within +/-0.3%, well within the estimated standard uncertainty of the reference stopping-power ratios (0.5%). Ionization chamber dosimetry of narrow beams at the photon qualities used in this work (6 MV) can therefore be based on stopping-power ratios data in dosimetry protocols. For a modulated 6 MV broad beam used in clinical IMRT, s(w,air) agrees within 0.1% with the value for 10 x 10 cm2, confirming that at low energies IMRT absolute dosimetry can also be based on data for open reference fields. At higher energies (24 MV) the difference in s(w,air) was up to 1.1%, indicating that the use of protocol data for narrow beams in such cases is less accurate than at low energies, and detailed calculations of the dosimetry parameters involved should be performed if similar accuracy to that of 6 MV is sought.
立体定向技术和调强放疗子野中使用的窄光子束电离室绝对剂量测量，因辐射场中缺乏电子平衡而受到限制。对于在宽光子束和准电子平衡条件下获得的剂量测量规程中的阻止本领比，能否用于窄野剂量测量并将不确定度保持在与加速器校准中使用的宽束相同水平，这是值得怀疑的。已对两台配备了放射外科施源器和多叶准直器的6兆伏临床加速器（医科达SL - 18和西门子Mevatron Primus）进行了蒙特卡罗模拟。模拟了窄圆形和Z形的轴上和离轴野，以及调强放疗配置的宽束，同时还模拟了参考10×10平方厘米的射野。利用相空间数据生成了三维剂量分布，并与实验轮廓（电离室、二极管和胶片）进行了令人满意的比较。计算了水中不同深度处的光子和电子能谱，随后进行了斯宾塞 - 阿蒂克斯（δ = 10千电子伏）阻止本领比计算，并与国际原子能机构TRS - 398实用规程中使用的结果进行了比较。对于水/空气和聚甲基丙烯酸甲酯/空气的阻止本领比，10×10平方厘米射野的结果在0.1%以内相符。对于放射外科施源器和窄多叶准直器束，计算得到的s(w,air)值与参考值在±0.3%以内相符，完全在参考阻止本领比估计的标准不确定度（0.5%）范围内。因此，在本工作所使用的光子品质（6兆伏）下，窄束的电离室剂量测量可以基于剂量测量规程中的阻止本领比数据。对于临床调强放疗中使用的调制6兆伏宽束，s(w,air)与10×10平方厘米射野的值在0.1%以内相符，这证实了在低能量下，调强放疗绝对剂量测量也可以基于开放参考射野的数据。在较高能量（24兆伏）时，s(w,air)的差异高达1.1%，这表明在这种情况下，将规程数据用于窄束的准确性不如低能量时，若要达到与6兆伏类似的精度，则应进行所涉及剂量测量参数的详细计算。
