Holla Raghavendra, Khanna D, Pillai Bhaskaran K, Jafar Ali K V, Renil Mon P S, Clinto C O, Ganesh Tharmarnadar
Department of Physics, Karunya Institute of Technology and Sciences, Coimbatore, Tamil Nadu, India.
Department of Medical Physics, Amrita Institute of Medical Sciences, Amrita Vishwa Vidyapeetham, Kochi, Kerala, India.
J Med Phys. 2019 Jul-Sep;44(3):145-155. doi: 10.4103/jmp.JMP_17_19.
An experimental method using the linear portion of the relative film dose-response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields.
The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values.
The maximum percentage deviation of PDD with PDD for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDD with PDD for electron beams was within 3% for energy range studied of 8-15 MeV. The gamma passing rates of PDD with PDD were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDD with PDD were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROF was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDD and PDD was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROF using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone.
The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.
提出一种利用射线照相胶片和放射变色胶片的相对胶片剂量响应曲线线性部分的实验方法，该方法可用于确定各种非常小、中等和大辐射野中的相对深度剂量以及小射野的相对输出因子（ROF）。
胶片斜率（FS）法成功应用于在10 cm×10 cm光子束参考条件和标称10 cm×10 cm尺寸电子束限光筒条件下，获取来自Synergy直线加速器（瑞典斯德哥尔摩Elekta AB公司）的光子和电子外照射束的百分深度剂量（PDD）。对于小射野剂量测定，FS法应用于来自直线加速器（瑞典斯德哥尔摩Elekta AB公司）的6 MV光子束的EDR2胶片（美国纽约罗切斯特Carestream Health公司）以及直径为5、7.5、10、12.5和15 mm的小型圆形放射外科限光筒（瑞典斯德哥尔摩Elekta AB公司）。在源皮距100 cm处确定所有这些限光筒的ROF以及直径为5、10和15 mm限光筒的中心轴PDD。使用该技术和Gafchromic EBT3胶片（美国新泽西州阿什兰）确定射波刀系统小射野的ROF。将PDD和ROF与电离室（IC）和蒙特卡罗（MC）模拟值进行比较。
对于4、6和15 MV光子束，PDD与PDD的最大百分比偏差分别在1.9%、2.5%和1.4%以内，深度可达20 cm。在所研究的8 - 15 MeV能量范围内，电子束PDD与PDD的最大百分比偏差在3%以内。PDD与PDD的γ通过率高于96.5%，最大γ值>2，出现在4、6和15 MV光子的零深度处。对于电子束，PDD与PDD之间的γ通过率高于97.7%，最大γ值0.9、1.3和0.7分别出现在8、12和15 MeV的零深度处。对于5 mm限光筒的小射野，ROF为0.665±0.021，而MC法为0.674。对于5 mm和1 mm限光筒，PDD与PDD之间的最大百分比偏差分别为3%和2%，1Dγ通过率分别为95.5%、96%和98%。对于射波刀系统，使用EBT3胶片的ROF与MC公布值在5 mm限光筒时相差在0.2%以内。
作者开发了一种用于光子和电子束相对剂量测定的新颖且更准确的方法。这提供了一种独特的方法，可在剂量梯度陡峭尤其是小射野中以高空间分辨率确定PDD和ROF。
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