Wilcox Ellen E, Daskalov George M
Radiation Oncology Department, Saint Francis Hospital, 114 Woodland Street, Hartford, Connecticut 06105, USA.
Med Phys. 2007 Jun;34(6):1967-74. doi: 10.1118/1.2734384.
External beam therapy (EBT) GAFCHROMIC film is evaluated for dosimetry and characterization of the CyberKnife radiation beams. Percentage depth doses, lateral beam profiles, and output factors are measured in solid water using EBT GAFCHROMIC film (International Specialty Products, Wayne, NJ) for the 6 MV radiation beams of diameter 5 to 60 mm produced by the CyberKnife (Accuray, Sunnyvale, CA). The data are compared to those measured with the PTW 60008 diode and the Wellhofer CC01 ion chamber in water. For the small radiation field sizes used in stereotactic radiosurgery, lateral electronic disequilibrium and steep dose gradients exist in a large portion of these fields, requiring the use of high-resolution measurement techniques. For small beams, the detector size approaches the dimensions of the beam and adversely affects measurement accuracy in regions where the gradient varies across the detector. When film is the detector, the scanning system is usually the resolution-limiting component. Radiographic films based upon silver halide (AgH) emulsions are widely used for relative dosimetry of external radiation treatment beams in the megavoltage energy range, because of their good spatial resolution and capability to provide integrated dosimetry over two dimensions. Film dosimetry, however, has drawbacks due to its steep energy dependence at low photon energies as well as film processor and densitometer artifacts. EBT radiochromic film, introduced in 2004 specifically for IMRT dosimetry, may be a detector of choice for the characterization of small radiosurgical beams, because of its near-tissue equivalence, radiation beam energy independence, high spatial resolution, and self developing properties. For radiation beam sizes greater than 10 mm, the film measurements were identical to those of the diode and ion chamber. For the smaller beam diameters of 7.5 and 5 mm, however, there were differences in the data measured with the different detectors, which are attributed to their different spatial resolution and non-water-equivalence.
对用于剂量测定和射波刀辐射束特性表征的外照射放疗（EBT）GAFCHROMIC薄膜进行了评估。使用EBT GAFCHROMIC薄膜（国际特种产品公司，新泽西州韦恩）在固体水中测量了由射波刀（Accuray公司，加利福尼亚州桑尼维尔）产生的直径为5至60mm的6MV辐射束的百分深度剂量、侧向射束剖面和输出因子。将这些数据与在水中使用PTW 60008二极管和韦尔霍费尔CC01电离室测量的数据进行比较。对于立体定向放射外科中使用的小辐射野尺寸，在这些野的很大一部分中存在侧向电子不平衡和陡峭的剂量梯度，这需要使用高分辨率测量技术。对于小射束，探测器尺寸接近射束尺寸，并且在梯度在探测器上变化的区域中会对测量精度产生不利影响。当使用薄膜作为探测器时，扫描系统通常是分辨率限制组件。基于卤化银（AgH）乳剂的射线照相胶片由于其良好的空间分辨率和在二维上提供积分剂量测定的能力，被广泛用于兆伏能量范围内的外照射治疗射束的相对剂量测定。然而，胶片剂量测定由于其在低光子能量下陡峭的能量依赖性以及胶片冲洗机和密度计伪影而存在缺点。2004年专门为调强放疗剂量测定引入的EBT放射变色薄膜，由于其接近组织等效性、辐射束能量独立性、高空间分辨率和自显影特性，可能是表征小放射外科射束的首选探测器。对于大于10mm的辐射束尺寸，薄膜测量结果与二极管和电离室的测量结果相同。然而，对于7.5mm和5mm的较小射束直径，不同探测器测量的数据存在差异，这归因于它们不同的空间分辨率和非水等效性。
