Laboratorio de Física Médica, Instituto Nacional de Neurología y Neurocirugía, México.
Med Phys. 2012 Oct;39(10):6111-7. doi: 10.1118/1.4752211.
Radiosurgery uses small fields and high-radiation doses to treat intra- and extracranial lesions in a single session. The lack of a lateral electronic equilibrium and the presence of high-dose gradients in these fields are challenges for adequate measurements. The availability of radiation detectors with the high spatial resolution required is restricted to only a few. Stereotactic diodes and EBT radiochromic films have been demonstrated to be good detectors for small-beam dosimetry. Because the stereotactic diode is the standard measurement for the dosimetry of radiosurgical beams, the goal of this work was to perform measurements with the radiochromic film Gafchromic(®) EBT2 and compare its results with a stereotactic diode.
Total scatter factors, tissue maximum, and off-axis ratios from a 6 MV small photon beams were measured using EBT2 radiochromic film in a water phantom. The film-measured data were evaluated by comparing it with the data measured with a stereotactic field diode (IBA-Dosimetry).
The film and diode measurements had excellent agreement. The differences between the detectors were less than or equal to 2.0% for the tissue maximum and the off-axis ratios. However, for the total scatter factors, there were significant differences, up to 4.9% (relative to the reference field), for field sizes less than 1.0 cm.
This work found that the Gafchromic(®) EBT2 film is adequate for small photon beam measurements, particularly for tissue maximum and off-axis ratios. However, careful attention must be taken when measuring output factors of small beams below 1.0 cm due to the film's energy dependence. The measurement differences may be attributable to the film's active layer composition because EBT2 incorporates higher Z elements (i.e., bromide and potassium), hence revealing a potential energy dependence for the dosimetry of small photon beams.
放射外科使用小射野和高剂量射线单次治疗颅内和颅外病变。这些射野缺乏侧向电子平衡，且存在高剂量梯度，这对充分的测量提出了挑战。具有所需高空间分辨率的辐射探测器的可用性仅限于少数几种。立体定向二极管和 EBT 光致变色胶片已被证明是小射束剂量测量的良好探测器。由于立体定向二极管是放射外科射束剂量测量的标准测量仪器，因此这项工作的目的是使用光致变色胶片 Gafchromic(®) EBT2 进行测量，并将其结果与立体定向二极管进行比较。
在水模体中使用 EBT2 光致变色胶片测量 6MV 小光子射束的总散射因子、组织最大剂量和离轴比。通过将胶片测量数据与立体定向场二极管（IBA-Dosimetry）测量的数据进行比较来评估胶片测量数据。
胶片和二极管测量结果具有很好的一致性。对于组织最大剂量和离轴比，两个探测器之间的差异小于或等于 2.0%。然而，对于总散射因子，在小于 1.0cm 的射野尺寸下，存在显著差异，最大可达 4.9%（相对于参考射野）。
这项工作发现 Gafchromic(®) EBT2 胶片适合于小光子射束测量，特别是对于组织最大剂量和离轴比。然而，在测量小于 1.0cm 的小射束输出因子时，必须格外小心，因为胶片存在能量依赖性。测量差异可能归因于胶片的活性层组成，因为 EBT2 中包含更高 Z 元素（即溴化物和钾），因此对于小光子射束的剂量测量可能存在潜在的能量依赖性。
