Department of Human Oncology, University of Wisconsin-Madison, Madison, Wisconsin 53792, USA.
Med Phys. 2011 Jul;38(7):4081-5. doi: 10.1118/1.3597834.
Small field dosimetry is prone to uncertainties due to the lack of electronic equilibrium and the use of the correct detector size relative to the field size measured. It also exhibits higher sensitivity to setup errors as well as large variation in output with field size and shape. Radiochromic film is an attractive method for reference dosimetry in small fields due to its ability to provide 2D dose measurements while having minimal impact on the dose distribution. Gafchromic EBT2 has a dose range of up to 40 Gy; therefore, it could potentially be useful for high dose reference dosimetry with high spatial resolution. This is a requirement in stereotactic radiosurgery deliveries, which deliver high doses per fraction to small targets.
Targets of 4 mm and 12 mm diameters were treated to a minimum peripheral dose of 21 Gy prescribed to 80% of the maximum dose in one fraction. Target doses were measured with EBT2 film (both targets) and an ion chamber (12 mm target only). Measured doses were compared with planned dose distributions using profiles through the target and minimum peripheral dose coverage.
The measured target doses and isodose coverage agreed with the planned dose within +/-1 standard deviation of three measurements, which were 2.13% and 2.5% for the 4 mm and 12 mm targets, respectively.
EBT2 film is a feasible dosimeter for high dose per fraction reference 2D dosimetry.
由于缺乏电子平衡以及相对于所测量的场大小使用正确的探测器尺寸，小场剂量测定容易出现不确定性。它还表现出对设置误差以及随场大小和形状的输出变化的更高敏感性。由于其能够提供二维剂量测量，同时对剂量分布的影响最小，因此，在小场中，光致变色胶片是一种有吸引力的参考剂量测定方法。Gafchromic EBT2 的剂量范围高达 40Gy；因此，它有可能用于具有高空间分辨率的高剂量参考剂量测定。这是立体定向放射外科输送的要求，其每部分向小靶区输送高剂量。
以最小外周剂量 21Gy 治疗 4mm 和 12mm 直径的靶区，该剂量规定为一个部分中最大剂量的 80%。使用 EBT2 胶片（两个靶区）和离子室（仅 12mm 靶区）测量靶区剂量。通过靶区和最小外周剂量覆盖的剖面比较测量剂量与计划剂量分布。
测量的靶区剂量和等剂量覆盖率与计划剂量在三个测量值的正负 1 个标准差内一致，对于 4mm 和 12mm 靶区，分别为 2.13%和 2.5%。
EBT2 胶片是一种可行的高剂量部分参考二维剂量测定剂量计。
