Medical Physics Unit, McGill University, Montreal General Hospital (L5-113), 1650 Cedar Avenue, Montreal, Quebec H3G 1A4, Canada.
Med Phys. 2010 Jun;37(6):2404-13. doi: 10.1118/1.3392247.
To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields.
A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [Alfonso et al., Med. Phys. 35, 5179-5186 (2008)], a candidate plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10 x 10 cm2 field was measured using three radiation detectors: Gafchromic EBT films, a diamond detector, and a guarded liquid-filled ionization chamber developed in-house (GLIC-03). Pcsr correction factors [formula in text] were determined for five different types of air-filled ionization chambers (Exradin A12, NE-2571, Exradin A1SL, Exradin A14, and PinPoint 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery).
The combined standard uncertainty in measuring the correction factor [formula in text] using the three dosimetry techniques was 0.3%. For all the air-filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than +/- 0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955-0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922-1.0048. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber-type dependence.
The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factors [formula in text] for air-filled ionization chambers at the 0.3% 1sigma uncertainty level. These techniques can be used to determine criteria for the selection of plan-class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.
为非标准复合场中的参考剂量测量建立准确的实验剂量学技术。
构建了一个充满水的圆柱形 PMMA 体模，在体模中心测量头颈部调强放疗的参考水吸收剂量。基于用于非标准场参考剂量学的新形式主义[Alfonso 等人，医学物理学 35,5179-5186（2008）]，在体模的 CT 图像上创建了典型头颈部调强放疗的候选计划类特定参考（pcsr）场。使用三种辐射探测器：Gafchromic EBT 胶片、金刚石探测器和内部开发的带保护的液体填充电离室（GLIC-03），将 pcsr 场中的水吸收剂量归一化为参考 10×10 cm2 场中的水吸收剂量。为五种不同类型的空气填充电离室（Exradin A12、NE-2571、Exradin A1SL、Exradin A14 和 PinPoint 31006）确定了 pcsr 校正因子[文本中的公式]，一种是在完全旋转的输送中，另一种是在相同的 MLC 设置和权重下，但从单个旋转角度输送（塌陷输送）。
使用三种剂量学技术测量校正因子[文本中的公式]的组合标准不确定度为 0.3%。对于所有测试的空气填充电离室和 pcsr 场，校正因子的差异不超过 +/- 0.8%。对于完全旋转输送，校正因子在 0.9955-0.9986 的窄范围内，而在塌陷输送中，它们在稍宽的 0.9922-1.0048 范围内。在塌陷输送中，Farmer 型电离室（Exradin A12 和 NE2571）的校正因子非常相似（分别为 0.9922 和 0.9931），而较小电离室的校正因子则表现出更明显的电离室类型依赖性。
作者已经建立了三种实验剂量学技术，可在 0.3%1sigma 不确定度水平下对空气填充电离室的非标准场校正因子[文本中的公式]进行参考测量。这些技术可用于确定计划类特定参考场的选择标准，并最终改善非标准场的临床参考剂量学。
