Département de Physique, de Génie Physique et d'Optique, Université Laval, Québec, Québec G1K 7P4, Canada.
Med Phys. 2012 Jan;39(1):429-36. doi: 10.1118/1.3666765.
To compare the performance of plastic scintillation detectors (PSD) for quality assurance (QA) in stereotactic radiosurgery conditions to a microion-chamber (IC), Gafchromic EBT2 films, 60 008 shielded photon diode (SD) and unshielded diodes (UD), and assess a new 2D crosshair array prototype adapted to small field dosimetry.
The PSD consists of a 1 mm diameter by 1 mm long scintillating fiber (BCF-60, Saint-Gobain, Inc.) coupled to a polymethyl-methacrylate optical fiber (Eska premier, Mitsubishi Rayon Co., Ltd., Tokyo, Japan). Output factors (S(c,p)) for apertures used in radiosurgery ranging from 4 to 40 mm in diameter have been measured. The PSD crosshair array (PSDCA) is a water equivalent device made up of 49 PSDs contained in a 1.63 cm radius area. Dose profiles measurements were taken for radiosurgery fields using the PSDCA and were compared to other dosimeters. Moreover, a typical stereotactic radiosurgery treatment using four noncoplanar arcs was delivered on a spherical phantom in which UD, IC, or PSD was placed. Using the Xknife planning system (Integra Radionics Burlington, MA), 15 Gy was prescribed at the isocenter, where each detector was positioned.
Output Factors measured by the PSD have a mean difference of 1.3% with Gafchromic EBT2 when normalized to a 10 × 10 cm(2) field, and 1.0% when compared with UD measurements normalized to the 35 mm diameter cone. Dose profiles taken with the PSD crosshair array agreed with other single detectors dose profiles in spite of the presence of the 49 PSDs. Gamma values comparing 1D dose profiles obtained with PSD crosshair array with Gafchromic EBT2 and UD measured profiles shows 98.3% and 100.0%, respectively, of detector passing the gamma acceptance criteria of 0.3 mm and 2%. The dose measured by the PSD for a complete stereotactic radiosurgery treatment is comparable to the planned dose corrected for its SD-based S(c,p) within 1.4% and 0.7% for 5 and 35 mm diameter cone, respectively. Furthermore, volume averaging of the IC can be observed for the 5 mm aperture where it differs by as much as 9.1% compared to the PSD measurement. The angular dependency of the UD is also observed, unveiled by an under-response around 2.5% of both 5 and 35 mm apertures.
Output Factors and dose profiles measurements performed, respectively, with the PSD and the PSDCA were in agreement with those obtained with the UD and EBT2 films. For stereotactic radiosurgery treatment verification, the PSD gives accurate results compared to the planning system and the IC once the latter is corrected to compensate for the averaging effect of the IC. The PSD provides precise results when used as a single detector or in a dense array, resulting in a great potential for stereotactic radiosurgery QA measurements.
比较立体定向放射外科条件下塑料闪烁探测器（PSD）、微电离室（IC）、Gafchromic EBT2 胶片、60008 型屏蔽型光子二极管（SD）和非屏蔽型二极管（UD）在质量保证（QA）中的性能，并评估一种新的适用于小野剂量测量的二维十字线阵列原型。
PSD 由一根直径为 1 毫米、长 1 毫米的闪烁光纤（BCF-60，Saint-Gobain，Inc.）和一根聚甲基丙烯酸甲酯光纤（Eska premier，Mitsubishi Rayon Co.，Ltd.，Tokyo，Japan）组成。已经测量了直径为 4 至 40 毫米的立体定向放射外科中使用的孔径的输出因子（S(c,p)）。PSD 十字线阵列（PSDCA）是一个水当量装置，由 49 个包含在 1.63 厘米半径区域内的 PSD 组成。使用 PSDCA 对立体定向放射外科野进行了剂量分布测量，并与其他剂量计进行了比较。此外，在一个球形模体上进行了典型的使用四个非共面弧的立体定向放射外科治疗，其中放置了 UD、IC 或 PSD。使用 Xknife 计划系统（Integra Radionics Burlington，MA），在等中心点处规定了 15 Gy 的剂量，每个探测器都放置在等中心点处。
与 Gafchromic EBT2 相比，PSD 测量的输出因子归一化到 10×10 cm(2) 场时平均差异为 1.3%，与 UD 测量归一化到 35 毫米直径圆锥时的差异为 1.0%。尽管存在 49 个 PSD，但使用 PSD 十字线阵列进行的剂量分布与其他单探测器的剂量分布一致。与 Gafchromic EBT2 和 UD 测量的一维剂量分布相比，使用 PSD 十字线阵列获得的伽马值分别为 98.3%和 100.0%，符合伽马接受标准（0.3 毫米和 2%）。对于完整的立体定向放射外科治疗，PSD 测量的剂量与基于其 SD 的 S(c,p) 校正后的计划剂量相差 1.4%和 0.7%，分别用于 5 和 35 毫米直径的圆锥。此外，对于 5 毫米孔径，可以观察到 IC 的体积平均值，与 PSD 测量值相比，它的差异高达 9.1%。UD 的角度依赖性也被观察到，5 和 35 毫米孔径的响应分别降低了约 2.5%。
分别使用 PSD 和 PSDCA 进行的输出因子和剂量分布测量结果与 UD 和 EBT2 胶片的测量结果一致。对于立体定向放射外科治疗验证，PSD 与计划系统和 IC 相比提供了准确的结果，而 IC 则需要进行校正以补偿 IC 的平均效应。PSD 作为单个探测器或在密集阵列中使用时提供精确的结果，因此在立体定向放射外科 QA 测量中具有很大的潜力。
