Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, 2500, Australia.
Illawarra Cancer Care Centre, Wollongong Hospital, Wollongong, NSW, 2500, Australia.
Med Phys. 2017 Oct;44(10):5402-5412. doi: 10.1002/mp.12469. Epub 2017 Aug 12.
The aim of in vivo skin dosimetry was to measure the absorbed dose to the skin during radiotherapy, when treatment planning calculations cannot be relied on. It is of particularly importance in hypo-fractionated stereotactic modalities, where excessive dose can lead to severe skin toxicity. Currently, commercial diodes for such applications are with water equivalent depths ranging from 0.5 to 0.8 mm. In this study, we investigate a new detector for skin dosimetry based on a silicon epitaxial diode, referred to as the skin diode.
The skin diode is manufactured on a thin epitaxial layer and packaged using the "drop-in" technology. It was characterized in terms of percentage depth dose, dose linearity, and dose rate dependence, and benchmarked against the Attix ionization chamber. The response of the skin diode in the build-up region of the percentage depth dose (PDD) curve of a 6 MV clinical photon beam was investigated. Geant4 radiation transport simulations were used to model the PDD in order to estimate the water equivalent measurement depth (WED) of the skin diode. Measured output factors using the skin diode were compared with the MOSkin detector and EBT3 film at 10 cm depth and at surface at isocenter of a water equivalent phantom. The intrinsic angular response of the skin diode was also quantified in charge particle equilibrium conditions (CPE) and at the surface of a solid water phantom. Finally, the radiation hardness of the skin diode up to an accumulated dose of 80 kGy using photons from a Co-60 gamma source was evaluated.
The PDD curve measured with the skin diode was within 0.5% agreement of the equivalent Geant4 simulated curve. When placed at the phantom surface, the WED of the skin diode was estimated to be 0.075 ± 0.005 mm from Geant4 simulations and was confirmed using the response of a corrected Attix ionization chamber placed at water equivalent depth of 0.075 mm, with the measurement agreement to within 0.3%. The output factor measurements at 10 cm depth were within 2% of those measured with film and the MOSkin detector down to a field size of 2 × 2 cm . The dose-response for all detector samples was linear and with a repeatability within 0.2%. The skin diode intrinsic angular response showed a maximum deviation of 8% at 90 degrees and from 0 to 60 degree is less than 5%. The radiation sensitivity reduced by 25% after an accumulated dose of 20 kGy but after was found to stabilize. At 60 kGy total accumulated dose the response was within 2% of that measured at 20 kGy total accumulated dose.
This work characterizes an innovative detector for in vivo and real-time skin dose measurements that is based on an epitaxial silicon diode combined with the Centre for Medical Radiation Physics (CMRP) "drop-in" packaging technology. The skin diode proved to have a water equivalent depth of measurement of 0.075 ± 0.005 mm and the ability to measure doses accurately relative to reference detectors.
体内皮肤剂量测定的目的是测量放射治疗期间皮肤的吸收剂量，此时不能依赖于治疗计划计算。在分次立体定向模式下，这一点尤其重要，因为过量的剂量会导致严重的皮肤毒性。目前，用于此类应用的商业二极管的水当量深度范围为 0.5 至 0.8 毫米。在这项研究中，我们研究了一种基于硅外延二极管的新型皮肤剂量计，称为皮肤二极管。
皮肤二极管是在薄的外延层上制造的，并使用“插入式”技术进行封装。它的特性包括百分深度剂量、剂量线性度和剂量率依赖性，并与 Attix 电离室进行了基准测试。研究了 6MV 临床光子束 PDD 曲线的建库区中皮肤二极管的响应。使用 Geant4 辐射传输模拟来模拟 PDD，以估计皮肤二极管的水当量测量深度（WED）。在水当量体模的等中心处，在 10cm 深度和表面处，使用皮肤二极管测量的输出因子与 MOSkin 探测器和 EBT3 胶片进行了比较。还在 CPE 条件下和在实心水模体的表面量化了皮肤二极管的固有角响应。最后，使用来自 Co-60 伽马源的光子评估了皮肤二极管的辐射硬度，直到累积剂量达到 80kGy。
使用皮肤二极管测量的 PDD 曲线与等效的 Geant4 模拟曲线的误差在 0.5%以内。当放置在体模表面时，根据 Geant4 模拟，皮肤二极管的 WED 估计为 0.075±0.005mm，并通过将校正后的 Attix 电离室放置在水当量深度为 0.075mm 处来确认，测量结果的一致性在 0.3%以内。在 10cm 深度处的输出因子测量结果与胶片和 MOSkin 探测器的测量结果相差在 2%以内，直至 2×2cm 的小照射野。所有探测器样本的剂量响应均呈线性，重复性在 0.2%以内。皮肤二极管的固有角响应在 90 度时最大偏差为 8%，而在 0 至 60 度时小于 5%。在累积剂量为 20kGy 后，辐射灵敏度降低了 25%，但随后发现其稳定下来。在 60kGy 总累积剂量下，响应与在 20kGy 总累积剂量下的响应相差在 2%以内。
这项工作对一种基于外延硅二极管和 CMRP“插入式”封装技术的新型体内实时皮肤剂量测量探测器进行了特征描述。皮肤二极管的水当量测量深度被证明为 0.075±0.005mm，并且能够与参考探测器准确地测量剂量。
