North West Cancer Centre, Tamworth, New South Wales, Australia.
Department of Radiation Oncology, Chris O'Brien Lifehouse, Sydney, New South Wales, Australia.
Med Phys. 2022 Jun;49(6):4082-4091. doi: 10.1002/mp.15543. Epub 2022 Mar 1.
Kilovoltage (kV) X-ray beams are an essential modality in radiotherapy. Solid state detectors are widely available in radiotherapy departments, but their use for kV dosimetry has been limited to date. This study aimed to evaluate the dosimetric performance of a range of solid state detectors for kV dosimetry.
Percentage depth doses (PDDs) and relative output factors (ROFs) were measured on an XStrahl 300 unit (XStrahl-Ltd., UK) using 60, 100, 150, and 300 kVp X-ray beams. The fields were defined by circular applicators with field sizes of 2, 5, 8, and 10 cm diameter and square applicators of field sizes 10 × 10 and 20 × 20 cm . The following Physikalisch-Technische Werkstätten (PTW) dosimeters were used for measurements: Advanced Markus, PinPoint 3D and Semiflex ionization chambers; photon, electron, and stereotactic radiosurgery (SRS) diodes plus the microDiamond detector. All PDDs were normalized at 5 mm depth, and ROFs were measured at 3 mm depth to avoid collisions with the end of the applicators. ROFs measured using chambers were corrected for polarity and ion-recombination effects.
PDD measurements for 60, 100, and 150 kVp beams exhibited good agreement between all diodes and the ionization chambers over the entire range of depths except in the first few millimeters near the surface. However, for the 300 kVp, all diode detectors exhibited an overresponding behavior compared to reference depth dose data measured with the Advanced Markus chamber. ROFs with the diodes were higher than the Advanced Markus chamber at low energy, and the magnitude of these differences is inversely proportional to the field sizes. The PTW P diode showed the highest variation of up to 15% in the output factor compared to the Advanced Markus chamber.
This study evaluated the dosimetric performance of a range of solid state detectors in kV relative dosimetry. This study showed that diode detectors are a suitable replacement for ionization chambers for the PDD measurement of low energy kV beams (60-150 kVp) except for the PDD of 60 kVp with the smaller field sizes. However, an overresponding behavior of diode detectors at 300 kVp beams shows that diode detectors are not suitable for the PDD measurement of high energy kV beams. Generally, all solid state detectors overresponded to ROF measurements, indicating that it is not suitable for ROF measurements. In general, both shielded and unshielded diodes produced a similar dosimetric response, which demonstrates that the energy dependence of solid state detectors should be considered before they are used for any kV relative dosimetric measurements.
千伏 (kV) X 射线束是放射治疗中的一种重要模式。固态探测器在放射治疗部门中广泛可用，但迄今为止，它们在 kV 剂量测量中的应用一直受到限制。本研究旨在评估一系列固态探测器在 kV 剂量测量中的剂量性能。
在 XStrahl 300 装置（XStrahl-Ltd.，英国）上使用 60、100、150 和 300 kVp X 射线束测量百分深度剂量 (PDD) 和相对输出因子 (ROF)。使用圆形适形器定义场，其场尺寸为 2、5、8 和 10 cm 直径以及方形适形器的场尺寸为 10×10 和 20×20 cm。使用以下 Physikalisch-Technische Werkstätten (PTW) 剂量计进行测量：高级 Markus、PinPoint 3D 和 Semiflex 电离室；光子、电子和立体定向放射外科 (SRS) 二极管以及 microDiamond 探测器。所有 PDD 均在 5 mm 深度归一化，ROF 在 3 mm 深度测量以避免与适形器末端碰撞。使用室测量的 ROF 进行了极性和离子复合效应的校正。
60、100 和 150 kVp 束的 PDD 测量在整个深度范围内除了在表面附近的前几个毫米处，所有二极管和电离室之间都显示出良好的一致性。然而，对于 300 kVp，所有二极管探测器与使用高级 Markus 室测量的参考深度剂量数据相比表现出过响应行为。在低能量下，二极管的 ROF 高于高级 Markus 室，这些差异的幅度与场尺寸成反比。PTW P 二极管的输出因子与高级 Markus 室相比最高变化可达 15%。
本研究评估了一系列固态探测器在 kV 相对剂量测量中的剂量性能。本研究表明，二极管探测器是低能 kV 束（60-150 kVp）PDD 测量的电离室的合适替代品，除了较小场尺寸的 60 kVp 的 PDD 外。然而，二极管在 300 kVp 束中的过响应行为表明，二极管探测器不适合高能 kV 束的 PDD 测量。一般来说，所有固态探测器对 ROF 测量都有过响应，表明它不适合 ROF 测量。一般来说，屏蔽和非屏蔽二极管产生相似的剂量响应，这表明在将固态探测器用于任何 kV 相对剂量测量之前，应考虑其能量依赖性。
