Faculty of Science, University of Zurich, Zurich, Switzerland.
Radiotherapy Hirslanden, Hirslanden Medical Center, Aarau, Switzerland.
Med Phys. 2017 Jul;44(7):3788-3793. doi: 10.1002/mp.12293. Epub 2017 May 22.
The use of X-ray imaging in radiation therapy can give a substantial dose to the patient. A Cobalt machine combined with an magnetic resonance imaging (MRI) was recently introduced to clinical work. One positive aspect of using non-ionizing imaging devices is the reduction of the patient exposure. The purpose of this study was to quantify the difference in out-of-field dose to the patient between the image guided radiation therapy (IGRT) treatment applied with a linear accelerator with cone beam CT (CBCT) equipment and a Cobalt machine combined with an MRI.
The treatment of a rhabdomyosarcoma in the prostate was planned and irradiated using different modalities and radiation therapy machines. The whole-body dose was measured for a 3D-conformal radiation therapy (3DCRT), an intensity-modulated radiation therapy (IMRT), and a volumetric-modulated arc therapy plan applied with a conventional linear accelerator operated at 6 MV beam energy. Additionally, the dose of an IMRT plan applied with a Co machine combined with an MRI was measured. Furthermore, the dose of one CBCT scan using the linear accelerator's on-board imaging system was determined. The 3D dose measurements were performed in an anthropomorphic phantom containing 168 slots for thermoluminescence dosimeters (TLDs). A combination of LiF:Mg,Ti (TLD100) and LiF:Mg,Cu,P (TLD100H) was used to accurately determine the in- and out-of-field dose. The plans were rescaled to different fractionation schemes (2 Gy, 3 Gy, and 5 Gy fraction dose) and the dose of one CBCT scan was additionally added to the treatment dose per fraction applied with the linear accelerator. The resulting absorbed doses per fraction of the two machines were compared.
In the target region, all measured treatment plans presented the same magnitude of dose, while the CBCT dose was a factor of 100 smaller. Close to the planned target volume (PTV), the dose from the Co machine was a factor of two higher compared with the 3DCRT + CBCT dose. Up to 45 cm from the PTV, the treatment applied with the Co-sources showed an increased out-of-field dose compared to the linear accelerator + CBCT IGRT treatments. Further away from the PTV in the region where leakage from the gantry head is dominating, the out-of-field dose of the Cobalt machine was smaller compared to the linear accelerator + CBCT.
The peripheral dose of the Co machine combined with an MRI is larger up to 45 cm from the PTV and further away, it is lower than the dose from a linear accelerator + CBCT treatment. The presented fractionation schemes had a marginal impact on the results.
放射治疗中使用 X 射线成像会给患者带来大量剂量。最近，一种钴机与磁共振成像（MRI）相结合的设备已引入临床应用。使用非电离成像设备的一个积极方面是减少患者的辐射暴露。本研究的目的是定量比较使用带有锥形束 CT（CBCT）设备的直线加速器和钴机与 MRI 相结合的图像引导放疗（IGRT）治疗的患者场外剂量差异。
对前列腺横纹肌肉瘤进行了计划和放射治疗，使用了不同的模式和放射治疗设备。对 3D 适形放疗（3DCRT）、调强放疗（IMRT）和容积调强弧形治疗计划进行了全身剂量测量，这些计划均应用于能量为 6MV 的常规直线加速器。此外，还测量了应用钴机与 MRI 相结合的 IMRT 计划的剂量。此外，还确定了使用直线加速器机载成像系统进行的一次 CBCT 扫描的剂量。在包含 168 个用于热释光剂量计（TLD）的插槽的人体模型中进行了 3D 剂量测量。使用 LiF:Mg，Ti（TLD100）和 LiF:Mg，Cu，P（TLD100H）的组合来准确确定场内和场外剂量。将计划缩放为不同的分割方案（2Gy、3Gy 和 5Gy 分割剂量），并将直线加速器每次分割应用的一次 CBCT 扫描的剂量添加到治疗剂量中。比较了两台机器的每分割吸收剂量。
在靶区，所有测量的治疗计划都具有相同的剂量大小，而 CBCT 剂量则小了 100 倍。在接近计划靶区（PTV）的地方，钴机的剂量比 3DCRT+CBCT 剂量高两倍。在 PTV 外 45cm 以内，钴源治疗的场外剂量高于直线加速器+CBCT IGRT 治疗。在 PTV 更远的区域，由于龙门架头部泄漏占主导地位，钴机的场外剂量小于直线加速器+CBCT。
钴机与 MRI 结合的外周剂量在 PTV 外 45cm 以内较大，而在更远的地方，其剂量低于直线加速器+CBCT 治疗。所提出的分割方案对结果的影响不大。
