Institut Curie, PSL Research University, Radiation Oncology Department, Paris, France.
Institut Curie, PSL Research University, Radiation Oncology Department, Paris, France.
Phys Med. 2021 May;85:42-49. doi: 10.1016/j.ejmp.2021.04.023. Epub 2021 May 6.
Bone cement used for vertebroplasty can affect the accuracy on the dose calculation of the radiation therapy treatment. In addition the CT values of high density objects themselves can be misrepresented in kVCT images. The aim of our study is then to propose a streamlined approach for estimating the real density of cement implants used in stereotactic body radiation therapy.
Several samples of cement were manufactured and irradiated in order to investigate the impact of their composition on the radiation dose. The validity of the CT conversion method for a range of photon energies was investigated, for the studied samples and on six patients. Calculations and measurements were carried out with various overridden densities and dose prediction algorithms (AXB with dose-to-medium reporting or AAA) in order to find the effective density override.
Relative dose differences of several percent were found between the dose measured and calculated downstream of the implant using an ion chamber and TPS or EPID dosimetry. If the correct density is assigned to the implant, calculations can provide clinically acceptable accuracy (gamma criteria of 3%/2 mm). The use of MV imaging significantly favors the attribution of a correct equivalent density to the implants compared to the use of kVCT images.
The porosity and relative density of the various studied implants vary significantly. Bone cement density estimations can be characterized using MV imaging or planar in vivo dosimetry, which could help determining whether errors in dose calculations are due to incorrect densities.
用于椎体成形术的骨水泥会影响放射治疗剂量计算的准确性。此外，高密度物体本身的 CT 值在千伏 CT 图像中可能会被误判。因此，我们的研究旨在提出一种简化的方法来估算立体定向体部放射治疗中使用的水泥植入物的实际密度。
为了研究其组成对辐射剂量的影响，我们制造并辐照了几种水泥样本。研究了 CT 转换方法对一系列光子能量的有效性，针对研究样本和六名患者进行了研究。为了找到有效的密度覆盖，使用各种覆盖密度和剂量预测算法（AXB 具有剂量到介质报告或 AAA）进行了计算和测量。
使用离子室和 TPS 或 EPID 剂量测定下游的植入物进行剂量测量和计算时，发现剂量存在几个百分点的差异。如果正确的密度分配给植入物，计算可以提供临床可接受的准确性（伽马标准为 3%/2 毫米）。与使用千伏 CT 图像相比，MV 成像的使用明显有利于为植入物分配正确的等效密度。
各种研究植入物的孔隙率和相对密度差异很大。可以使用 MV 成像或平面体内剂量测定来表征骨水泥密度估计，这有助于确定剂量计算中的误差是否是由于不正确的密度造成的。
