Rijken James D, Colyer Christopher J
Adelaide Radiotherapy Centre, Flinders Private Hospital, Bedford Park, South Australia.
J Appl Clin Med Phys. 2017 Sep;18(5):301-306. doi: 10.1002/acm2.12167. Epub 2017 Aug 30.
The dosimetric uncertainties associated with radiotherapy through hip prostheses while overriding the implant to a set density within the TPS has not yet been reported. In this study, the uncertainty in dose within a PTV resulting from this planning choice was investigated. A set of metallic hip prosthetics (stainless steel, titanium, and two different Co-Cr-Mo alloys) were CT scanned in a water bath. Within the TPS, the prosthetic pieces were overridden to densities between 3 and 10 g/cm and irradiated on a linear accelerator. Measured dose maps were compared to the TPS to determine which density was most appropriate to override each metal. This was shown to be in disagreement with the reported literature values of density which was attributed to the TPS dose calculation algorithm and total mass attenuation coefficient differences in water and metal. The dose difference was then calculated for a set density override of 6 g/cm in the TPS and used to estimate the dose uncertainty beyond the prosthesis. For beams passing through an implant, the dosimetric uncertainty in regions of the PTV may be as high as 10% if the implant composition remains unknown and a set density override is used. These results highlight limitations of such assumptions and the need for careful consideration by radiation oncologist, therapist, and physics staff.
在放疗计划系统（TPS）中将髋关节假体的密度覆盖设定值时，与通过髋关节假体进行放疗相关的剂量学不确定性尚未见报道。在本研究中，对这种计划选择导致的计划靶体积（PTV）内剂量的不确定性进行了研究。一组金属髋关节假体（不锈钢、钛以及两种不同的钴铬钼合金）在水浴中进行CT扫描。在TPS内，将假体部件的密度覆盖设定在3至10 g/cm之间，并在直线加速器上进行照射。将测量的剂量分布图与TPS进行比较，以确定覆盖每种金属的最合适密度。结果表明，这与已报道的密度文献值不一致，这归因于TPS剂量计算算法以及水和金属中总质量衰减系数的差异。然后计算了TPS中密度覆盖设定为6 g/cm时的剂量差异，并用于估计假体之外的剂量不确定性。对于穿过植入物的射束，如果植入物成分未知且使用设定的密度覆盖，则PTV区域的剂量学不确定性可能高达10%。这些结果凸显了此类假设的局限性，以及放射肿瘤学家、治疗师和物理人员进行仔细考虑的必要性。
