Depauw Nicolas, Pursley Jennifer, Lozano-Calderon Santiago A, Patel Chirayu G
Department of Radiation Oncology, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts.
Department of Orthopaedic Surgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts.
Pract Radiat Oncol. 2023 May-Jun;13(3):256-262. doi: 10.1016/j.prro.2023.01.009. Epub 2023 Feb 2.
Impending and actual pathologic fractures secondary to metastatic bone disease, lymphoma, or multiple myeloma often require intramedullary fixation followed by radiation therapy. Because of carbon's low atomic number, there are reduced computed tomography (CT) imaging artifacts and dose perturbation when planning postoperative radiation for carbon fiber (CF) rods. Herein, we characterize the dosimetric properties of CF implants compared with titanium alloy (TA) for proton and photon.
TA and CF samples were acquired from an implant manufacturer. Material characteristics were evaluated by CT scans with and without metal artifact reduction (MAR). Relative stopping power (RSP) was determined from the range pull-back of each sample in a 20-cm range proton beam. Photon transmission measurements were made in a solid water phantom and compared with the modeled dosimetry from the RayStation planning system.
CF caused no visible CT artifacts, and MAR was not necessary for Hounsfield unit (HU) determination (median, 364 HU) or contouring, whereas TA (median, 3071 HU) caused substantial artifacts, which were improved, but not eliminated by MAR. The proton RSP was measured as 3.204 for TA and 1.414 for CF. For 6 MV photons, the measured transmission was 89.3% for TA and 98% for CF. CF RSP calculation and transmission from CT HU showed a physical density overestimate compared with measurements, which would cause a slight, but acceptable, dose uncertainty (<10% proton range or 1% photon transmission).
With a density similar to bone, CF implants did not cause imaging artifacts and minimal dose perturbation compared with TA. Although the CF proton RSP is underestimated and the photon attenuation is overestimated by the HU, both effects are relatively small and may be most easily accounted for by planning with a 2-mm expansion around organs at risk beyond or in close proximity to the implant.
继发于转移性骨病、淋巴瘤或多发性骨髓瘤的即将发生和实际的病理性骨折通常需要髓内固定，随后进行放射治疗。由于碳的原子序数低，在为碳纤维（CF）棒规划术后放疗时，计算机断层扫描（CT）成像伪影和剂量扰动会减少。在此，我们比较了CF植入物与钛合金（TA）在质子和光子方面的剂量学特性。
从植入物制造商处获取TA和CF样本。通过有无金属伪影减少（MAR）的CT扫描评估材料特性。从20厘米射程质子束中每个样本的射程回缩确定相对阻止本领（RSP）。在固体水模体中进行光子透射测量，并与RayStation计划系统的模拟剂量学进行比较。
CF未引起可见的CT伪影，确定Hounsfield单位（HU）（中位数，364 HU）或勾画轮廓时无需MAR，而TA（中位数，3071 HU）引起大量伪影，虽经MAR改善但未消除。TA的质子RSP测量值为3.204，CF为1.414。对于6兆伏光子，TA的测量透射率为89.3%，CF为98%。与测量值相比，CF的RSP计算和CT HU的透射显示物理密度高估，这将导致轻微但可接受的剂量不确定性（质子射程<10%或光子透射率<1%）。
CF植入物密度与骨相似，与TA相比，不引起成像伪影且剂量扰动最小。尽管CF的质子RSP被低估，HU对光子衰减的估计过高，但两种影响都相对较小，在植入物之外或紧邻植入物的危及器官周围规划时扩大2毫米可能最容易解决这些问题。
