(P 261) EVALUATION OF CT ARTIFACTS AFTER SPINAL OLIGOMETASTATIC DISEASE RESECTION AND RECONSTRUCTION: A COMPARATIVE STUDY OF CARBON-FIBER VS. TITANIUM IMPLANTS

Objective: Spinal metastases are common in oncology, with about 70% of all bony metastases occurring in the spine. Spinal oligometastatic disease (OMD) refers to patients with a limited number (≤ 5) of treatable spinal metastases. Treatment involves a multidisciplinary approach, tailored to factors such as tumor histology, location, symptoms, radiosensitivity, and prior treatments. Surgery is indicated for mechanical pain, decompression, instability correction, or oncological cytoreduction. Combining surgery with postoperative radiotherapy is a common practice for local control in limited spinal OMD.

A possible improvement for spinal OMD treatment is to change traditional titanium implants to innovative carbon-fiber implants. Carbon-fiber implants could improve radiotherapy planning and lead to more accurate delivery of the calculated dose compared to traditional implants. However, it has been difficult to quantify the difference in computed tomography (CT) artifacts after implementation of carbon-fiber and titanium implants.

The objective of this study is to quantitatively compare CT artifacts produced by carbon-fiber and titanium implants in patients with spinal OMD undergoing postoperative stereotactic ablative radiotherapy (SABR).

Methods: This retrospective single-center study included patients aged 18 years or older with spinal OMD who received carbon-fiber or titanium spinal implants between 2018 and 2020. Patients were closely matched by gender and age to minimize the influence of osteoporotic changes on CT artifact measurements. Exclusion criteria included traumatic or inflammatory conditions, previous fusion surgery, and the use of bone cement (i.e., polymethyl methacrylate or PMMA).

Surgery was recommended as a curative treatment for spinal OMD. Most patients presented with pain or spinal cord compression and had insufficient response to non-surgical treatment. Decompression and fixation surgery was performed, with the choice between carbon-fiber or titanium implants made through shared decision-making and surgeon preference. Patients were treated with FDA-approved and CE-marked carbon-fiber (CarboFix Orthopedics, Herzliya, Israel) and titanium implants (Stryker Corporation, Michigan, USA).

CT artifacts were measured preoperatively and within the first postoperative week using Hounsfield units (HU). Measurements were taken using PACS integrated software (Phillips Medical Systems, Eindhoven, The Netherlands) under a standard CT spine protocol (median tube potential 140 kVp, median tube current 60 mA, 2.5 mm slice thickness). Regions of interest (ROI) were selected by a neurosurgeon and checked by an orthopedic surgeon specialized in spine surgery.

Pre- and postoperative HU measurements were performed at the vertebral body, pedicle, and spinal cord at three different levels: the lower instrumented vertebra, the level of metastatic spinal cord compression, and at an uninvolved level. These locations were chosen because the lower instrumented vertebra received the implant. The level of metastatic spinal cord compression is the level where radiotherapy is directed, and usually no screws are inserted at this level, meaning that HU changes come from scattering from different levels. Additionally, an uninvolved level served as an internal control. Areas were measured as a circle in the vertebral body and spinal canal, while an ellipse was used for the pedicle. CT images were analyzed for the carbon-fiber as well as the titanium implant group, and HU areas measured at each level were delicately matched between pre- and postoperative images (< 10% difference in the examined area measured in cm²).

Statistical analyses were performed using SPSS version 25 (IBM Corp, Somer, NY, USA). For continuous data, the Kolmogorov-Smirnov test was used to assess the normal distribution assumption. Mean HU were calculated for each group pre- and postoperatively, and a paired t-test was used to compare the mean HU pre- versus postoperatively. Significance was set as a p-value ≤ 0.05.

Results: A total of 22 patients were included, with 11 patients each in the carbon-fiber and titanium implant groups. The carbon-fiber group had a mean age of 54 years (range 20-70) and included six males and five females. The titanium group had a mean age of 56 years (range 36-66) and included five males and six females. Most lesions were located at the thoracic level.

Carbon-fiber implants
Postoperative scatter artifacts were observed with carbon-fiber implants, showing a significant increase in HU at the vertebral body, pedicle, and spinal cord at the lower instrumented vertebra level (p = 0.012, 0.015, and 0.014, respectively). HU increased up to 1.67-fold compared to preoperative values. No significant HU increase was noted at the metastatic spinal cord compression level or the uninvolved level.

Titanium implants
Postoperative scatter artifacts were more pronounced with titanium implants, showing a significant increase in HU at the vertebral body, pedicle, and spinal cord at the lower instrumented vertebra level (up to 5.65-fold increase, p-values < 0.001). Although not statistically significant, there was a trend towards higher HU at the metastatic spinal cord compression level (average 2.51-fold increase). No significant HU increase was observed at the uninvolved level.

Conclusion: A greater increase in HU was observed in the group treated with titanium spine implants than in the group treated with carbon-fiber spine implants. This relatively high postoperative HU-increase indicates a trend towards more CT artifacts after reconstruction with titanium implants.

Our quantitative measurements could be used in future studies to assess if less CT artifacts due to treatment with carbon-fiber implants lead to time savings during radiotherapy treatment planning and, potentially, better tumoricidal effects and decreased adverse effects due to accurate delivery of the calculated dose.