Perturbation effects of the carbon fiber‐PEEK screws on radiotherapy dose distribution

Abstract Radiation therapy, in conjunction with surgical implant fixation, is a common combined treatment in cases of bone metastases. However, metal implants generally used in orthopedic implants perturb radiation dose distributions. Carbon‐Fiber Reinforced Polyetheretherketone (CFR‐PEEK) material has been recently introduced for production of intramedullary nails and plates. The purpose of this work was to investigate the perturbation effects of the new CFR‐PEEK screws on radiotherapy dose distributions and to evaluate these effects in comparison with traditional titanium screws. The investigation was performed by means of Monte Carlo (MC) simulations for a 6 MV photon beam. The project consisted of two main stages. First, a comparison of measured and MC calculated doses was performed to verify the validity of the MC simulation results for different materials. For this purpose, stainless steel, titanium, and CFR‐PEEK plates of various thicknesses were used for attenuation and backscatter measurements in a solid water phantom. For the same setup, MC dose calculations were performed. Next, MC dose calculations for titanium, CFR‐PEEK screws, and CFR‐PEEK screws with ultrathin titanium coating were performed. For the plates, the results of our MC calculations for all materials were found to be in good agreement with the measurements. This indicates that the MC model can be used for calculation of dose perturbation effects caused by the screws. For the CFR‐PEEK screws, the maximum dose perturbation was less than 5%, compared to more than 30% perturbation for the titanium screws. Ultrathin titanium coating had a negligible effect on the dose distribution. CFR‐PEEK implants have good prospects for use in radiotherapy because of minimal dose alteration and the potential for more accurate treatment planning. This could favorably influence treatment efficiency and decrease possible over‐ and underdose of adjacent tissues. The use of such implants has potential clinical advantages in the treatment of bone metastases.

investigate the perturbation effects of the new CFR-PEEK screws on radiotherapy dose distributions and to evaluate these effects in comparison with traditional titanium screws. The investigation was performed by means of Monte Carlo (MC) simulations for a 6 MV photon beam. The project consisted of two main stages. First, a comparison of measured and MC calculated doses was performed to verify the validity of the MC simulation results for different materials. For this purpose, stainless steel, titanium, and CFR-PEEK plates of various thicknesses were used for attenuation and backscatter measurements in a solid water phantom. For the same setup, MC dose calculations were performed. Next, MC dose calculations for titanium, CFR-PEEK screws, and CFR-PEEK screws with ultrathin titanium coating were performed. For the plates, the results of our MC calculations for all materials were found to be in good agreement with the measurements. This indicates that the MC model can be used for calculation of dose perturbation effects caused by the screws. For the CFR-PEEK screws, the maximum dose perturbation was less than 5%, compared to more than 30% perturbation for the titanium screws. Ultrathin titanium coating had a negligible effect on the dose distribution. CFR-PEEK implants have good prospects for use in radiotherapy because of minimal dose alteration and the potential for more accurate treatment planning. This could favorably influence treatment efficiency and decrease possible over-and underdose of adjacent tissues.
The use of such implants has potential clinical advantages in the treatment of bone metastases.

| INTRODUCTION
The optimal treatment of spinal and para-spinal tumors necessitates a multidisciplinary approach, including surgery and radiation (adjuvant) therapy. Surgery plays a primary management role and permanent metallic hardware is often required. 1 Spinal fixation, using pedicle screws and implant rods, has recently been used for rigid connections in the vertebrae of the spine. 2 Radiotherapy is administered postoperatively to improve local control, especially in the setting of close margins or microscopically positive margins. 3 Unfortunately, delivery of adjuvant radiation therapy can be compromised due to the presence of the postoperative hardware.
Traditional orthopedic implant materials are usually stainless steel or titanium and their use poses two major obstacles for accurate radiotherapy planning and delivery. Second, metal implants distort dose distributions from therapeutic megavoltage beams. A 5% to 10% dose reduction to tissues in regions behind stabilization rods has been reported, due to the attenuation effect. [5][6][7] As a result, some patients who might benefit from radiotherapy may be treated with lower doses unlikely to provide long-term local control. On the other hand, enhanced scattering from the high-Z materials 8 in combination with high doses of radiation prescription may lead to the development of myelopathy, the most feared complication of radiotherapy.
Approaches to reducing photon dose calculation errors near metal implants were described in a recent publication 9 and are out of the scope of this paper. However, it should be noted that correct dose calculation in the vicinity of metal implants is still problematic, even with the use of modern TPS and new image artifacts reduction methods.
Unlike metals, carbonaceous materials have low atomic numbers, good biocompatibility, chemical stability, good mechanical properties, and modulus of elasticity similar to human bones. Carbon-Fiber Reinforced Polyetheretherketone (CFR-PEEK) material has been recently introduced for production of intramedullary nails and plates and now is broadly accepted as a radiolucent alternative to metallic biomaterials in the spine community. 10

2.C | MC simulations
All MC simulations in this study were performed with the EGSnrc code 13 for coupled electron and photon transport. This code has been extensively validated in the past and was selected as best suited for our study. Unlike commercial TPS, EGSnrc code is capable of accurate dose calculations near metal objects as it fully models all physical interactions of ionizing particles with the medium. The cut-off energies for photon transport (P cut) and for electron transport (E cut ) were set to 0.01 MeV and 0.7 MeV, respectively, in both accelerator simulation and phantom dose calculation. For all simulations, the boundary crossing algorithm was EXACT and the electron step algorithm was PRESTA-II. The user adjustable values for other parameters were set at their default values.
Spin effect (which turns off/on spin effects for electron elastic scattering) was set to ON, as it is necessary for accurate backscattering calculations. 15

2.D | Screws
The screws investigated in this work are shown in Fig. 1. The screws were modeled as 0.6 cm 9 0.6 cm 9 6 cm parallelepiped. Three screw compositions were studied in this work. At the first stage, homogeneous screws made of titanium and CFR-PEEK were considered. Then, CFR-PEEK screws with ultrathin (thickness of less than 0.01 cm) titanium coating were modeled. Titanium coating is used for improved and direct osseointegration. 16 In addition, this titanium shell improves screw visibility under fluoroscopy.  horizontal error bars correspond to AE 1% variation in the dose and AE 0.5 mm variation in the distance, respectively. The error bars were added to the measured data points to allow an easy visual comparison between the results.

2.E |
The dose at the entrance surface of the plates was increased (the effect of backscatter) by 22% for stainless steel, 18% for titanium, and less than 1% for CFR-PEEK. There was only minor dependence (less than 1%) on the thickness of the plate. For the same plates, the dose at the exit surface was reduced by 10, 13, and 17% for steel and by 8, 10, and 13% for titanium for the thickness of 1, 3, and 6 mm, correspondingly. For the CFR-PEEK plates, the dose was increased by less than 2%.
The results of attenuation measurements are shown in Fig. 3, together with MC calculated percent depth dose curves in water for 1 mm, 3 mm, and 6 mm plates of steel, titanium, and CFR-PEEK It can be seen that all measurements were within 1%/1 mm agreement with the MC calculations.

3.B | MC simulations for the screws
Dose difference maps for screws made of titanium (TI), CFR-PEEK, and CFR-PEEK with titanium coating (CFR-PEEK-TI) are shown in Figs. 4 and 5, for the beam coming along (from the top) and across (from the right) the screws, respectively. Dose differences at the points defined in Fig. 2   Maximum overdose due to backscatter was 10% for the Ti screws and effectively zero for the CFR-PEEK screws (both with and without titanium coating). Maximum underdose due to attenuation was about 30% for the Ti screws and about 5% for the CFR-PEEK screws. As expected, the largest perturbation occurred when the beam direction was along to the screw axis.
It can be seen that the dose overdose due to backscatter decreases quickly with distance and becomes negligible at a few millimeters from the screw. However, there might be situations where screws are placed in very close vicinity of the spinal cord, as illustrated in Fig. 6. It can also be seen here that the ultrathin titanium shell allows acquisition of CT images with good screw visualization and minimum appearance of image arifacts.
Titanium screws introduced large distortion in the radiation dose distribution. CFR-PEEK screws caused minimal alteration of dose distribution. Ultrathin titanium coating had a negligible effect on the dose distribution.
Only two scenarios were considered in this work, when screws are either parallel or perpendicular to the beam directions. These scenarios correspond to the maximum and minimum perturbations introduced by the screws; when screws are oriented at other angles to the beam, dose perturbation will be within the range of the minimum and maximum values given in our results for perpendicular and parallel screw orientations.
In our work, we concentrated on the dose perturbation due to the screw presence. Image artifacts which can influence clinical dosimetry were not considered here and should be investigated separately.
Unlike the recent publication of Ni et al., 17 our work concentrated on the comparison of perturbation effects of pedicle screws with different compositions, as these effects can be of special importance due to proximity to the spinal cord. This comparison was performed for 6 MV photon beam only, because dose perturbation effects of metal implants are even greater for higher photon energies used in radiotherapy 18 and, therefore, the advantages of CFR-PEEK screws over metal screws would be more pronounced. T A B L E 1 Dose differences in percents at the points defined in Fig. 2.

| CONCLUSION
F I G . 6. CT image of pedicle screws with titanium coating inserted into vertebra.