Scattering of therapeutic radiation in the presence of craniofacial bone reconstruction materials

Abstract Purpose Radiation scattering from bone reconstruction materials can cause problems from prolonged healing to osteoradionecrosis. Glass fiber reinforced composite (FRC) has been introduced for bone reconstruction in craniofacial surgery but the effects during radiotherapy have not been previously studied. The purpose of this study was to compare the attenuation and back scatter caused by different reconstruction materials during radiotherapy, especially FRC with bioactive glass (BG) and titanium. Methods The effect of five different bone reconstruction materials on the surrounding tissue during radiotherapy was measured. The materials tested were titanium, glass FRC with and without BG, polyether ether ketone (PEEK) and bone. The samples were irradiated with 6 MV and 10 MV photon beams. Measurements of backscattering and dose changes behind the sample were made with radiochromic film and diamond detector dosimetry. Results An 18% dose enhancement was measured with a radiochromic film on the entrance side of irradiation for titanium with 6 MV energy while PEEK and FRC caused an enhancement of 10% and 4%, respectively. FRC‐BG did not cause any measurable enhancement. The change in dose immediately behind the sample was also greatest with titanium (15% reduction) compared with the other materials (0–1% enhancement). The trend is similar with diamond detector measurements, titanium caused a dose enhancement of up to 4% with a 1 mm sample and a reduction of 8.5% with 6 MV energy whereas FRC, FRC‐BG, PEEK or bone only caused a maximum dose reduction of 2.2%. Conclusions Glass fiber reinforced composite causes less interaction with radiation than titanium during radiotherapy and could provide a better healing environment after bone reconstruction.


| INTRODUCTION
Treatment of advanced head and neck cancer requires a multimodal approach with surgery and radiotherapy typically given with concurrent chemotherapy. Nearly 60% of patients are diagnosed with locally advanced but non-metastatic disease for whom combined modality treatment is regarded as standard. 1,2 The sites of head and neck squamocellular carcinoma treated with radiotherapy are surrounded by or adjacent to bony structures that during operative treatment can require resection and subsequent reconstruction. This is equally true for advanced brain tumors, which are also often treated with the combination of surgery and post-operative radiotherapy. [3][4][5][6] Surgical treatment often involves the use of foreign material in the reconstruction of both anatomy and function. 7 The reconstructive material is thereafter present at the time of postoperative radiotherapy. The presence of foreign metallic material can cause problems for radiotherapy due to radiation scattering and absorption. 8 The complications of radiation dose enhancement can vary from local irritation and impaired wound healing to osteoradionecrosis. 9,10 Soft tissue atrophy due to scattered radiation has also been reported as a complication when using metallic reconstructive implants. 11 An implant material that does not impact on dose distribution of radiotherapy or interfere with diagnostic imaging such as computer tomography (CT) or magnetic resonance imaging (MRI) would be ideal. Materials used in reconstruction include autologous bone, titanium and its alloys, polyether ether ketone (PEEK) and fiber-reinforced composites (FRC). 12 It is important to know how these materials interact with ionizing radiation where risk for underor overdosing may contribute to failure of treatment.
It is known that titanium causes radiation scattering and absorption resulting in dose enhancement or reduction in the surrounding tissue. 13 Titanium also interferes with diagnostic imaging and causes imaging artifacts that can be problematic in postoperative followup. [14][15][16] There is need for a non-metallic reconstructive material that is both durable and biocompatible. Glass fiber reinforced composites (FRC) were first introduced as a reconstructive material for dental hard-tissue and are now also in use in craniofacial surgery. [17][18][19][20][21][22] Particles of bioactive glass (BG) have been added to FRC implants to improve osteoconductivity, osteogenicity and antimicrobial properties. [23][24][25][26][27] Glass FRC and cortical bone have very similar radio-opacity and therefore glass FRC does not cause artifacts in diagnostic imaging unlike metallic materials. 28 The aim of this study was to examine the effects of the different materials used in reconstructive surgery on the surrounding environment during radiotherapy using measurements with film dosimetry and diamond detector dosimetry. The materials tested were titanium, PEEK, glass fiber reinforced composite with and without bioactive glass S53P4 and bone. In composite materials theoretical backscatter calculations are not trivial and measurements are an appropriate alternative. The hypothesis was that FRC-BG causes less scattering and absorption than titanium and that the composite material and bone interact similarly with ionizing radiation.

| MATERIALS AND METHODS
Sandwich-like glass FRC-BG implant simulating specimens were used in this study. Titanium, polyether ether ketone (PEEK) and bone were used as controls. The materials used in preparation of the FRC samples are listed in Table 1

2.A | Film dosimetry
The absorption and scattering of the radiation in studied materials were measured using a Gafchromic EBT3 film (Ashland ISP, Wayne, NJ, USA) in a water equivalent solid material. Three films were placed near the specimen plates. One in contact with the plates above them, giving the information about backscattering, and two other films behind the plates, one in contact and one at 1.5 mm distance giving information about the scattering and absorption (Fig. 2).
The films were exposed at the dose level of 2 Gy. Since the EBT3 film is a radiochromic film, it has self-developing feature, and therefore no chemical, thermal or physical processing was needed. Films were scanned with a flatbed scanner (Epson Perfection V700 scanner, Seiko Epson Corporation, Tokyo, Japan), and analyzed with OmniPro I'mRT software (IBA Dosimetry GmbH, Germany). The dose values were determined by calculating the mean value within a 1.5 cm 2 ROI in the center of the sample plate.

2.B | Diamond detector dosimetry
The radiation dose behind the sample material was measured using a single crystal diamond detector (microDiamond 60019, PTW Freiburg GmbH, Germany). The microDiamond measurements were made in a water phantom (BluePhantom2, IBA Dosimetry GmbH, Germany). The detector was mounted in a vertical orientation ( Fig. 3) and the dose was measured in seven different points (1-20 mm) behind the sample material.

3.A | Film dosimetry
The results are shown in

| DISCUSSION
The effects of bone reconstruction materials on photon radiation were measured for 5 different materials. Main interest was in the effects of the novel composite material compared especially to titanium which is the gold standard in bone reconstruction.
The thickness and required strength of the reconstruction material used in head and neck surgery vary with site and function.
Load-bearing sites such as the mandible are subject to much greater forces than, for example, the calvarium. Therefore, the requirements for reconstructive materials vary a lot and depend on the site of implantation. In addition, the size of the reconstructed area sets certain requirements for the reconstruction material. In per gram (EDG, number of electrons per gram) x physical density (PD, g/cm 3 ), it has been shown that the backscatter dose increases with EDV and thus with PD. 31,32 Usually high Z material has high physical density but this is not a consistent phenomenon. The physical density of pure titanium is 4.50 g/cm 3 , PEEK 1.32 g/cm 3 , FRC 1.20 g/cm 3 , bioactive glass 2.6 g/cm 3 and bone 1.85 g/cm 3 while the density for soft tissue is 1.00 g/cm 3 . The Z for PEEK is lower than the other tested composites but the attenuation and back scatter results with tested radiation energies were similar. [33][34][35] The high density of titanium in comparison with the other tested materials accounts for the higher backscattering dose.
The radiation dose increases most on top of the implant and the effect of metallic reconstruction materials on radiation is inevitable even with careful planning. 36 The scattering caused by titanium plates during radiotherapy is greater with thicker plates than with measuring the effects of titanium on radiation. 13,39,40 In our measurements the glass fiber reinforced composite did not cause marked dose enhancement or reduction. The changes in radiation dose were greatest with titanium. The use of a composite reconstruction material could thus provide the surgical site a better environment for healing during and after radiotherapy.
The results with the novel material are promising considering a reconstructive material that has proven to work in cranial reconstruction 22 and causes less imaging artifacts in CT and MRI compared to titanium. 41,42 The development of nonmetallic implants for craniofacial bone reconstruction has led to the introduction of implants made of composite materials. 43 Composite implants allow radiotherapy but in order to be detectable in CT and MRI they need radio-opacifying filling material which in turn can cause imaging artefacts. 28 Glass fiber reinforced composite has the advantage of being radio-opaque and allowing radiotherapy without dose enhancement.
It is important to understand the effects of the reconstruction material on radiation dose both on the entrance and on the exit side of the implant. The effects can have an impact on treatment outcome and the healing of the operated area. Increase in the radiation dose in the bone and soft tissue adjacent to the implant material can contribute to difficulties in healing. 44 There is no specific amount of radiation that is known to cause osteoradionecrosis but the risk is shown to increase with dose. 45 Our results show that the dose reduces most with titanium compared to other tested reconstruction materials. The possibility to provide a better healing environment is promising. Fiber-reinforced composite does not cause radiation scattering to a measurable account and seems in that perspective to be a good alternative to titanium.
A limitation to the study is that the results are not directly comparable to a clinical setting. The study setting was kept simple in order to eliminate measurement artifacts thus making it easier to understand the differences between the tested materials. Single direct photon beams are not typically used in radiation therapy anymore, instead almost all curative intent head and neck radiation treatments are delivered with multiple IMRT beams or dynamically rotating fields (VMAT). However we considered it a good way of comparing the performance of the materials. Assessing the clinical effect of each material separately with a modern radiation treatment plan, using an anthropomorphic phantom and measurements or Monte Carlo simulation, would make it harder to compare the differences between the materials since the plan would vary with different materials present.
We conclude that glass fiber reinforced composite is a promising material based on its minimal interaction with photon radiation. We found that this material is safer than titanium in terms of tissue healing and predictability of dose distribution during radiotherapy. Glass fiber reinforced composite may be a material of choice for craniofacial surgery for patients undergoing multimodal treatment of head and neck cancer.

ACKNOWLEDG MENTS
The authors thank Hanna Mark for her assistance with the preparation of the FRC samples.

CONFLI CT OF INTEREST
Author Pekka Vallittu is a member of the Board and shareholder of Skulle Implants Corporation.