Dentomaxillofacial Radiology (2006) 35, 14-17
© 2006 British Institute of Radiology
doi: 10.1259/dmfr/28125805
Evaluation of scatter dose of dental titanium implants exposed to photon beams of different energies and irradiation angles in head and neck radiotherapy
M Beyzadeoglu*,1,
B Dirican1,
K Oysul1,
J Ozen2 and
O Ucok3
1 Department of Radiation Oncology, Gülhane Military Medical Academy, Ankara, Turkey;
2 Department of Prosthetic Dentistry, Gülhane Military Medical Academy, Ankara, Turkey;
3 Department of Oral Diagnosis and Radiology, Gülhane Military Medical Academy, Ankara, Turkey
*Correspondence to: Dr Murat Beyzadeoglu, Gulhane Military Medical School, Department of Radiation Oncology, 06018, Etlik, Ankara, Turkey; E-mail: muratbeyzadeoglu{at}yahoo.com
Received 21 December 2004;
revised 27 April 2005;
accepted 13 June 2005
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Abstract
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Objectives: In this study, quantification of backscatter doses between scattering titanium dental implant and the thermoluminescent dosimeter (TLD100) radiation detector at axial beam irradiation-angle range of 0–90° in head and neck radiotherapy is done to evaluate irradiation angle dependency of dose enhancement contributing to osteoradionecrosis.
Methods: A cylindrical titanium dental implant with diameter of 4 mm and length of 9 mm was implanted into a specially-designed human mandible phantom with a TLD100 chip placed on the buccal site and irradiated with 6 MV X, 25 MV X and Co-60 gamma sources at 19 different irradiation angles.
Results: Dose enhancement on a buccal site of the titanium implant depends on the incident beam angle. At angles of 65°, 60° and 40° the maximum detected scatter doses over the titanium implant are 36%, 32% and 23% for Co-60 gamma, 6 MV X-ray and 25 MV X-ray, respectively. The dose enhancement at different beam angles was less pronounced in 25 MV X and more pronounced in Co-60 gamma irradiation.
Conclusions: For the different radiation beams studied, the irradiation angle between scattering titanium dental implants and the central axis does not significantly affect the total dose that may lead to osteoradionecrosis of the mandible.
Keywords: backscatter radiation; titanium dental implant; head and neck cancer; osteoradionecrosis
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Introduction
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Because head and neck cancer patients tend to be over the age of 50 years, they constitute a group likely to have dental prostheses. Prosthesis-associated trauma and ulcers, neurovascular impingement,1,2 and masticatory efficiency loss may lead to the replacement of conventional removable prostheses with those supported by dental implants. These implants are made of titanium and titanium alloys forming an oxide layer to provide protection from corrosion.3,4 An interesting aspect of titanium implants is the effect of these implants on the surrounding bone during radiotherapy of oral, nasal and paranasal neoplasms.
Radiation scatter from high atomic number (Z) materials is established to cause both soft tissue and bony complications in the oral cavity, making scattered radiation an important factor in head and neck region radiotherapy planning. Osteoradionecrosis, a dreaded complication of the use of radiation therapy in the treatment of head and neck cancer, has clinically been defined in literature as irradiated bone that has failed to heal in 3 months; more specifically, it is best described as a slow-healing radiation-induced ischaemic necrosis of the bone with associated soft tissue necrosis of variable extent occurring in the absence of tumour necrosis, recurrence, or metastatic disease.5
Clinical manifestations of osteoradionecrosis may include pain, orocutaneous fistula, exposed necrotic bone, pathological fracture, and suppuration.6–8 It is more commonly seen in the mandible than in the maxilla due to the relatively decreased vascularity and increased bone density of the mandible. The mandible often receives a greater dose of radiation than the maxilla. Radiation dose is a contributing factor to the development of osteoradionecrosis as well as tumour location, dental trauma, the pre-morbid state of dentition and concomitant chemoradiotherapy.9–12
Radiation therapy treatment plans may be designed to avoid metallic implants when possible or multiple fields may be used to minimize the scatter dose. Nevertheless, the main issue is judging whether or not there is a need to remove titanium implants from patients to be irradiated. Implant removal is traumatic and yields the patients toothless. Measuring of the scatter factor for the bone–titanium interface also provides grounds for further research of the effects of radiation on titanium implants.8
It is widely known that when metal objects are in the path of megavoltage X-ray and gamma ray beams, a significant amount of scattered radiation results.13 The dose enhancement magnitude is dependent on the atomic number of the scatterer for both electron14–16 and photon17–20 beams as well as the beam angle.
Quantitative backscatter dose factor (BSDF) is defined by the equation:
where Di is the dose at the interface upstream and Dh is the dose at the same point in a homogeneous tissue-equivalent phantom for the same primary photon fluence, whereas E is the energy parameter of the photon beam, A is the field size at the point measurement, w is the width of the inhomogeneous material, d is the thickness of medium above the inhomogeneous material, t is the thickness of this material, x is the upstream distance away from the interface, Z is the atomic number of the inhomogeneous medium, and 
is the angle of the photon beam.21,22
In the present study, we evaluate the dose enhancement caused by a titanium implant at different irradiation angles between 0 and 90° for Co-60 gamma, 6 MV X-ray and 25 MV X-ray in order to detect whether irradiation angle difference in head and neck radiotherapy restricts the tumour target dose or increases the normal tissue radiomorbidity that may result in osteoradionecrosis.
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Materials and methods
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A special phantom, a human mandible (effective Z=7.8) surrounded with water simulating soft tissue density (effective Z=7.4), was used for the measurements. A pure titanium (Z=22) root-form cylindrical implant with diameter of 4 mm and length of 9 mm was implanted into the human mandible. Thermoluminescent dosimeter (TLD100) chips with 3.2 mm x3.2 mm x0.9 mm size and thermoluminescent dosimeter reader (Victoreen 2800, USA) were used for dose measurements.
In our previous study evaluating the scatter factor of different dental implants, we had found that the backscatter factor is not significant 2 mm from the implant.8 So, in this study we decided to use only one TLD100 chip in the buccal site of the implant.
The phantom was irradiated with three different energies of Co-60 gamma (1.25 MeV), 6 MV X-rays and 25 MV X-rays. A Co-60 teletherapy machine (Theratron 780, Nordion, Canada) and a dual energy linear accelerator (Philips SL-25, UK) were the sources for irradiation (Figure 1
). The radiation field size was 10 cm x 10 cm with source–axis distance (SAD) of 100 cm for the linear accelerator (6 MV and 25 MV X-ray) and 80 cm for the Co-60 teletherapy machine. The large surface of the TLD100 was placed on the titanium dental implant in the mandibular buccal site and the centre of the TLD100 chip was adjusted to the isocentre of the incident radiation beam. The phantom was irradiated at 19 different angles with 5° intervals between central beam axis being perpendicular to the surface of TLD100 at 0° and parallel to TLD100 surface at 90° (Figure 2A, B). The implant was given a total dose of 2 Gy and measurements were repeated three times. The scatter factor was determined by the ratio of measured scattered dose with implant to measurement without implant.
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Figure 1 Special phantom; human mandible with TLD100 and titanium implant placed and surrounded with water in actual Philips SL-25 irradiation machine
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Results
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Scatter measurement results for each energy are given in Table 1. Maximum backscatter factors were measured at 65°, 60°, 40° for Co-60 gamma, 6 MV X-rays and 25 MV X-rays, respectively. Backscatter factor for Co-60 gamma rays was 1.19 at 0° and 1.00 at 90°, whilst maximum backscatter factor was noted as 1.36 at 65°. For 6 MV X-rays, backscatter was 1.18 at 0° and 1.00 at 90°, whereas the maximum backscatter was 1.32 at 60°. For 25 MV X-rays, maximum backscatter factor was 1.23 at 40° and 1.16 at 0° and 1.00 at 90°. The percentage difference between 0° and maximum backscatter angle is shown in Table 2.
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Table 1 Average backscatter factor (BSF) with standard deviation (SD) of different degrees for three radiation energies
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Figure 2 A, Mandibular assembly for measurement of scattered radiation. 1=implant position B, Implant cylinder with TLD100 chip and different irradiation angles a, implant cylinder; b, TLD100 chip
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Discussion
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In this study, backscatter dose from titanium implants due to obliquely incident photon beams for different energies and irradiation angles was studied. Backscatter doses at a buccal site for titanium implants were measured using a TLD100 detector at 19 different angles. Backscatter dose measurements reported in the literature have usually been done under the condition of perpendicular beam angle between the scattering material and radiation field (zero angle). However, the radiation fields and surface of the scattering materials are not always perpendicular in the clinical radiotherapeutic setting. In an article by Nadrowitz,20 backscatter dose was found to increase with varying beam angles from 0° to 90° up to a maximum at 55° for copper and silver implants, whereas we have studied titanium implant backscatter dose that may contribute to radiation morbidity in an actual radiotherapy setting. Our experimental study allowed us to determine backscatter doses at varying beam angles. Backscatter factor was determined as 1.19 at 0° and 1.00 at 90°, whilst maximum backscatter factor was noted as 1.36 at 65° for Co-60 gamma ray. We also measured maximum backscatter factors at irradiation angles of 65°, 60°, 40° for Co-60 gamma, 6 MV X-rays and 25 MV X-rays, respectively.
Our results with the same irradiation angle of zero on the buccal site are consistent with the results of Wang et al.12 However, we have used a third source of irradiation, Co-60, in addition to higher energy Linac X-rays, with the lower radiation energy of Co-60 preferred for clinical head and neck radiotherapy, with different irradiation angles allowing us to state that "as energy decreases, scatter dose enhancement increases and as irradiation angle increases, scatter factor increases until a peak angle in the range of 40° to 65°, then decreases".
The interaction of high energy X-rays and gamma rays with matter is the result of the Compton Effect, in which the photon collides with electrons in the material to produce a broad spectrum of secondary electrons by inelastic collision processes and by pair production in which the photon is absorbed and a positron-electron pair is produced. The degree of Compton and high-energy electron dose enhancement at an interface depends primarily on electron scattering differences at the bone-titanium interface. The Compton scattering process is independent of the atomic number (Z) of the material and pair production scattering increases with the Z2 while Compton scattering process increases with the electron density of the material.23 We have used pure titanium implants with effective atomic number of 22 and 4.5 g cm–3 electron density.12 Due to these chemical and physical properties, the soft tissue dose on the scatter side of the interface exceeds the dose to soft tissue resulting from the scattering cascade of secondary electrons from the higher atomic number material. The scatter factor differs at varying angles due to secondary electrons which are scattering laterally or in opposite direction of the photons. Our results are consistent with the described scattering process as we measured a smaller range for the backscatter electrons at a beam angle of 0° compared with 65° for Co-60. The distribution of backscatter electrons correlates well with the dose peak at 65°. If the beam axis is parallel to the surface of the implant (beam axis angle is 90°), a smaller backscatter dose is measured compared with the beam which is perpendicular to the implant axis (0°) due to the fact that the beam diverges and the implant acts as an absorber for secondary scattered electrons.
In conclusion, for the different radiation beams tested, the irradiation angle between scattering titanium dental implants and the central axis does not significantly affect the total dose that may lead to osteoradionecrosis of the mandible. Thus, removal of the implants before radiotherapy may not be indicated.
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Abstract
Introduction
