161Radiation absorption of orbital implants

Correspondence andreprint requests to:Zeynel A. Karcioglu, MDTulane Univ. Sch. Med.1430 Tulane Ave., Room 5020New Orleans, LA 70112USATel.: +1-504-588-2261Fax: +1-504-584-2684

Acknowledgements:The paper was presented in part at theARVO Meeting in Ft. Lauderdale,Florida in May, 1995. This work hasbeen supported in part by anunrestricted grant provided by the St.Giles Foundation of New York City,NY (Dr. Karcioglu).

Clinical research

Orbit 0167-6830/98/US$ 12.00

Orbit – 1998, Vol. 17, No. 3,pp. 161-167© Æolus PressBuren (The Netherlands) 1998

Accepted 10 November 1997

Radiation absorption properties of orbitalimplants

Zeynel A. Karcioglu, M.D.1,3,4

Hassan Al-Ghamdi, BS.2

Abdulkarim Al-Bateri, PhD2

Assem Rostem, M.D.2

1King Khaled Eye Specialist Hospital and 2King Faisal SpecialistHospital & Research Centre, Riyadh, Saudi Arabia, 3Departmentof Ophthalmology and 4Cancer Center, Tulane University, New

Orleans, LA, USA

Abstract

purpose To determine the radiation absorption properties (RAP) ofthree commonly used orbital implant materials, namely, methylmetha-crylate (MM), hydroxyapatite (HA), and porous polyethylene (PP).

methods Eighteen (18)-mm spheres of MM, HA and PP were testedwith 1.25 MV ®-rays from cobalt-60, 6 MV X-rays, and 9 MeV, 12MeV, and 16 MeV electron beams. The implants were immersed in awater phantom, and the measurements were obtained on X-omat V®film; the scanning was done with a computerized laser densitometer,CADSCAN®.

results The RAP of all three materials appeared to be very close tothose of water. The density of the MM implant was calculated to be theclosest to that of water at all photon and electron energies. PP had ahigher transmission than water at all electron energies (9, 12 and 16MeV); the transmission through HA, however, was lower than throughwater.

conclusion When postoperative radiation is indicated for an orbitcontaining an implant, the RAP of the allograft material play a signif-icant role in the planning of the radiation treatment. Our study indicat-ed that MM implants have RAP equivalent to those of water whentreatment of orbital tumors is undertaken at the commonly used photonand electron energies. The RAP of the other allografts were eitherhigher or lower, which may lead to unreliability in irradiation planning.

Key words Orbital implant; orbital tumor; porous implant; radia-tion absorption; retinoblastoma

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Introduction Enucleation is a commonly utilized form of treat-ment in many cases of intraocular and adnexal tumors.1 The loss of aneye and surrounding orbital tissues has always been regarded as a dis-aster, usually followed by exhaustive efforts at restoration, functionallyand cosmetically. Many surgical techniques and materials are utilizedto improve the reconstruction of the anophthalmic socket and minimizethe complications of enucleation and orbital surgery.2,3 Although com-plications such as infection, implant migration, extrusion and socketcontracture are discouraging, the most dreaded one is tumor recurrencefollowing enucleation due to intraocular or orbital malignancy.4 In somecases, the orbital implant is not inserted at the time of enucleationbecause of planned postoperative irradiation or the high probability oftumor recurrence in the anophthalmic socket. In other instances, how-ever, the implantation of the orbital sphere is done at the time of sur-gery. If tumor recurrence takes place in the latter situation, irradiationmust be planned and applied in the presence of the orbital implant.Spheres used in orbital reconstruction are made of different materials,with different radiation absorption properties (RAP). The importance ofthe effect that the implant type may have on the planning of irradiationdosage and delivery prompted us to investigate the RAP for the threemost commonly used orbital implants: methylmethacrylate (MM), hy-droxyapatite (HA), and porous polyethylene (PP).

Materials and methods The experiment was designed to deter-mine the RAP of orbital implants relative to water by using differentphoton and electron energies. Water was chosen as the reference stan-dard because its density closely approximates that of normal tissue.Eighteen (18)- millimeter spheres made of MM, HA and PP were tested(Fig. 1).

Several types of radiation were used to carry out the study, namely,1.25 MV gamma rays from cobalt 60, 6 MV X-ray beams, and 9 MeV,12 MeV, and 16 MeV electron beams from a linear accelerator. The

Fig. 1. Orbital implant materialsinvestigated for their radiationabsorption properties, from left toright: methylmethacrylate,hydroxyapatite and porouspolyethylene.

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163Radiation absorption of orbital implants

measurements were done in a small water phantom using Kodak X-omat V® film and a computerized laser densitometer (CADSCAN®).The implants were immersed in sufficient water to cover their surface(Fig. 2). X-ray films were placed under the water phantom and theimplants were exposed to the various radiation beams. A film wastaken for each type of radiation and scanned using a CADSCAN®densitometer.

Fig. 2. Diagrammatic illustration ofthe methodology.

Results The radiation absorption of all three implants was veryclose to that of water for Co-60 and 6-MV X-ray beams (Table 1 andFig. 3).

The density of the MM orbital implant was calculated to be the clos-est to that of water at all photon and electron energies. The transmis-sion of the HA and PP spheres, however, was significantly differentfrom that of water at all electron energies (Fig. 4). Thus, PP showed ahigher transmission than water at all electron energies (9, 12, and 16MeV); although the absorption increased with increasing electron ener-gy. The transmission through HA was lower than that through water atall electron energies (9, 12, and 16 MeV), but here, the transmissionincreased with increasing electron energy.

Discussion Today, different radiation sources, including cobalt-60units, betatrons and linear accelerators, are utilized to treat intraocularand orbital tumors.5 In all irradiation techniques, however, the goal isthe same: the accurate delivery of an adequate treatment dose to thetarget area and the minimization of the scatter onto healthy tissues.6,7

When postoperative irradiation is indicated in the orbit, the RAP of anyallograft material between the radiation source and the target field areextremely important for effective treatment. The target volume is usu-ally located with the help of computerized tomographic cuts throughthe tumor mass and a two- or three-dimensional plan is produced withthe aid of a planning computer. During the planning process, the im-planted prosthesis is handled by the computer as tissue of normal den-sity. However, if the allograft has a higher or lower transmission thannormal tissues, an incorrect treatment plan may result. This could resultin the delivery of either too high a dose to the normal tissues due to

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ENERGY MM HA PP

Co-60 0 -2.50% +3.50%6-MV X-ray 0 0 09-MeV electron beam 0 -28% +17%12-MeV electron beam 0 -25% +13%16-MeV electron beam 0 -13% + 7%

table 1. Transmission of orbitalimplant materials relative to water.

Fig. 3, a&b. Comparison of theradiation absorption of implantmaterials and water when exposed tophoton energies.

higher implant transmission or too low a dose to the targeted tumor dueto lower implant transmission.

Hatjigiorgis et al.8 conducted an in vitro study to determine the influ-ence of the HA allograft on the dose distribution of a 15-MeV electronbeam. Their results indicated that the maximum dose enhancementcaused by HA as a bone substitute was approximately 5%.

Arora et al.9 studied the effects of an hydroxyapatite allograft on dosedistribution during radiation treatment using a 4-MV photon beam. Theiranalyses indicated that the HA implants caused no significant change inthe attenuation or scattering properties of the photon beam. They there-fore concluded that an HA implant can be ignored in the planning of

MV

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165Radiation absorption of orbital implants

Fig. 4, a,b&c. Comparison of theradiation absorption of implantmaterials and water when exposed toelectron energies.

radiation treatment. We agree with their conclusion with respect tophoton energies.

Fontenla et al.10 studied the attenuation and scattering effects of sili-cone, MM and HA orbital implants on the dose distributions of 6, 9,and 12 MeV electron beams. They concluded that lower-energy elec-tron beams are unsuitable for the treatment of tumors behind some

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ocular implants, particularly those made of silicone.10 They reported upto 40% and 10% attenuation for the 6 and 12 MeV electron beams,respectively, thereby indicating that lower-energy electrons lose a sig-nificant amount of their effectiveness when they are targeted to tumorsthrough some orbital implants. In agreement with Fontenla et al., wefound the MM implant to be the closest to normal body density. Ittherefore is the least likely to alter the dose distribution.

In our experiments we used water, the density of which is equivalentto that of normal tissue, to evaluate the RAP of different allograftmaterials. The results indicate that MM has a density equivalent to thatof water when exposed to different photon and electron energies whichare commonly used to treat orbital tumors. The densities of the othertwo allografts were found to be either higher or lower. The results ofdose calculations prior to radiation therapy may therefore be unreliable.

The transmission of high electron energy (16 MeV) through orbitalimplants made of PP and HA was found to be approximately 7% higherand 13% lower, respectively, than through the water controls. Theseresults indicate that the attenuation of lower energy electrons duringirradiation treatment may vary significantly depending on the type oforbital implant. The transmission of PP was found to be much higherthan that of HA at all electron energy levels; this may indicate a needfor lower doses of radiation in cases where the target area for treatmentis behind a polyethylene allograft.

The determination of the RAP of different implant materials has sig-nificance not only for orbital spheres, since many of these allografts arealso used in orbital and craniofascial surgery for other reconstructivework, including the repair of congenital defects, injuries, enophthalmos,etc.11

In the treatment of retinoblastoma patients, the attenuation character-istics of orbital implants become even more significant because of cer-tain peculiarities of the disease.12 In bilateral cases, if one eye is re-moved and replaced with an orbital implant and the other eye requiresfurther radiation treatment, the transmission characteristics of the im-plant become very important since the radiation may need to be deliv-ered via the socket which contains the implant.

Another issue in retinoblastoma is the development of second primarymalignancies.13,14 There is general agreement that radiotherapy increas-es the incidence of second primaries in patients with retinoblastoma.14,15

In the study of Roarty et al., the 30-year incidence of second malignantneoplasms following radiation therapy for retinoblastoma was reportedto be 35.1%; 29.3% of these neoplasms developed within the field ofradiation.15 In the same series, only 5.8% of the patients who did notreceive radiotherapy developed second malignancies. Therefore, treat-ment with high-energy radiation delivered via implants should not al-low the soft tissues behind the implant to receive unnecessarily highdoses.

In summary, special attention should be paid to the RAP of allograftmaterials when tumor patients with orbital implants receive radiationtreatment. The nature of the implant may affect the dosage calculationsin the radiotherapy of recurrent orbital malignancies or second prima-ries and in the treatment of the fellow eye through an opposite port in

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3 Danz W. Mobility implants: a review.In: Bosniak SL, editor. Advances inOphthalmic Plastic andReconstructive Surgery - TheAnophthalmic Socket. New York:Pergamon, 1990:46-54.

4 Karcioglu ZA, Mullaney PB, MillarLC. Extrusion of porous polyethyleneorbital implant in recurrentretinoblastoma. Ophthalm PlastReconstr Surg 1998;14:37-44.

5 Alberti WE, Schipper J, SagermanRH. Radiation techniques for thetreatment of retinoblastoma andorbital tumors. In: Alberti WE,Sagerman RH, editors. Radiotherapyof Intraocular and Orbital Tumors,Berlin: Springer-Verlag, 1993:332-45.

6 Schipper J. An accurate and simplemethod for megavoltage irradiationtherapy of retinoblastoma. RadiotherOncol 1983;1:31-41.

7 McCormick B, Ellsworth R,Abramson D, et al. Radiation therapyfor retinoblastoma: Comparison ofresults with lens-sparing versus lateralbeam techniques. Int J Radiat Oncol

cases of bilateral retinoblastoma, where one eye has been replaced withan orbital allograft. However, the data obtained from the literature, aswell as our results, are based on the testing of allografts before theirimplantation into the orbit. This does not take into account the fi-brovascular ingrowth which takes place postoperatively in porousspheres. Studies to determine the attenuation properties of orbital spheresfollowing implantation are under way in our laboratory.

Biol Phys 1988;15:567-74.8 Hatjigiorgis CG, Shiu AS, Fleming

TJ, Kramer DC. Influence of calciumphosphate hydroxyapatite on dosedistribution in electron beam radiationtherapy. Int J Oral MaxillofacImplants 1987;2:65-8.

9 Arora V, Weeks K, Halperin EC,Dutton JJ. Influence of corallinehydroxyapatite used as an ocularimplant on the dose distribution ofexternal beam photon radiationtherapy. Ophthalmology 1992;99:380-2.

10 Fontenla DP, Ahmed M, Chuic S, etal. Effect of ocular implants ofdifferent materials on the dosimetry ofexternal beam radiation therapy. Int JRadiat Oncol Biol Phys1995;32:1477-80.

11 Fries PD. Autogenous, alloplastic,integrated and resorbable implants fororbital blow-out fracture repair. Orbit1994; 13:135-45.

12 Karcioglu ZA, Mullaney PB, Al-Mesfer AS. Porous polyethyleneorbital implant in retinoblastomapatients. Ophthalmology 1998; inpress.

13 Abramson DH, Frank CM. Secondnonocular tumors in survivors ofbilateral retinoblastoma.Ophthalmology 1998;105:573-80.

14 Karcioglu ZA, Mullaney PB, Al-Mesfer SA. Porous polyethyleneorbital implant in retinoblastomapatients. Ophthalmology 1998;105:1311-6.

15 Roarty JD, McLean IW, ZimmermanLE. Incidence of second neoplasms inpatients with bilateral retinoblastoma.Ophthalmology 1988;95:1583-7.

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