British Journal of Oral and Maxillofacial Surgery (2003) 41, 416 © 2003 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0266-4356(03)00139-6, available online at www.sciencedirect.com SHORT COMMUNICATION Stereolithographic modelling and radiation dosage Andrew J. Gibbons, ∗ Christian Duncan, † Hiroshi Nishikawa, ‡ Anthony D. Hockley, § M. Stephen Dover ¶ ∗ Specialist Registrar in Maxillofacial Surgery; †Fellow; ‡Consultant Plastic Surgeon; §Consultant Neurosurgeon; ¶Consultant Maxillofacial Surgeon, Craniofacial Surgery Unit, Birmingham Children’s Hospital, Steelhouse Lane, Birmingham B4 6HN, UK Hughe’s paper 1 highlights the use of stereolithographic models to facilitate custom-made titanium prosthesis for simple orbital floor reconstructions. In more complex or- bital, defects mirror imaging the normal orbit can be help- ful in determining the correct shape and volume of the damaged orbit. 2 The benefits of these techniques must be weighed against the risks of increased radiation exposure. Com- puted tomography (CT) now accounts for 50% of total radiation from the medical use of ionising radiation. 3 There are various measures of radiation. The ‘tissue absorbed dose’ unit is measured in Grays and is used when discussing the adverse effect of radiation on tissues function, such as cataracts. The ‘equivalent dose’ is the absorbed dose multiplied by a radiation weighting factor and is measured in Sierverts. The radiation weighting factor for X-rays is one, so the equivalent dose equals the absorbed dose. Finally, the ‘effective dose’ takes into consideration which organs have been irradiated and their relative sensitivities to radiation. It is calculated by mul- tiplying the equivalent dose by an organ-specific tissue weighting factor and then summing these values for all organs irradiated. The effective dose is also measured in Sieverts. With improvements in CT, the doseof radiation needed to make accurate stereolithographic models has fallen. 1,2,4 However, the dose to the lens of the eye may be high in scanning orbits and the potential for inducing cataracts must be borne in mind. 3 The radiation-related risk of a patient developing can- cer during their lifetime is inversely proportional to their age; the younger the patient, the greater the risk. Whilst it is important to minimize radiation exposure to all pa- tients, it is particularly important for children. Radiation doses can be reduced by minimising the number of radiographs taken prior to any CT scan, op- timising the settings for scan acquisition and scanning the smallest area required to make the stereolithographic model.If a CT scan is needed to assess orbital trauma consideration should be given as to whethera stere- olithographic model is indicated so that only one CT scan is taken. Shielding of radiosensitive organs is also beneficial. 5 The concept of making an accurate customised plate for orbital reconstruction from a stereolithographic model is attractive. However, it can involve increased exposure to radiation. Whether it gives improved surgical outcomes remains to be established. REFERENCES 1. Hughes CW, Page K, Bibb R, Taylor J, Revington P. The custom-made titanium orbital floor prosthesis in reconstruction for orbital floor fractures. Br J Oral Maxillofac 2003; 41: 50–53. 2. Perry M, Banks P, Richards R, Friedman EP, Shaw P. The use of computer-generated three-dimensional models in orbital reconstruction. Br J Oral Maxillofac Surg 1998; 36: 275–284. 3. Royal College of Radiologists. Making the Best of Department of Clinical Radiology Guidelines for Doctors. 4th ed. London: Royal College of Radiologists, 1998. 4. Arvier JF, Barker TM, Yau YY, D’Urso PS, Atkinson RL, Mc Dermant GR. Maxillofacial biomodelling. Br J Oral Maxillofac Surg 1994; 32: 276–283. 5. Rehani MM, Berry M. Radiation doses in computed tomography. BMJ 2000; 320: 593–594. The Authors Andrew J. Gibbons MA (Cantab), FDSRCS, FRCS Specialist Registrar in Maxillofacial Surgery Christian Duncan MPhil, FRCS Fellow Hiroshi Nishikawa MA (Cantab), MD, FRCS (Plast) Consultant Plastic Surgeon Anthony D. Hockley FRCS Consultant Neurosurgeon M. Stephen Dover FDSRCS, FRCS Consultant Maxillofacial Surgeon Craniofacial Surgery Unit, Birmingham Children’s Hospital Steelhouse Lane, Birmingham B4 6HN, UK Correspondence and requests for offprints to: Dr Andrew J. Gibbons MA (Cantab), FDSRCS, FRCS, Specialist Registrar in Maxillofacial Surgery, Craniofacial Surgery Unit, Birmingham Children’s Hospital, Steelhouse Lane, Birmingham B4 6HN, UK Accepted 3 July 2003 416