British Journal of Radiology (2005) 78, 637-638
© 2005 British Institute of Radiology
doi: 10.1259/bjr/74393791
A local diagnostic reference level for velopharyngeal investigations
L R Bridge, BSc, MSc
1
D S Hillier, DCR(R)
2
D E Bonnett, MSc, PhD
1 and
N Bowley, FRCR
3
1 The Medical Physics Department, The Kent Oncology Centre, Maidstone Hospital, Kent ME16 9QQ, 2 Diagnostic Imaging Department, Surrey and Sussex Healthcare NHS Trust and 3 Medical Imaging Department, Queen Victoria Hospital, East Grinstead, West Sussex RH19 3DZ, UK
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Abstract
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The purpose of this study was to derive an initial local diagnostic reference level for velopharyngeal investigations carried out as standard radiological practice in the Medical Imaging Department, Queen Victoria Hospital, East Grinstead. This is a specialist video-fluoroscopic radiological technique used to evaluate velo-pharyngeal dysfunction, especially for paediatric patients. A retrospective analysis over a period of 7 months involving 50 examinations yielded dose–area product values ranging from 0.04 Gy cm2 (minimum) to 0.37 Gy cm2 (maximum) with a mean value of 0.11 Gy cm2 and 3rd quartile value of 0.12 Gy cm2. The maximum effective dose was estimated as 0.016 mGy. An initial local diagnostic reference level of 0.12 Gy cm2 has been levied.
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Introduction
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Velopharyngeal investigation (VPI) is a video-fluoroscopic radiological technique used to evaluate velopharyngeal dysfunction [1–3]. This is a speech impairment which results in the sufferer speaking with a severe nasal tone. Velopharyngeal dysfunction can be congenital or a secondary symptom of cleft palate surgery. Following the examination, patients can be treated with surgery, or occasionally will respond purely from speech therapy. The investigation is often repeated for post treatment evaluation. This specialized technique is carried out routinely by The Medical Imaging Department at the Queen Victoria Hospital, East Grinstead, therefore a local diagnostic reference level (LDRL) was required for the implementation of Regulation 4 of The Ionising Radiation (Medical Exposure) Regulations (IR(ME)R) 2000 [4].
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Methods
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All VPI procedures were performed, using a common radiological facility. This was a Toshiba GCU Fluoroscopy/Radiography system (Toshiba Corporation, Tokyo, Japan), consisting of a KXO-80F generator, DRX3624-HC undercouch X-ray tube (0.6/1.2 mm focal spot), DT-GCU 90°/90° tilting table with a triple field (23 cm, 17 cm and 11.5 cm) Model RTP9211G-P6 image intensifier. The total filtration of the incident X-ray beam emerging from the patient couch was estimated to be 3.5 mm Al by measurement of the half value layer and the use of published data [5]. The fluoroscopic image was fed into a Panasonic AG-5260 video cassette recorder (VCR). Additionally an AKG gun microphone is positioned to record the patients speech during the procedure.
A Diamentor dose–area product (DAP) meter (PTW, Freiburg, Germany) has been retrospectively fitted to the undercouch tube assembly, to routinely monitor patient doses. The DAP meter was calibrated following the procedure described in the National Protocol for Patient Dose Measurement in Diagnostic Radiology [6], whereby the DAP meter readout was adjusted to agree with the DAP value derived from a dose measurement using a Radcal 2025 electrometer with 3 ml ionization chamber (Radcal Corporation, Monrovia, CA). This took account of the attenuation of the beam by the couch top. The uncertainty in the DAP measurement was assessed as ±7%.
With the patient in the erect position, a lateral fluoroscopic image centred at the angle of the mandible is viewed. The normal (23 cm diameter) field of view is usually employed. The X-ray beam is minimally collimated to include the soft palate, tongue and epiglottis and it is normal practice for the image of the collimators to be well contained within the imaged field of view. Further dose optimization is practised by manual deselection of the antiscatter grid.
During the procedure the fluoroscopic image and patient's verbal response to a series of requests from a Speech Therapist are recorded on the VCR. The X-ray tube exposure is under semi-automatic control with the tube current manually selected at 0.5 mA, and the tube kilovoltage (kV) under automatic adjustment. In practice because of the fixed nature of the patient geometry, the tube kV selected remains virtually constant during the exposure. The IR(ME)R Operator controlling radiation exposure is a Registered Radiographer. Other Operators present in the X-ray room include a Consultant Plastic Surgeon and a Speech Therapist. For paediatric patients a comforter and carer (patient's parent or guardian) is usually present to comfort the patient.
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Results and discussion
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Data were analysed over a period of 7 months, for 50 examinations. The majority of patients (90%) were less than 20 years of age, illustrating that most referrals are for paediatrics and teenagers. The youngest patient was 4 years of age and the oldest patient was 44 years of age with the a third quartile value of 11 years of age.
Although the procedure could be protracted due to poor compliance with the Speech Therapist's requests, the technique should, in general, be a low dose technique, because of short fluoroscopic exposure times, together with the use of close collimation and absence of an anti-scatter grid. DAP values ranged from 0.04 Gy cm2 (minimum) to 0.37 Gy cm2 (maximum) with similar mean and third quartile values (0.11 Gy cm2 and 0.12 Gy cm2, respectively). Fluoroscopic exposure times ranged from 15 s (minimum) to 70 s (maximum) with mean and third quartile values of 37 s and 43 s, respectively. The mean DAP value of 0.11 Gy cm2 is less than all national reference doses for complete examinations (adult and paediatric patients) recommended by the NRPB [7]. A summary of DAP and fluoroscopy exposure time data is shown in Table 1
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Table 1. Summary of dose–area product (DAP) and fluoroscopy exposure time for velopharyngeal investigation procedures during the analysis period
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Using X-dose [8], the effective dose was estimated by simulation of this procedure using a lateral head radiographic projection. Although X-dose calculates the effective dose for radiographic projections from an input of the DAP value, it was used for this fluoroscopic procedure because of the fixed geometry and orientation of the X-ray beam. This yielded a value of 0.016 mGy at maximum.
An initial LDRL of 0.12 Gy cm2 has been arrived at based on the latest national guidance [9], combined with a pragmatic approach taking into account the levels of radiation dose and risk involved.
The effective dose is very much less than 0.1 mSv and is similar to that for a chest PA radiograph, and may be considered rated as a "1 Trivial" Level of Risk [10], however a literature survey yielded a dearth of information regarding dose information for this procedure, therefore the figures included in this survey may be of use for other Departments carrying out this procedure.
Received for publication December 1, 2004.
Revision received February 3, 2005.
Accepted for publication February 21, 2005.
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References
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- Pigott RW. An analysis of the strengths and weaknesses of endoscopic and radiological investigations of velopharyngeal incompetence based on a 20 year experience of simultaneous recording. Br J Plast Surg 2002;55:32–4.[Medline]
- Shprintzen RJ, Golding-Kushner KJ. Evaluation of velopharyngeal insufficiency. Otolaryngol Clin North Am 1989;22:519–36.[Medline]
- Pigott RW, Makepeace AP. Some characteristics of endoscopic and radiological systems used in elaboration of the diagnosis of velopharyngeal incompetence. Br J Plast Surg 1982;35:19–32.[Medline]
- Ionising Radiation (Medical Exposure) Regulations 2000 SI 2000/1059. London: HMSO, 2000.
- Data for estimating x-ray tube total filtration. The Institute of Physical Sciences in Medicine Report No. 64. York: IPSM, 1991.
- Dosimetry Working Party of the Institute of Physical Sciences in Medicine. National protocol for Patient Dose Measurements in Diagnostic Radiology. Chilton: NRPB, 1992.
- Hart D, Hillier MC, Wall BF. Doses to Patients from Medical X-ray Examinations in the UK – 2000 Review. NRPB-W14, 2002.
- Le Heron JC. XDOSE X-ray Radiography Dosimetry Program using NRPB SR-262 Organ Doses. Christchurch, New Zealand: National Radiation Laboratory. Ministry of Health, 1994.
- Guidance on the establishment and use of diagnostic reference levels for medical X-ray examinations. Joint Working Party Report 88. London: IPEM, 2004.
- International Commission on Radiological Protection. Radiological protection in biomedical research. ICRP Publication 62. Ann ICRP 22 No 3, 1991.
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Abstract
Introduction
