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Nerilee Baker, RN, Intensive Therapy Certificate. Luke Bromilow, RN. Intensive Therapy Certificate. Joan Costigan, RN (General and Psych), Intensive Therapy Certificate. This study was undertaken as part of the Intensive Therapy Course, 1989 - 90 at St. Vincent's Hospital, Sydney. |
A radiation safety study was conducted in the Intensive Therapy Unit of St Vincent's Hospital, Sydney. Scatter of ionising radiation around the bed area during chest X-rays was measured with an ion chamber to determine the safe distance for staff to stand to minimize radiation exposure. The results show that scatter is detectable up to and including two metres from the bed and that within this area cumulative exposure over one year is unlikely to exceed the annual dose-equivalent limit. Measures should be taken to increase staff awareness of mandatory precautions during X-ray examinations.
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An occupational hazard of working as a nurse in an intensive therapy unit is the possible exposure to ionizing radiation from portable diagnostic X-rays. The number of portable chest X-rays taken in the Intensive Therapy Unit of St Vincent's Hospital, Darlinghurst, during the month of May in which our study took place was 311. These include routine monitoring, post operative, post procedure X-rays, and those required to assess the patient's changing condition.
A study conducted in the Netherlands found that several misunderstandings, misconceptions, and erroneous beliefs exist in regard to in-hospital radiation hazards, and concluded that there was likely to be ignorance among hospital personnel outside the Netherlands also (Janssen and Wellens, 1989: 36-38).
X-rays are a form of ionizing radiation produced by firing electrons at a heavy metal target. The electrons then release energy as X-rays. When ionizing radiation penetrates living tissue it can destroy living cells or make them function abnormally. It does this by physically removing an electron from water molecules in the cell converting them to free radicals. These in turn break the DNA chain or scramble its coding. Although most chromosomes repair normally, impaired chromosome coding may continue if mitosis occurs before the repair process. The cells that are least differentiated and multiply the most rapidly are the most radiosensitive. Alteration of DNA in cells can lead to cancer. genetic mutation, or direct tissue death.
Ionising radiation exposure has a cumulative effect, and as a result, annual maximum permissible dose equivalents have been determined by the National Health and Medical Research Council (NH&MRC, 1984: 172).
Sources of secondary radiation during mobile X-rays include scatter from the patient and X-ray cassette in all directions, from the primary beam, and leakage from the X-ray tube. (refer figure 1) (National Health and Medical Research Council (NH&MRC, 1984: 168).
Assumptions have been made that this secondary radiation is uniform around the bed area and uniform at each bed area in the Intensive Therapy Unit.
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Figure 1. Diagram of potential sources of ionising radiation during a portable chest X-ray (adapted from Johns, 1961: 624, Figure XVII-I).
The hypothesis of this study is that during the taking of portable, diagnostic, anteroposterior, erect chest X-ray films it is safe to stand two metres from the bed and the cumulative exposure closer to the bed is negligible over one year.
Measurement of the area of scatter of ionizing radiation at points around each bed area in the Intensive Therapy Unit will provide objective data on which to base education of personnel regarding the avoidance of unnecessary exposure to ionizing radiation during portable diagnostic X-rays.
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Westmead and Parramatta Hospitals' Medical Physics Department conducted a study in 1989 which aimed to measure ionizing radiation exposure to nursing staff involved in or adjacent to mobile chest X-rays. A chest phantom was placed in the bed and exposed to 75 kVp for 6mAs. Readings were taken using a mdh dosimeter with a 180 cc scatter chamber at arbitrary points around a bed, ranging from 0.5 - 2.Om from the phantom at gonad level.The study concluded that there was no radiation safety hazard to staff in excess of two metres from the patient and for closer distances there was minimal exposure (Medical Physics Department, Notes, Westmead Hospital, 1989).
In 1989, Lawrence Sim of Princess Alexandra Hospital, Brisbane, conducted a study of scattered radiation during per-operative screening. He simulated a hip pinning procedure using an Alderson diagnostic radiology phantom and a Phillips BV25 mobile image intensifier. The measurements of scatter were taken at both eye and thyroid level from the floor, using a Pitman 37-D dosimeter and 350 cc Air Equivalent ionizing chamber. The results showed that scatter diminished rapidly with distance from radiation source, and increased in a linear relationship with an increase in the amount of exposure. (Sim, 1989).
A survey of private general dental practitioners in Australia was undertaken in 1986 to determine measures taken to reduce X-ray exposure. The study covered the use of protective devices such as lead aprons, but of particular interest was a diagram of X-ray scatter around the patient at the gonad level of the operator. taken from the work of Rolofson, Hamel, and Stewart. It showed exposure to occur anywhere from 0-3m from the patient, depending on the direction the X-ray was taken. The study found that a considerable number of dentists stand closer than two metres to the patient during the taking of an X-ray (Monsour, 1988).
Studies using thermoluminescent dosimeter badges to detect exposure to ionising radiation of nursing staff over a longer period were conducted in the USA. In Boston, the CCU nurses were monitored over a 3 year period for cumulative exposure to ionising radiation from the potential sources in their unit such as portable X-rays, fluoroscopy, and radiopharmaceuticals. The recommended dose limit equivalent was 125mR over a three month period for non-occupationally exposed workers. The results of the study showed there was "no cumulative exposures over SOMR during the entire study period". (Jankowski. 1984, pp 55-58) The University Medical Centre of the University of Arizona conducted a 9 year retrospective study of the exposure of Emergency Department personnel to ionising radiation. The personnel were divided into groups according to their job. 'The average exposure to nursing personnel was 0.70 mrem/month.' They recommended maximum exposure for people not occupationally exposed was 500 mrem/year (Grazer, et al, 1987).
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The design of this study is based on the method and results of the Westmead Hospital study, mentioned previously, and conducted with the advice and assistance of the Radiation and Safety Officer of St Vincent's Hospital, and the Chief Physicist of Westmead Hospital. The aim of the study is to quantitatively describe the field of scatter of ionising radiation around the bed area during portable, diagnostic, antero-posterior, erect chest X-rays.
Several differences exist between this study and that of Westmead Hospital. This study was conducted in the clinical setting of the Intensive Therapy Unit of St Vincent's Hospital, over a period of 13 days. Data was collected during the routine morning and post operative chest X-ray examinations of patients capable of sitting upright. X-ray energies varied according to patient requirements, whereas the Westmead Hospital study was conducted in a more controlled environment using a chest phantom at only two X-ray energies (Medical Physics Department, Notes, Westmead Hospital, 1989).
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Ten points of measurement around the bed area were chosen arbitrarily, based on data from the Westmead Hospital study which recommended a safe distance of two metres from the patient. Between four and eleven readings were taken at each point. More readings were taken at points considered to be significant, examples being, next to the bed where nursing staff may be required to stand during chest X-rays to support the patient, and two metres from the bed where scatter was anticipated to be negligible and would verify the safe distance as recommended by the Westmead Hospital study.
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Measurements were taken with a MDH industries 2025 series X-ray monitor incorporating 180ce air-equivalent ion chamber and a digital processor. (refer figure 3) This monitor is designed for verification of proper operation of diagnostic X-ray machines, fluoroscopes. and other radiation sources. It measures ionising radiation by, converting the charge of ions created by X-rays passing through the chamber, to current. and expressing this as a Radiation Absorbed Dose in milliroentgens (mR). Response of the ion chamber is dependent on ambient temperature and pressure and should be recalibrated when used in varying environments. (MDH Industries, Information Leaflet). Calibration of this monitor had been done at room temperature (22°C) and sea level on 21/2/90.
Other methods of data collection were considered. including, thermoluminescent dosimetry badges (MD), film badges, and personal dosimeter. The TLD badges have limited accuracy at the level of exposure anticipated in the study and would have required 20 - 30 X-rays at each point to obtain valid data. Similarly, the film badges have a sensitivity 10 times less than the TLD badges and are intended for monitoring radiation exposure over long periods (1 - 3 months). A sufficient number of both types of badges were unavailable for a satisfactory study using this method. The personal dosimeter is substantially more sensitive than the TLD badges (20 times) and uses a ion chamber that is much smaller and therefore not as accurate as the 180cc ion chamber. Had the 180cc ion chamber been unavailable, the personal dosimeter would have been suitable for data collection (Eberl. 1990).
The X-ray machine used in St. Vincent's Intensive Therapy Unit, is a condensor-discharge type manufactured by Showa, model SCM-ZM. Its maximum output is 125 kilovolts (KV) and 50 milliampere-seconds (mAs). The machine is subject to routine checks every six months to ensure proper function within the limits required by the hospital.
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The ion chamber was placed on a variable height, mobile table, secured, and the Rate key pressed to allow the chamber to warm up and give a OmR reading. Prior to X-ray exposure, the patient was positioned upright in bed with the X-ray cassette behind his/her back. The bed was levelled and adjusted to a predetermined height (60 cm). For each X-ray exposure, the radiographer positioned the X-ray machine. The following measurements were taken using a tape measure and spirit level (refer Figure 2).
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Figure 2. Diagram of the measurements taken at each bed area
One of the arbitrary points was chosen and its location determined by measuring the distance from the edge of the bed, either perpendicular to the side or end of the bed, or at a 45° angle from the corner of the bed (refer Figure 6). The ion chamber was positioned at this point, at the same height as the mid-sternum of the patient, correlating with the centre of the primary X-ray beam, with the flat surface facing the X-ray cassette.
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Figure 3: Diagram of the points around each bed area at which readings were taken
Immediately prior to the X-ray being taken, the Exposure key on the ion chamber was pressed. Following exposure of the X-ray, the reading was recorded from the ion chamber in mR, and the X-ray technique ascertained from the radiographer in KV and mAs.
The patient weight was determined using the patient's medical notes, by questioning the patient, or by estimation where weighing the patient was not possible.
Extraneous information which was recorded included date/time of X-ray, Medical Record Number of patient, bed number, and location of recording point, left or right side of the bed.
Readings were taken at each bed area over two one-week periods. Measurements were taken and recorded by the researchers, but X-rays were taken by several radiographers over the 13 days.
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The scatter dose at each point, measured in milliroentgen (mR), is a measurement of ionisation produced in air by the radiation scatter. The limits on exposure to radiation set down by the NH&MRC are given in the SI unit of millisievert (mSv) which is a measure of the biological effect of radiation in tissue rather than its effect in air.
To convert results the following equation is used:
DOSE EQUIVALENT (µSv) = EXPOSURE (mR) x 8.7
After conversion to SI units, the highest reading and mean at each point were used to calculate the number of X-rays staff may be exposed to without exceeding the limit of lmsv as determined by the National Health and Medical Research Council. (NH&MRC, 1984: 171).
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| (Mean) | (Highest dose) | ||||
| A | 0 | 0 | 0 | Unlimited | Unlimited |
| B | 0.012 | 0.004 | 0.1044 | 9,579 | 5,747 |
| C | 0.189 | 0.073 | 1.644 | 608 | 383 |
| D | 0.002 | 0.0042 | 0.0174 | 57,471 | 11,494 |
| E | 0.0075 | 0.005 | 0.06525 | 15,326 | 11,494 |
| F | 0.0589 | 0.02 | 0.5124 | 1,952 | 11,494 |
| G | 0.006 | 0.0056 | 0.0522 | 19,157 | 11,494 |
| H | 0 | 0 | 0 | Unlimited | Unlimited |
| I | 0.01 | 0.009 | 0.087 | 11,494 | 5,747 |
| J | 0.0037 | 0.007 | 0.03219 | 31,065 | 5,747 |
Table 1: Results.
Figure 4: Graphic representation of scatter around bed area
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Limits on exposure to ionising radiation as stated by the NH&MRC are 50 millisieverts per year for radiation workers and 5 millisieverts per year for members of the public (persons present during radiographic procedures but not classed as persons occupationally exposed). If exposure to this group is greater than 5mSv over a number of years, the limit is decreased to 1mSv. Nurses working in critical care areas fall within this category as determined by the Radiation Safety Officer of the hospital.
The results also demonstrate that exposure at points near the bed, where nursing staff may be required to stand to support patients, would only pose a potential radiation hazard if this became standard practice during all chest X-rays over the year. Even then, it is unlikely that the stated limits would be exceeded. It must be noted however, that the NH&MRC recommends that all practical steps be taken to minimise exposure to ionising radiation; this is referred to as the ALAPA principle (as low as reasonably achievable). (NH&MRC, 1984: 170).
The findings of this study correlate with those of the Westmead Hospital study. This study was conducted in the clinical setting rather than in a controlled environment, to reflect the actual potential exposure of staff. Radiation scatter varies significantly depending on the voltage and current of the X-ray and the radiographers technique. These factors are influenced by patient weight and size, the pathology being examined, and the position of the patient in relation to the X-ray machine (Collins, 1988: 61).
Limitations of this study include:
Further research into these areas is recommended.
In keeping with the ALARA principle, it is recommended that:
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