6 RADIATION DOSE, RADIATION PROTECTION AND THE IONISING RADIATIONS REGULATIONS

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6 RADIATION DOSE, RADIATION PROTECTION AND THE

IONISING RADIATIONS REGULATIONS

Overview

This chapter discusses the issues of radiation dose, radiation protection and the relevant aspects of United Kingdom legislation as they apply to bone densitometry investigations, concentrating in particular on the technique of dual X-ray absorptiometry (DXA). DXA scanning gives very low radiation dose to patients, and this is an important factor in its popularity as a method of measuring bone mineral density (BMD). An understanding of dose is useful for reassuring patients who may be anxious about radiation risks, while the recording of individual patient exposure is a requirement of the Ionising Radiations Medical Exposure (IRMER) Regulations, and detailed information about dose is needed to complete the new Central Office for Research Ethics Committees (COREC) application form for research studies. The introduction of fan-beam DXA systems increased dose compared with older pencil-beam machines and for certain models (the Lunar Expert in particular) required a re-evaluation of the radiation protection of staff operating equipment. Finally, this chapter reviews the steps required to implement the Ionising Radiations Regulations in departments providing a DXA scanning service.

Radiation Dose in DXA

Studies of radiation exposure to patients from DXA scans have confirmed that the doses involved are small compared with most other radiological investigations using ionising radiation.1-18 However, clinicians requesting scans and staff performing them should be aware that any exposure to radiation carries a potential risk. With diagnostic examinations this is generally very small, and especially for DXA scans where radiation levels for some types of equipment are so low that they are difficult to measure.

The radiation hazards involved in the diagnostic use of X-rays are carcinogenesis (the induction of cancer by exposure to radiation), and in men and women with child bearing potential, an increased risk of diseases caused by genetic abnormalities occurring in their future children.19 Both these hazards are examples of the

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stochastic effects of radiation. The word stochastic means a process governed by the laws of random chance. For radiation this means that in any individual there is a small chance (with a risk that increases with dose) that exposure to X-rays will have a harmful effect. A familiar example of a process governed by the laws of chance is winning the National Lottery. Most people playing the Lottery fail to win anything at all, and by analogy an overwhelming majority of patients having X-ray examinations do not come to any harm from exposure to radiation. However, just as there is a certain tiny chance of winning the Lottery, there is similarly a very small chance that exposure to X-rays may cause cancer in an unlucky individual.

Unlike the Lottery where the exact chances of winning can be mathematically calculated, it has not proved so easy to quantify the risks of radiation induced cancer. This is because cancer is a relatively common disease with many other causes unrelated to radiation. Except in a few special circumstances, such as the studies of the survivors of the atomic bombings of the Japanese cities of Hiroshima and Nagasaki during the Second World War,19 these other causes of cancer are much more common and prevent us from detecting the tiny number of extra cases caused by radiation. The few circumstances in which has been possible to quantify the radiation risk all involve high doses of radiation much larger than those used in diagnostic X-ray examinations. It is therefore necessary to extrapolate the risks from these high doses down to the much lower doses used in medical practice, and in general we do this by assuming that the risk increases in proportion to the dose. Less is known about the risks of causing genetic disease in children of people exposed to radiation because no study of a human population has ever detected this effect. However, we know from experiments with plants and insects that such effects do exist and in principle must occur in human beings too.

Table 1 lists some risks in life comparable to the risk of developing cancer after receiving a radiation dose of 1 microsievert (1 Sv). This is comparable to the dose received by a patient having a spine and hip DXA examination on a GE-Lunar Prodigy system.

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Table 1

Some activities carrying a risk of death comparable to receiving an effective dose of 1 Sv * ___________________________________________________________________

Exposure to natural background radiation for 4 hours Smoking one-tenth of a cigarette

Travelling 3 miles by car

Travelling 15 miles in an airliner Rock climbing for 5 seconds Canoeing for 20 seconds

Working in a factory for half a day Being a woman aged 30 for 60 minutes Being a woman aged 40 for 20 minutes Being a woman aged 50 for 8 minutes Being a woman aged 60 for 3 minutes Being a woman aged 70 for 1 minute

___________________________________________________________________

*

The effective dose for a spine and hip DXA examination is 1.4 Sv on a GE-Lunar

Prodigy system and 7.5 Sv on a Hologic Discovery (see Table 4). Data for other risks in life are taken from E E Pochin 28

Units of Radiation Dose

All accurate measurements of radiation dose are made with the help of ionisation chambers. These measure the amount of electric charge released in a given volume of air when the ionisation chamber is placed in the ray beam. The intensity of an X-ray beam measured in this way is called exposure, and is measured in units called roentgens (symbol, R). Manufacturers' specification sheets for DXA systems often give figures for the X-ray beam intensity expressed in milliroentgens (mR) (1 mR equals one thousandth of a roentgen). This is a simple measurement to make because it only involves performing scans of an ionisation chamber. However, it is not a particularly useful measurement of the radiation risk to patients.

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Measurements of exposure express the effect of the radiation on air. To find the radiation dose to the human body, we need to relate this to the effect in tissue. The first step in doing this is to convert the measurement of exposure in air into the absorbed dose in the human body. Absorbed dose expresses the radiation dose to the body in terms of the energy absorbed from the X-ray beam per unit mass of tissue. The idea behind this is that the more energy that is absorbed from the radiation beam by the body the more damage is done to the tissue. On the molecular scale, this damage results in broken atomic bonds in biologically important molecules such as DNA. The unit of absorbed dose is the gray (symbol, Gy), and 1 Gy is an ab-sorbed dose of 1 joule of energy per kilogram of tissue. One gray is a relatively large dose of radiation, and in diagnostic radiology absorbed dose is usually expressed in milligray (mGy) (1 mGy equals one thousandth of a gray). With DXA systems, dose is often so low that a more convenient unit is the microgray (µGy) (1 Gy equals one millionth of a gray). Because the air in an ionisation chamber consists of atoms with a similar atomic number to the atoms in soft tissue, measurements of exposure in air expressed in mR are readily converted into absorbed dose in soft tissue expressed in µGy with a factor that varies only slightly with the X-ray photon energy. For X-rays generated at 100 kVp, the conversion factor for lean tissue is 9.2 µGy/mR.4

An important measurement usually expressed as absorbed dose is the entrance surface dose. This is the absorbed dose to the skin at the point where the X-ray beam enters the patient's body. Some figures for entrance surface dose for widely used DXA systems are listed in Table 2. As the X-ray beam passes through the pa-tient's body the radiation is attenuated due to the absorption and scattering of the X-ray photons. Internal organs therefore receive progressively lower doses the more shielded they are by overlying tissues, and therefore the average dose to the human body from the X-ray beam is considerably smaller than the entrance surface dose.

Because absorbed dose measures purely the energy transferred by the radiation to the tissue, it turns out not to be the most appropriate way of expressing the biological effects of the radiation such as the risk of cancer induction. The problem is that there are several different types of ionising radiation, including alpha rays, beta rays, gamma rays, X-rays and neutrons. For the same absorbed dose, some types of

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Table 2

Measurements of entrance surface dose (ESD) for different DXA systems. The scan modes are identified by either the X-ray tube current or the scan time.

___________________________________________________________________

DXA System Scan Mode ESD (Gy)

___________________________________________________________________

GE-Lunar DPX Medium (0.75 mA) 10

GE-Lunar Prodigy Thin (0.75 mA) 10

GE-Lunar Prodigy Standard (3.0 mA) 40

GE-Lunar Prodigy Thick (3.0 mA) 80

GE-Lunar Expert-XL 2 mA fast (12 sec) 320 GE-Lunar Expert-XL 5 mA fast (12 sec) 800

Hologic QDR-1000 Quick mode (4 min) 30

Hologic QDR-1000 Performance mode (8 min) 60 Hologic Discovery Express mode (10 sec) 100

Hologic Discovery Fast mode (30 sec) 150

Hologic Discovery Array mode (60 sec) 300

___________________________________________________________________

radiation such as alpha rays and neutrons are more likely to cause harm that others such as X-rays. The biological harm caused by radiation is therefore expressed as the equivalent dose (sometimes also referred to as the radiation weighted dose), which is derived from absorbed dose by multiplying by the radiation weighting factor:

Equivalent dose = Absorbed Dose  Radiation Weighting Factor (1)

The radiation weighting factor is a measure of the relative ability of each particular type of ionising radiation to do biological damage. This ability to cause harm is measured relative to X-rays. Thus for X-rays the radiation weighting factor equals 1.0

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by definition, and the equivalent dose is always numerically equal to the absorbed dose. For those types of radiation that are inherently more hazardous than X-rays such as alpha rays and neutrons it is important to include the radiation weighting factor to accurately express the risks involved. To differentiate between measure-ments of equivalent dose and those of absorbed dose, the former are quoted in units called sieverts (symbol, Sv). As with the gray, 1 sievert is a relatively large dose. Hence in diagnostic radiology, equivalent dose is usually expressed in millisievert (mSv) (1 mSv equals one thousandth of a sievert). For DXA scans, dose is often so low that a more convenient unit is the microsievert (µSv) (1 Sv equals one millionth of a sievert).

Even when we express dose measurements as equivalent dose, we have still not arrived at a way of measuring dose that is particularly useful for expressing the radiation risk to patients. The problem is that different types of X-ray examinations (for example, DXA scans, chest X-rays and skull X-rays) involve exposure of different parts of the human body, and different organs have different sensitivities to the harmful effects of radiation. The final step to arrive at a way of measuring radiation dose that reflects the real radiation risk to patients is to convert the equivalent dose figures for each organ into the effective dose. This is done by multiplying the equivalent dose for each organ by a tissue weighting factor proportional to the sensitivity of that organ to the stochastic effects of radiation, and summing over all the organs exposed:

Effective Dose =

EDT · wT (2)

T

where EDT and wT are respectively the equivalent dose and tissue weighting factor for the Tth organ. The advantage of using effective dose is that the radiation risk from any type of radiological investigation can be summarised in a single figure: thus we can directly compare the radiation hazard to the patient from a DXA scan with a chest X-ray, a CT scan of the abdomen, a radionuclide bone scan, or any other investigation that uses ionising radiation.

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The tissue weighting factors wT are chosen so their sum is unity, i.e.:

wT = 1 (3)

Tj

With this in mind it can be seen from Equation 2 that if every organ in the body is uniformly exposed to the same equivalent dose, the effective dose would equal this uniform dose. Thus effective dose can be defined as the uniform dose to the whole body that carries the same stochastic risks to the patient as the given radiological investigation. Like equivalent dose, effective dose is also expressed in units of sieverts.

Table 3

Tissue weighting factors from ICRP-60 19

___________________________________________________ Tissue Type Weighting factor (wT)

___________________________________________________

Ovaries 0.20

Bone marrow (red) 0.12

Colon 0.12 Lung 0.12 Stomach 0.12 Bladder 0.05 Breast 0.05 Liver 0.05 Oesophagus 0.05 Thyroid 0.05 Skin 0.01 Bone surfaces 0.01 Remainder 0.05 * ___________________________________________________

*

The following 10 tissues are included in the remainder with

weighting factors 0.005 each: adrenal, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen thymus, uterus.

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The above scheme for calculating effective dose was established by the International Commission on Radiological Protection (ICRP) and is explained in greater detail in ICRP Publication 60.19 The list of tissue weighting factors published by the ICRP is summarised in Table 3. The larger the value of the weighting factor the more sensitive the organ is to the harmful effects of radiation. Thus X-ray examinations that involve exposure of the trunk and abdomen are inherently more hazardous because they include organs such as lung, stomach, colon and bone marrow that have high tissue weighting factors. In contrast X-ray examinations that involve exposure of the extremities are less hazardous because they include organs such as skin, muscle and bone that have low weighting factors. The weighting factors listed in Table 3 replaced an earlier set published in 1977 20 and are based on updated information about cancer induction by radiation. In 2005 the ICRP proposed a further revision of the tissue weighting factors, the main differences being a decrease in the factor for the gonad dose expressing the genetic risk, and an increase in the factor for breast tissue reflecting the breast cancer risk.21

Patient doses from DXA

Effective dose is the preferred method of specifying patient dose from DXA investigations because it relates directly to the radiation risk involved. A number of studies have published figures for Hologic1,3,4,5,10,12,13,15,17,18 and GE-Lunar2,6,7,8,9,11,13 DXA systems and results are summarised in Tables 4 to 6. The most common method of estimating the effective dose is to scan a human shaped phantom containing thermoluminescent (TLD) dosimeters to measure the organ doses and then calculate the effective dose using Equation 2.

There is general agreement that the effective dose for a spine and hip DXA examination is very small. For current models of DXA scanner it is generally between 1 µSv and 10 µSv depending on the make, model and scan mode used. Effective doses for the GE-Lunar Prodigy system (1.4 µSv for spine and hip using the Standard scan mode) are lower than for the Hologic Discovery (7.5 µSv using the Express scan mode). Figure 1 shows these DXA doses compared with some other frequently performed X-ray examinations.22 One useful way of explaining the doses

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given by different types of X-ray examination is to compare them in terms of the length of time one must be exposed to natural background radiation to give the same dose. We are all unavoidably exposed to natural sources of radiation, for example cosmic rays from outer space and naturally occurring radioactive isotopes in the environment and our own bodies. Together these give a cumulative effective dose of about 2.5 mSv per year, equivalent to 7 Sv a day. Some more complicated examinations such as CT scans give the equivalent of several years exposure to natural background radiation, while a spine and hip DXA examination gives the equivalent of 1 day or less.

Figure 1: Comparison of the effective dose to the patient from a spine and hip DXA examination on a GE-Lunar Prodigy (using the Standard mode) and a Hologic Discovery (using the Express mode) with some other common radiological

examinations. Doses for other procedures are taken from Wall and Hart22, Kalender3 and the Administration of Radioactive Substances Advisory Committee Notes for Guidance.

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Table 4

Effective doses for DXA spine and hip examinations in adults for different makes, models and scan modes. 6,9,11,18

___________________________________________________________________

DXA Model Scan mode PA Spine (Sv) Hip (Sv)

___________________________________________________________________

GE-Lunar DPX Medium 0.2 0.15

GE-Lunar Prodigy Thin 0.2 0.15

GE-Lunar Prodigy Standard 0.8 0.6

GE-Lunar Prodigy Thick 1.6 1.2

GE-Lunar Expert 5 mA fast 59.0 56.0

Hologic QDR1000 Quick 1.3 0.9

Hologic QDR1000 Performance 2.6 1.8

Hologic Discovery Express 4.4 3.1

Hologic Discovery Fast 6.7 4.7

Hologic Discovery Array 13.3 9.3

___________________________________________________________________

Effective doses for spine and hip examinations performed on the GE-Lunar Prodigy and the Hologic Discovery are summarised in Table 4 together with figures for some older models of DXA scanner. The lowest dose is for the Lunar DPX, an older type of pencil-beam machine for which a spine scan gives a dose of 0.2 µSv. At the other extreme, doses for the Lunar Expert fan-beam system are more than 100 times greater than the DPX with a dose of 60 µSv for a spine scan,7,8,13 equivalent to around 3 chest X-rays. The higher doses for examinations on the fan-beam models

4,6,10,11,13,16,18

may relate in part to the improvement in image resolution. Some centres also perform whole-body DXA scans and Table 5 lists effective dose figures for this examination.

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Table 5

Effective doses for DXA total body examinations in adults for different makes and models. 4,9,11,18

___________________________________________________________________

DXA Model Total body (Sv)

___________________________________________________________________ GE-Lunar DPX < 0.1 GE-Lunar Prodigy 0.1 GE-Lunar Expert 75.0 Hologic QDR1000 4.6 Hologic Discovery-A 4.2 Hologic Discovery-W 8.4 ___________________________________________________________________

Relatively few studies have been published of the radiation dose to children from paediatric DXA examinations.9,17,18 If children are scanned using the ordinary adult scan modes they will receive a higher dose than adults because their thinner bodies means that their internal organs receive less protection from the attenuation of X-rays by overlying tissue. Also, if the scan area used for paediatric scans is the same physical area as for adults a relatively larger proportion of the child’s body is exposed to the X-ray beam.18 Table 6 lists some DXA doses in children that can be compared with the adult figures in Tables 4 and 5. The software on GE-Lunar systems includes paediatric scan modes that keep child doses low by using a lower X-ray tube current. Njeh et al. studied the effective dose to 5- and 10-year-old children scanned using the paediatric modes on the Lunar DPX-L system9 and found figures only marginally greater than for adult scans.6 Hologic machines do not have paediatric scan modes and as a result spine and hip doses are several times larger for children compared with adults. The Hologic spine and hip data in Table 6 assume that the appropriate scan length is set in children so that scan acquisitions are not allowed to run for the default adult scan length. For paediatric scans performed on the Hologic Discovery the radiation dose should be kept as low as possible by using the Express mode rather than the Fast or Array modes, and by making sure that the scan is stopped

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before it runs for the adult scan length.18 It is worth noting that even if the effective dose is the same for children and adults (as with GE-Lunar models), children still face a risk of injury that is up to 3-times higher than adults.19 This is because they have their full life expectancy before them, and because growing tissues are more sensitive to the harmful effects of radiation, and the calculation of effective dose does not take these factors into account.

Table 6

Effective doses for paediatric DXA examinations for a 5 y old and a 10 y old child ___________________________________________________________________

DXA System Scan mode Child aged 5 y Child aged 10 y

___________________________________________________________________

GE-Lunar DPX (a) Spine 0.28 Sv 0.20 Sv

Total body 0.03 Sv 0.02 Sv

Hologic Discovery (b) Express spine 9.1 Sv 7.1 Sv

Express hip 7.4 Sv 5.9 Sv

Total body 5.2 Sv 4.8 Sv

___________________________________________________________________ (a) Data from Njeh et al. 11

(b) Data from Blake et al. 18

Patient dose from DXA scans of the peripheral skeleton are exceptionally low. Lewis et al. reported an effective dose of 0.07 µSv for a forearm scan on a Hologic QDR-1000,4 while Patel et al. found a similar figure for forearm studies performed on the Osteometer DTX-200 peripheral DXA system.14 Doses for peripheral DXA scans are particularly low because: (1) the thinner thickness through the limb compared with the trunk means that the X-ray beam intensity required is less; (2) the scan area is generally smaller than for spine and hip examinations; (3) the scan area does not generally include any of the more radiosensitive tissues with higher weighting factors.

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Occupational Doses from DXA

The occupational dose to DXA scan operators arises because as the X-ray beam passes through the patient’s body some X-ray photons are scattered out of the beam and irradiate the whole room including the operator. Allowing for factors such as how close the operator is seated to the patient during scanning, there is a relationship between patient dose and operator dose because the more X-rays pass through the patient’s body the more scattered photons are produced. Because patient dose is so low DXA is a relatively safe technique for operators too, and those scanner models that give lower dose to patients will generally also give lower dose to the operator.

Several studies 6,10,11,16,23 have evaluated the occupational dose to staff performing DXA investigations. In the United Kingdom, under the revised Ionising Radiations Regulations (IRR 1999) the maximum permitted annual dose for a non-classified radiation worker is 6 mSv.24 Assuming a working year of 2000 hours (8 hours/day  5 days/week  50 weeks/year) this corresponds to an average dose of 3 µSv/hour in the work area. If dose rates around the DXA scanning table approach this limit, then the work area should be defined as a Controlled Area. In this circumstance the Ionising Radiations Regulations require the employer to appoint a suitably trained member of staff as the Radiation Protection Supervisor, staff doses should be monitored using a film badge, and Local Rules should be written that outline safe methods of working. For members of the public (i.e. people not working with radiation) the annual dose limit set by the regulations is 1 mSv/year. If this lower limit is translated into the work place, 1 mSv/year corresponds to 0.5 µSv/hour in the work area. Experience with monitoring of occupational dose levels in Radiology and Nuclear Medicine Departments shows that in practice this lower figure of 0.5 µSv/hour is readily achievable for the large majority of hospital staff working with radiation. There is therefore no reason why the occupational dose of staff involved with a relatively safe technique like DXA scanning should exceed 1 mSv/year.

Results of monitoring dose at a distance of 1 metre from the centre of the scanning table for different models of DXA systems are shown in Figure 2 where they are compared with the Controlled Area limit (equivalent to 3 Sv/hour) and the lower figure of 0.5 Sv/hour set by the dose limit to members of the public. The data shown

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in Figure 2 assume that the operator is scanning 2 patients an hour for the pencil-beam machines and 4 patients an hour for the fan-pencil-beam systems (4000 to 8000 patients a year, which represents a relatively high work load). For the two pencil-beam systems (GE-Lunar DPX and Hologic QDR1000), the dose to staff is extremely low even with the operator sat as close as 1 metre from the patient during scanning. For fan-beam devices, however, the dose is higher and if the number of patients approaches the full capacity of the system, occupational exposures may approach the dose limit of 3 Sv/hour set by the Ionising Radiations Regulations. The simplest way to reduce the occupational dose is to take advantage of the inverse square law and position the operator further away from the patient during scanning. With the operator sat 2 metres from the patient, the occupational dose is one-quarter of that shown in Figure 2, and this is the working arrangement recommended for osteoporosis centres using fan-beam DXA machines.

Figure 2: Comparison of the time averaged scatter dose to an operator positioned 1 metre from the centre of the scanning table for different models of DXA scanner. Results show the mean equivalent dose per hour assuming that 2 patients/hour are scanned on the pencil-beam systems (GE-Lunar DPX and Hologic QDR-1000) and 4 patients/hour on the fan-beam systems (GE-Lunar Prodigy, GE-Lunar Expert-XL and Hologic Discovery). Regulatory limits were taken from the United Kingdom revised Ionising Radiations Regulations24 (Environmental dose data from Patel et al.10,23).

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Radiation Safety Checks at Installation of a New DXA System

If you are installing a new DXA scanner or replacing an older machine, it is important to inform your local Radiation Protection Adviser early in the planning process. Advice should be sought on where to site the scanner and the writing of local rules on radiation safety. Although very compact room designs are possible (Figure 3), these are not desirable unless the patient workload is very light. With the room layout shown in Figure 3, the operator is about 1 metre from the patient during scanning. As discussed above, with older pencil-beam models such as the Lunar DPX occupational doses were extremely low and a compact room design such as this was acceptable. With fan-beam systems, however, greater care is needed with room layout to ensure the safety of staff and rooms should be large enough to ensure that the operator’s console can be placed a least 2 metres from the patient. If this is not possible then it may be advisable with some DXA models to consider the installation of a lead-plastic radiation barrier to protect the operator.

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After installation and before any patients are scanned, measurements should be made of the dose from scattered radiation (to protect the operator) and entrance surface dose (to protect the patient). An electrical safety check should also be made. Because of the low levels of radiation involved, writing Local Rules is straightforward. Some points that should be included are listed in Table 7. Note that in the United Kingdom all staff operating medical equipment delivering ionising radiation must have an adequate knowledge of the hazards of radiation and safe working practice.25 For radiographers this requirement is met as part of their professional training, but staff from other professional backgrounds should attend an IRMER training course.

Table 7

Some Radiation Protection Requirements for Bone Densitometry

___________________________________________________________________ When planning a new installation, the Radiation Protection Adviser

should be consulted

If staff operating equipment do not have the appropriate professional qualification they must attend an IRMER course

When not in use, equipment producing X-rays should be protected against misuse by switching off, locking, and placing the key in a secure place Operators should never expose themselves to the primary X-ray beam During a scan, only the patient should be within the Controlled Area limit * The PC monitor and operator's desk should be placed well outside the Controlled Area limit.* For pencil beam axial DXA systems this means at least 1 metre away, and for fan-beam systems at least 2 metres away. If this is not possible, a radiation barrier should be installed.

___________________________________________________________________

*

The Controlled Area is the region within which the time averaged dose rate

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The Ionising Radiations Regulations (IRR 1999 and IRMER 2000)

The Ionising Radiations Regulations (IRR 1999) and the Ionising Radiations (Medical Exposure) Regulations (IRMER 2000) were introduced in the United Kingdom in 1999 and 2000 respectively,24,25 replacing older regulations that have applied since the 1980's. This section reviews the ways in which the new regulations should be implemented in a department providing a bone density scanning service.

A major reason for introducing the IRR 1999 regulations was to give effect to the lower dose limits for radiation workers and members of the public recommended in ICRP Publication 60.19 Annual dose limits for non-classified workers were reduced from 15 mSv to 6 mSv for staff and from 5 mSv to 1 mSv for members of the public. As emphasised above, scatter dose from bone densitometry equipment is mostly very low, and the majority of operators should receive doses well below the new limits. However, these changes emphasise the need for care in the operation of some types of fan-beam DXA systems to ensure that operator dose is well below the new Controlled Area limit (Figure 2). For equipment giving higher doses it will be necessary to define the scanning table and the immediate surrounding area as a Controlled Area. Advice should be sought from the hospital Radiation Protection Adviser, who if necessary can help with performing radiation measurements and advise on drawing up appropriate Local Rules and appointing a Radiation Protection Supervisor.

The new IRMER regulations 25 came into effect in 2000 and replace the older POPUMET regulations that required staff without the relevant professional training in the use of radiation to attend a Core of Knowledge course. The revised regulations also lay down requirements for appropriate training, and in addition require staff to undertake continuing professional development to demonstrate that their knowledge is up-to-date. As mentioned above, staff operating a DXA scanner without previous professional training in the use of ionising radiation (for example, a nurse) are required to attend a suitable course. This will need to include aspects of radiation protection relevant to their duties as an operator (see below) as well as their specific area of practice (in this case, bone densitometry).

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A central aspect of the IRMER 2000 regulations is that they set down a requirement for the employer (usually an NHS Trust) to provide a framework for the safe use of radiation. The duties of the staff involved are divided into three roles, Practitioners, Referrers and Operators. The Practitioner (often the clinician in charge of the Unit) has a duty to ensure that the diagnostic information derived from each examination justifies the risk entailed in exposing the patient to radiation and should scrutinise and authorise all scan requests. In practice for straightforward procedures such as a bone density scan, the duty of authorising scan requests can be delegated to the Operator provided this is done under a written protocol. This might specify, for example, that scan requests are accepted provided that they meet the Royal College of Physicians Guidelines for the diagnostic use of bone densitometry.26 Referrers have a duty to provide sufficient clinical information about a patient on the request form to enable the Practitioner to justify the scan. If this information is not provided the request card should be returned to the referrer. The Operator (usually a Radiographer or Technologist) has a duty to perform the scan in a safe manner for the patient. These duties include following a set procedure for patient identification, making enquiries of patients of child bearing age whether they might be pregnant, and performing studies in a safe manner according to the written procedures of the Unit. The points included above are summarised in Table 8. Further information can be found in articles in professional journals and on the Department of Health web site.27

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Table 8

Suggested action points for implementing the revised Ionising Radiations Regulations 24,25,27

___________________________________________________________________

IRR 1999

Consult your Radiation Protection Adviser and review whether it is necessary to define a Controlled Area around your DXA scanner

Review your Local Rules to ensure they are up-to-date and appropriate. Ensure that a member of staff is appointed to the role of Radiation Protection Supervisor Ensure that proper instrument QC procedures are in place for scanning

equipment

IRMER 2000

Ensure that all members of staff operating DXA scanning equipment have either an appropriate professional qualification or have attended an appropriate course Ensure that the members of staff fulfilling the role of Practitioner and Operator are defined and that all requests for DXA studies are authorised against a set of appropriate criteria

Ensure that proper procedures are in place for patient identification and for enquiring of patients of child bearing age whether they might be pregnant Ensure that the types of study being performed are covered by written procedures for the operator

Ask your Medical Physics Department to measure the radiation output of your scanner and ensure that it is within limits for the make and model

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References

1. Pye DW, Hannan WJ, Hesp R, Effective dose equivalent in dual X-ray absorptiometry, Br J Radiol (1990) 63: 149

2. Laskey MA, Flaxman ME, Barber RW, et al, Comparative performance in-vitro and in-vivo of Lunar DPX and Hologic QDR-1000 dual energy x-ray

absorptiometers. Br J Radiol (1991) 64: 1023-1029.

3. Kalender WA, Effective dose values in bone mineral measurments by photon absorptiometry and computed tomography, Osteoporosis Int (1992) 2: 82-87. 4. Lewis MK, Blake GM, Fogelman I, Patient dose in dual x-ray absorptiometry,

Osteoporosis Int (1994) 4: 11-15.

5. Lewis MK, Blake GM, Patient dose in morphometric X-ray absorptiometry, Osteoporosis Int (1995) 5: 281-282.

6. Njeh CF, Apple K, Temperton DH, Boivin CM, Radiological assessment of a new bone densitometer: the Lunar Expert, Br J Radiol (1996) 69: 335-340.

7. Huda W, Morin RL, Patient dose in bone mineral densitometry, Br J Radiol (1996) 69: 422-425.

8. Bezakova E, Collins PJ, Beddoe AH, Absorbed dose measurements in dual energy X-ray absorptiometry, Br J Radiol (1997) 70: 172-179.

9. Njeh CF, Samat SB, et al, Radiation dose and in vitro precision in paediatric bone mineral density measurement using dual X-ray absorptiometry, Br J Radiol (1997) 70: 719-727.

10. Blake GM, Patel R, Lewis MK, et al, New generation dual x-ray absorptiometry scanners increase dose to patients and staff, J Bone Miner Res (1996) 11 (Suppl 1): S157.

11. Steel SA, Baker AJ, Saunderson JR, An assessment of the radiation dose to patients and staff from a Lunar Expert-XL fan beam densitometer, Physiol Meas (1998) 19: 17-26.

12. Lloyd T, Eggli DF, Miller KL, et al, Radiation dose from DXA scanning to reproductive tissues of females, J Clin Densitometry (1998) 1: 379-383.

13. Njeh CF, Fuerst T, Hans D, Blake GM, Genant HK, Radiation exposure in bone mineral density assessment, Applied Radiation and Isotopes (1999) 50: 215-236.

14. Patel R, Blake GM, Fogelman I, Radiation dose to the patients and operator from a peripheral dual X-ray absorptiometry system, J Clin Densitometry (1999) 2: 397-401.

15. Damilakis J, Perinakis K, Vrahoriti H, Kontakis G, Varveris H, Gourtsoyiannis N. Embryo/fetus radiation dose and risk from dual x-ray absorptiometry

examinations. Osteoporosis Int 2002;13:716-722.

16. Boudousq V, Kotzki PO, Dinten JM, Barrau C, Robert-Coutant C, Thoma E, et al. Total dose incurred by patients and staff from BMD measurements performed on a new 2D digital bone densitometer. Osteoporosis Int 2003;14:263-269.

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17. Thomas SR, Kalkwalf HJ, Buckley DB, Heubi JE. Effective dose of dual energy x-ray absorptiometry scans in children as a function of age. J Clin Densitometry 2005; 8:415-422

18. Blake GM, Naeem M, Boutros M. Comparison of effective dose to children and adults from dual x-ray absorptiometry examinations. Bone 2006 (in press).

19. ICRP Publication 60, 1990 recommendations of the International Commission on Radiological Protection, Annals of the ICRP (1991) 21: No. 1-3.

20. ICRP Publication 26, Recommendations of the International Commission on Radiological Protection, Annals of the ICRP (1977) 1: No. 3.

21. 2005 Recommendations of the International Commission on Radiological Protection (draft for consultation) http:/www.icro.org/docs/2005_recs_ CONSULTATION_draft1a.pdf.

22. Wall BF, Hart D. Revised radiation doses for typical X-ray examinations. Br J Radiol 1997;70:437-439.

23. Patel R, Blake GM, Batchelor S, et al, Occupational dose to the radiographer in dual X-ray absorptiometry: a comparison of pencil-beam and fan-beam systems, Br J Radiol (1996) 69: 539-543.

24. The Ionising Radiations Regulations 1999. Statutory Instrument 1999 No 3232 (www.legislation.hmso.gov.uk/si/si1999/19993232.htm).

25. The Ionising Radiations (Medical Exposure) Regulations 2000. Statutory Instrument 1999 No 1059 (www.legislation.hmso.gov.uk/si/si2000/20001059.htm).

26. Royal College of Physicians. Osteoporosis: clinical guidlines for prevention and treatment. RCP, London, 1999.

27. www.doh.gov.uk/irmer.htm.

Figure

Figure 1: Comparison of the effective dose to the patient from a spine and hip DXA  examination on a GE-Lunar Prodigy (using the Standard mode) and a Hologic  Discovery (using the Express mode) with some other common radiological

Figure 1:

Comparison of the effective dose to the patient from a spine and hip DXA examination on a GE-Lunar Prodigy (using the Standard mode) and a Hologic Discovery (using the Express mode) with some other common radiological p.9
Figure 2: Comparison of the time averaged scatter dose to an operator positioned 1  metre from the centre of the scanning table for different models of DXA scanner

Figure 2:

Comparison of the time averaged scatter dose to an operator positioned 1 metre from the centre of the scanning table for different models of DXA scanner p.14
Figure 3: Room layout from the manufacturer's data sheet for the Hologic Discovery.

Figure 3:

Room layout from the manufacturer's data sheet for the Hologic Discovery. p.15

References

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