The radiation emissions produced by X-ray tubes and radioactive substances are known as ionizing radiation because of their ability to disrupt atoms and molecules, an ability not shared by several other types of radiation such as microwaves, visible light (e.g. from most lasers), and ultrasound. Ionizing radiation can disrupt DNA in the cell nucleus, causing damage to genes and chromosomes. The ability to ionize is the basis of the unique damage to cells and tissues which X-rays and radioactive materials can create. The hazards of ionizing radiations became rapidly evident after their discovery, initially in the form of damage to the skin (erythema) and blood fractions, and later as cancer induction and genetic mutation.

Radiation emitted from X-ray tubes and radioactive materials is a form of energy transmitted through space, as are heat and light. When it encounters an absorbing medium, such as the human body, the energy is gradually absorbed. A certain fraction of the energy is absorbed in each layer of tissue. The energy absorbed by a unit mass of absorber is referred to as the ‘dose’.

The passage of X-rays into the body is illustrated in Fig. 1 2789. In this example 20 per cent of the energy in a typical diagnostic X-ray beam is absorbed in the first 1cm of tissue, and 20 per cent of the remaining energy is absorbed in each successive 1cm layer. The dose therefore decreases as the depth of tissue increases. When more energy enters the body (Fig. 1(b)) 2789 more energy is absorbed in each tissue layer. The dose received by portions of the body depends on the intensity of the energy flow (energy entering per second), the time for which it flows, and the depth of the tissue.

Two units of dose are currently used: the gray (Gy), defined as energy deposition of 1 J/kg and the rad, defined as energy deposition of 100 ergs/g. The rad is gradually being superseded by the Gy. One gray is equal to 100 rads and is considered to be a large dose since it can produce acute effects such as a reduction in blood count if received by a large fraction of the body. A dose of 1 rad has no immediately obvious effects. The range of dose received by the skin of a patient from radiography is 1 to 100 mGy (0.1 - 10 rads). Typical doses received by attending medical personnel are much smaller than those received by patients (e.g. 0.1 mGy/month).

For radiation protection purposes, a quantity referred to as the ‘equivalent dose’ is in general use; this has different units, the sievert (Sv) and the rem. Maximum equivalent doses specified in governmental regulations are denoted in these units (see Table 1 689). For X-rays and radiations from radioactive materials used in medicine the sievert and the gray are identical, as are the rem and the rad, and the mSv and the mGy.

Radiation effects fall into two main classes: those that occur only after exposure to large doses exceeding a threshold value and in essentially all such exposed persons after a sufficiently large dose, and those effects that may be caused by any dose (i.e., with no threshold), although the probability that they will occur is strongly dose-dependent and is extremely small at the lowest doses. Examples of the former are skin erythema, lens opacities, radiation sickness (nausea, vomiting), sterility, and even death. Effects in this class require doses greatly exceeding those associated with occupational exposure or diagnostic radiation use. At least 3 Sv, 2.5 Sv, and 1 Sv are needed to produce a skin erythema, a radiation cataract, and radiation sickness respectively. Examples of the second class are the many types of malignant disease which become manifest after a latent period of several years, and genetic anomalies which are expressed in future generations. Fetal developmental defects may be included in this class of effects: their incidence increases with radiation dose to the fetus, but there are probably minimum dose thresholds. These effects may be produced by the doses received from occupational exposure, diagnostic radiation use, and possibly by the ambient background radiation which everybody experiences.

The most recent risk estimates for occupational whole body exposure in a working population are those of the International Commission on Radiological Protection (ICRP). The lifetime risk of fatal cancer following exposure to radiation at low dose rates or to intermittent small doses is estimated at 4 per cent/Sv. This translates into four extra cancer deaths in 10000 persons all exposed to a 10 mSv dose over the whole body. The risk of serious genetic anomalies appearing in offspring of persons suffering gonadal exposure is estimated at 0.6 per cent/Sv; that is one child in 16000 will be affected following a gonadal dose of 10 mSv to the parents. The risk of cancer developing after in utero exposure to radiation is estimated to be a few times higher than that of an exposed adult, at about 1 in 1000 following a 10 mSv dose. Radiation exposure of a pregnant woman between weeks 8 and 15 of gestation appears to cause a loss of IQ in the child, estimated at 30 IQ points/Sv, but as noted above, a threshold dose of about 0.1 Sv probably pertains.

The risk estimates for radiation-induced cancer given above are several times larger than those published by international review bodies in the early 1980s, primarily due to the availability of long-term data from exposed populations, particularly the Japanese A-bomb survivors. There has also been downward revision of the doses received, and a change in the projection model for future cancer mortality in study populations. The maximum permissible doses, particularly for lifetime exposure, have also been recently reduced. The most recent dose limits propounded by the International Commission on Radiological Protection which guide most countries of the world, and by the National Council on Radiation Protection (NCRP) in the United States, are shown in Table 1 689. The significant new limits from both organizations concern the lifetime limits which are reduced by factors of 2.5 and about 3.5 respectively, and the limits for exposure of the general public, which are reduced by a factor of 5.

The method by which whole body dose is estimated has also been revised, primarily to take account of the different susceptibilities of internal organs to fatal malignant disease induced by radiation. The new ‘effective dose’ method applies dose weighting factors to each exposed organ (Table 2) 690; this is particularly relevant for partial body doses and for internal doses from radionuclides, which are usually markedly non-uniform. Leukaemia and cancers of the stomach, colon, and lung are estimated to account for one-half (48 per cent) of the total detrimental effects of radiation.

For occupational X-rays exposures when a lead apron is worn, the effective dose will be due primarily to head and neck dose, which is relatively lightly weighted compared to the dose to the shielded organs under the apron. In this situation the effective dose will be much smaller than the dose recorded by a dosimeter worn at neck or head level.

As a measure of the significance of a radiation exposure, the normal background radiation dose received by the average person is about 0.08 mSv/month, and the minimum dose above background detectable by a personnel film badge is 0.1 mSv (10 mrem).

In the operating room, X-ray exposure accrues from radiography and C-arm fluoroscopy. In both procedures exposure is almost entirely due to radiation scattered by the part of the patient's body which is in the primary X-ray beam. Leakage radiation that penetrates the housing of the X-ray tube has an intensity less than 1 per cent of that due to scattering.

The dose received from scattered radiation depends on several operational factors: first, the dose received by the patient; the level of scatter at a given distance from the patient is proportional to the patient dose; second, the X-ray field size incident upon the patient; the scatter is proportional to the field area; third, the distance of personnel from the patient during the use of the X-ray beam. This last factor is of major significance and depends on the ‘inverse square law’; doubling the distance reduces the scatter dose by a factor of 4; a 10-fold increase in distance reduces the scatter by a factor of 100.

Because radiography often does not require personnel to be close to the patient, personnel exposures are normally very small. Fluoroscopy, however, is typically conducted in close proximity to the patient and therefore the major exposures to operating personnel are received by the fluoroscopist. Doses received by operating room personnel from radiography are so low that personnel shielding (lead aprons) is usually not needed, except by persons such as anaesthetists, who remain close to the patient in procedures which require multiple abdominal or skull radiographs. The scatter dose to a person 1 m away from a patient is typically about 0.0005 mSv (0.05 mrem) for a single chest film, but 0.005 mSv (0.5 mrem) for a single abdominal film. At 3 m distance the doses are nine times smaller.

Scattered radiation doses at a distance of 30cm from the side of a patient during fluoroscopy can exceed 12 mSv/h (1200 mrem/h) with some orientations of a C-arm fluoroscope (see below; Fig. 2 2790). Protective clothing should always be worn by personnel when a fluoroscope is in use.

A video disc recording system with immediate readout offers a convenient low-dose alternative to both fluoroscopy and film radiography in a number of surgical procedures, such as hip nailing in orthopaedic surgery and electrode or needle positioning in neurosurgery, where a large number of images are required intermittently. This system, which is often supplied as an optional accessory to a C-arm fluoroscope, provides a radiographic image on a single television frame on demand. The patient dose per image is about 15 times smaller than that required for a single conventional radiographic film, and the scattered radiation doses to surrounding personnel are likewise reduced. Typical doses 1 m from the X-ray axis for 100 images obtained with a 15cm × 15cm field at the image intensifier during a cerebral procedure are 0.005 mSv (0.5 mrem) compared with 0.02 mSv (2 mrem) during 3 min of fluoroscopy—a 4-fold dose reduction.

During the last decade there has been an increase in the use of fluoroscopes with above-table X-ray tubes and below-table image intensifiers for guiding the surgeon in some operative procedures. The versatile C-arm fluoroscope readily lends itself to any angular orientation, including those in which the tube is overhead. The physician's head and neck are necessarily in close proximity to the region of interest in these instances, and the scatter from the patient and the X-ray dose to the operator's head are both maximized. The eye lens, the red bone marrow in the skull and cervical spine, and the thyroid gland are particularly at risk under these circumstances.

To quote a recent report from the International Commission on Radiological Protection: ‘With the operator wearing a protective apron and standing beside the patient, the dose from an over-couch screening set, compared with that from an under-couch set, can be 250 times higher to the hands, 100 times higher to the eyes, and 35 times higher to the whole body. For an operator with a heavy workload the dose to the lens of the eye can greatly exceed the Commission's recommended limit of 150 mSv (15 rem) in a year.’

Figure 2 2790 shows three orientations of a C-arm fluoroscope operating with the tube at 110 kVp and 2 mA with a 15-cm diameter field size at the face of the intensifier. The dose rates at the eye, thyroid, and waist of the operator are shown at a distance of 30cm from the side of a wax phantom 30cm × 30cm × 20cm thick, and vary strongly with C-arm orientation. The eye doses vary by a factor of almost 100, the thyroid doses by a factor of 60, and the waist level doses by a factor of about 40. Figure 2(d) 2790 shows dose rates to the eye for a fixed fluoroscope with an over-table X-ray tube operating with smaller X-ray factors than the C-arm device and includes the effect of a lead shield hanging from the X-ray tube.

In recent years intraoperative radiation therapy has become increasingly used. This involves treatment of a tumour by external beam radiotherapy after it has been surgically exposed and, in some cases, after resection. In initial trials the patient was transported from the operating room to the radiation therapy room; today some major medical centres have combined surgical/radiotherapy facilities, which use very high energy X-rays or electrons. As in all radiotherapy procedures, medical staff do not remain in the room during the radiation treatment phase. Surgeons usually remain in the shielded area outside the therapy room, where they receive a permissible level of occupational exposure.

Each year in the United States of America, radionuclides are administered about 10 million times for diagnostic purposes: about one patient in eight receives such an examination. A small fraction of patients admitted for surgery will therefore emit &ggr;-rays or characteristic X-rays, residual activity from a recent examination. These individuals are a source of radiation exposure to those persons near them for a period which depends on the effective half-life of the radiopharmaceutical in the body. By far the most commonly used diagnostic radionuclide is technetium-99m, which has a physical half-life of only 6 h: in about 2 days (8 half-lives) the radioactivity, even if not excreted, has essentially disappeared. Some radiopharmaceuticals, such as those used to image bone or kidney, are almost entirely excreted within 6 h of administration. Immediately after administration, however, the dose rate at the surface of the patient may range from 0.01 to 0.2 mSv/h: higher dose rates occur when relatively large amounts of radioactivity have been administered, such as 740 MBq (20 mCi) of technetium-99m for a bone scan or a radionuclide angiogram. The dose rate falls rapidly with distance from the patient: by factors of 6 and 30 at distances of 30cm and 1 m respectively, from the patient. At 1 m from a patient who has recently received the highest dose, the radiation level will not exceed 0.01 mSv/h.

If surgery is undertaken soon after administration of a radiopharmaceutical manual contact is made with body fluids and organs that may contain appreciable radioactivity. For example, following the administration of 110 MBq (3 mCi) of technetium-99m sulphur colloid for a liver scan, the dose rate at the surface of the liver is about 0.60 mSv/h. Similarly, following the oral administration of 7.4 MBq (200 &mgr;Ci) of iodine-123 for a thyroid scan, the dose rate at the surface of the thyroid gland approaches 0.6 mSv/h. Since surgery is rarely performed soon after a diagnostic test, the dose even in an hour of contact must be considered negligible in comparison with the maximum permissible hand dose of 500 mSv/year.

Although exposures to surgeons and nurses from diagnostic radionuclides use may be considered negligible, this may not be the case after administration of radioactive materials for treatment of cancer, or for thyroid ablation. Multiple sealed radioactive sources are implanted into a tumour by a radiotherapist; however, some internal organs such as the prostate and pancreas are first exposed by surgery. After insertion of the radioactive material the wound is closed, with resultant exposure of the surgeon to radiation. In a typical procedure, 20 to 30 sealed metal seeds each containing about 18 MBq (0.5 mCi) of activity in the form of iodine-125 are permanently implanted. Iodine-125 has a half-life of 60 days and emits low energy X- and &ggr;-rays, which are strongly absorbed by overlying tissue. The subsequent dose rate to persons at 1 m from such a patient is of the order 0.001 mSv/h.

During wound closure, the hands and eyes of the surgeon receive considerably larger, but not excessive doses. The hand dose has been measured as 0.75 mSv/h and the eye dose as 0.012 mSv/h from an implant of 450 MBq. These levels are small compared to the hand and eye weekly limits of 10 mSv and 3 mSv, respectively. Surgical removal of a tumour containing iodine-125 seeds 1 or 2 months after implantation could involve surface doses to the fingers of the order 20 mSv/h. If, however, the contact time was only 1 min, the finger dose would be 0.33 mSv.

Large doses of iodine-131 with a half-life of 8 days are conventionally used for the treatment of thyroid disorders. Quantities up to 370 MBq (10 mCi) are used to treat hyperthyroidism and up to 5500 MBq (150 mCi) are used to treat thyroid cancer. A hyperactive thyroid gland with a 50 per cent uptake could contain 185 MBq (5 mCi) during the first day after administration. Neck surgery on or close to the gland would involve &ggr;-ray dose rates to the hands at 5cm from the gland of 4.3 mSv/h. Hand contact with the surface of the exposed gland would produce a finger dose of about 500 mSv/h, mostly due to the &bgr;-rays of low penetration. Surgical gloves reduce this dose by a factor of about 2. The doses received from a patient soon after the administration of a tumouricidal dose are much greater. Within the first few hours after receiving 5500 MBq (150 mCi), the dose/h at the surface of the patient's abdomen is about 4.5 mSv; immediately over the thyroid it could be 250 mSv. At 30cm from the body surface it is about 0.75 mSv, and at 1 m it is about 0.15 mSv.

As much as 20 per cent of the administered activity may be taken up by metastatic disease in the lungs. The dose to the hands of a surgeon performing chest surgery in the first few days would be about 3 mSv/h if the hands are 10cm distant from the metastases. Surgery would also involve massive radioactive contamination of the surgical field, instruments, and drapes from spilled blood; this is particularly serious on the first day, with markedly lower contamination levels over the next week. Fortunately about 70 per cent of the radioactivity dose used for cancer treatment is excreted in the first 24 h. It is clear that surgery, if indicated, should be postponed as long as possible.

The cardinal factors that enhance radiation safety are distance from the radiation sources, minimizing the exposure time of the operator or other personnel, and interposing shields between the source and the operator.

Although doses are small, personnel who are not required to be close to the patient should retire to a distance of at least 2 m before each image is made. Protective clothing is advised only for persons who cannot remove themselves, and then only when a large series of images is to be made.

Protective garments (aprons 0.3mm to 0.5mm of lead equivalent thickness, including thyroid shields) are mandatory. The operator should use equipment with an under-table tube, if clinically feasible, use an X-ray field as small as possible for visualization of objects of interest, minimize the beam-on time, set the control panel for automatic brightness control of the image (i.e. avoid the manual setting of kV and mA), and keep the image intensifier as close as possible to the patient.

If an over-table X-ray tube is employed (for example, with a C-arm fluoroscope), additional measures should be taken to reduce the head and neck dose of the operator. Ideally a transparent shield of lead plastic with lead equivalent in the 0.3 to 0.5mm range should be suspended from the X-ray tube housing at least on the operator's side of the tube; it should be long enough to intercept the scattered radiation arising from the surface of the patient at the X-ray beam entrance portal (Fig. 2(d)) 2790. This will reduce the eye dose rate by a factor of about 30 for 80 kVp X-rays. An alternative is to install a ceiling-mounted lead glass shield for shielding the head and neck. Shielding may be positioned in a flexible fashion between the surgeon and the over-table X-ray tube to intercept the scatter. If such a shield is not provided, an operator with a heavy fluoroscopic workload (more than 0.5 h/week of beam-on time) is advised to wear glasses (goggles) with lead glass 1 to 2mm thick (0.25 to 0.5mm of lead equivalent), and not ordinary glass or plastic.

Exposure from the intact body of a patient following a diagnostic administration of a radiopharmaceutical requires no precautions. Unless there is a medical emergency, surgery should be delayed for at least 24 h after such an administration to minimize contamination of the surgical facility with radioactive blood or urine and to avoid the exposures of the hand that would attend surgery on some organs with long retention times (technetium-99m sulphur colloid in the liver; iodine-131 in thyroid).

Following radionuclide therapy, the greatest radiation exposure of a surgeon and his assistants occurs in the rare instance of surgery performed within 24 h of administration of iodine-131 for thyroid cancer treatment. The safest policy is to delay surgery for at least a few days, when more than 80 per cent of the activity has been excreted. Hand exposure over the thyroid gland should be avoided since the maximum permissible annual dose to the hands could be accumulated in a few hours. Excised thyroid tissue from the gland or metastatic disease should be handled with forceps and should be placed in a shielded container with lead walls at least 1cm thick. Shields against body exposure are impractical, since lead of thickness 3mm reduces the dose by only a factor of two. The essence of such a procedure is speed. Following the operation a detailed room survey of radioactive contamination levels should be made by radiation safety personnel and decontamination procedures instituted.

Surgeons operating fluoroscopes frequently or those performing surgery on patients following recent therapeutic radionuclide administrations should wear film badges, preferably two, one on the collar for monitoring head and neck exposure, and one at waist level, which should be under the lead apron in the case of fluoroscopy. A finger ring monitor (if acceptable) under the surgical glove is recommended for the hand primarily used for manipulation in fluoroscopic procedures or in procedures close to radioactive organs following radionuclide therapy.

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