The importance of radiation safety to the treatment of malignant disease with radiotherapy
The BTEC students need to know how to deal with radiation safely.
Effect of x-rays
Diagnostic X-rays (primarily from CT scans due to the large dose used) increase the risk of developmental problems and cancer in those exposed. X rays are classified as a carcinogen by the World Health Organization’s International Agency for Research on Cancer. It is estimated that 0.4% of current cancers in the United States are due to computed tomography (CT scans) performed in the past and that this may increase to as high as 1.5-2% with 2007 rates of CT usage.
Experimental and epidemiological data currently do not support the proposition that there is a threshold dose of radiation below which there is no increased risk of cancer. However, this is under increasing doubt. It is estimated that the additional radiation will increase a person’s cumulative risk of getting cancer by age 75 by 0.6–1.8%. The amount of absorbed radiation depends upon the type of X-ray test and the body part involved. CT and fluoroscopy entail higher doses of radiation than do plain X-rays.
To place the increased risk in perspective, a plain chest X-ray will expose a person to the same amount from background radiation that we are exposed to (depending upon location) every day over 10 days, while exposure from a dental X-ray is approximately equivalent to 1 day of environmental background radiation.
The risk of radiation is greater to unborn babies, so in pregnant patients, the benefits of the investigation (X-ray) should be balanced with the potential hazards to the unborn foetus. In the US, there are an estimated 62 million CT scans performed annually, including more than 4 million on children. Avoiding unnecessary X-rays (especially CT scans) will reduce radiation dose and any associated cancer risk.
The main problem with X-ray radiotherapy is that it covers a large area of the body and may adversely affect healthy tissue.
The Bragg Curve is a graph of the energy loss rate, or Linear Energy Transfer (LET) as a function of the distance through a stopping medium.
X-ray radiotherapy passes through the entire body, with dose decreasing only slowly as it progresses through. The peak of the dose delivered is near the surface, rarely where the cancer is. These effects both cause damage to healthy tissues. In order to get a much higher dose to the cancer, multiple beams are often used, intersecting at the tumour site. This gives a dose to the surrounding tissue in most directions.
Any of the different types of X-rays can do immense damage to biological material:
Water is ionised to form free radicals that are highly reactive. Free radicals can combine to make hydrogen peroxide which is a powerful oxidising agent and can damage the DNA of the chromosomes;
At the molecular level, enzymes, RNA and DNA are damaged, and metabolic pathways are interfered with.
At the sub-cellular level cell membranes are damaged, along with the nucleus, chromosomes, and mitochondria.
Cellular level, cell division is damaged. Cells can die, or be transformed to malignant growth.
Tissue and organ damage; There can be disruption to the central nervous system, death of bone marrow and the lining to the gastro-intestinal system, leading to sickness and death. Cancers may arise.
Whole animal can die; or life is shortened;
Populations: mutations can alter the genetic characteristics of populations.
X-ray doses are very carefully controlled and maximum limits are set to minimise the risks to patients. These limits are well below the doses that would cause the least harm. However the procedures, although very safe, always run a very slight risk of long term harm. So does watching the TV all day.
Energetic X-rays are used to treat cancer in a process called radiotherapy. The tumour is exposed to high energy X-rays and killed. However there can be side effects. Also the dose has to be worked out carefully. 10 percent less dose can leave a tumour unaffected, while ten percent more can damage the patient.
A chest CT scan gives a radiation dose of about 8 mSv, which is about the same as 3.5 years background exposure, but a person would receive the same dose from a four hour flight, about the time it takes to fly to Greece from London, as they are higher up and have less atmospheric protection from cosmic rays. This dose increases the risk of developing cancer by one in 2500, though the risk without ever having had an x-ray is already 1 in 3.
There are many ways to reduce the dangers from radiation. The first is only to use it when necessary. Before people realised it was dangerous, shoe shops used to x-ray people’s feet to check that new shoes fitted properly. This no longer happens as the benefit did not justify the risk. However, the benefits of seeing where a bone is broken so it can be safely and properly mended are considered worth the small extra risk.
Every x-ray examination has strict controls about the maximum radiation dose a patient can be given, and the patient can be covered with lead-rubber shield to protect the parts of them not being examined from the radiation. This is especially used to protect reproductive organs so there is less risk of a mutation being passed on.
People working with X-rays have to take care as they could accumulate a high dose as they work:
They wear a film badge to check the amount of radiation they get;
They wear lead aprons while the machine is turned on, leave the room or stand behind a lead shield. The risk of developing problems due to radiation exposure increases with total dose.
The machine is in an enclosed room and the controls are in a separate room.
Interlocks are arranged so that nobody can walk into the X-ray room while the machine is turned on. If that were to happen, the machine would be turned off immediately.
Being exposed to X-rays carries a theoretical risk of triggering cancer at a later date, as does exposure to background radiation.
However, this risk is very low. For example, the Health Protection Agency (HPA) has calculated that:
An X-ray of the chest, teeth, arms or feet is the equivalent of a few days’ worth of background radiation, and has a less than 1 in 1,000,000 chance of causing cancer
An X-ray of the skull or neck is the equivalent of a few weeks’ worth of background radiation, and has a 1 in 100,000-1,000,000 chance of causing cancer
An X-ray of the breasts (mammogram), hip, spine, abdomen or pelvis is the equivalent of a few months’ to a year’s worth of background radiation, and has a 1 in 10,000-100,000 chance of causing cancer
An X-ray that uses a contrast fluid, such as a barium meal, is the equivalent of a few years’ worth of background radiation, and has a 1 in 1,000-10,000 chance of causing cancer
The unique radiation exposure conditions that exist in computed tomography (CT), during which thin slices of the patient are irradiated by a narrow, fan-shaped beam of x rays emitted from the x-ray tube during its rotation around the patient, have required the use of special dosimetry techniques to characterize the radiation doses to patients and to monitor CT system performance. This section describes the basic dosimetry quantities used to indicate patient doses during CT.
Absorbed dose – The fundamental quantity for describing the effects of radiation in a tissue or organ is the absorbed dose. Absorbed dose is the energy deposited in a small volume of matter (tissue) by the radiation beam passing through the matter divided by the mass of the matter. Absorbed dose is thus measured in terms of energy deposited per unit mass of material. Absorbed dose is measured in joules/kilogram, and a quantity of 1 joule/kilogram has the special unit of gray (Gy) in the International System of quantities and units. (In terms of the older system of radiation quantities and units previously used, 1 Gy equals 100 rad, or 1mGy equals 0.1 rad.)
Equivalent dose – The biological effects of an absorbed dose of a given magnitude are dependent on the type of radiation delivering the energy (i.e., whether the radiation is from x rays, gamma rays, electrons (beta rays), alpha particles, neutrons, or other particulate radiation) and the amount of radiation absorbed. This variation in effect is due to the differences in the manner in which the different types of radiation interact with tissue.
The variation in the magnitude of the biological effects due to different types of radiation is described by the “radiation weighting factor” for the specific radiation type. The radiation weighting factor is a dimensionless constant, the value of which depends on the type of radiation. Thus the absorbed dose (in Gy) averaged over an entire organ and multiplied by a dimensionless factor, the radiation weighting factor, gives the equivalent dose. The unit for the quantity equivalent dose is the sievert (Sv). Thus, the relation is
equivalent dose (in Sv) = absorbed dose (in Gy) x radiation weighting factor
In the older system of units, equivalent dose was described by the unit rem and 1 Sv equals 100 rem or 1 mSv equals 0.1 rem.
For x rays of the energy encountered in CT, the radiation weighting factor is equal to 1.0. Thus, for CT, the absorbed dose in a tissue, in Gy, is equal to the equivalent dose in Sv.
Effective dose – The risk of cancer induction from an equivalent dose depends on the organ receiving the dose. A method is required to permit comparison of the risks when different organs are irradiated. The quantity “effective dose” is used for this purpose. The effective dose is calculated by determining the equivalent dose to each organ irradiated and then multiplying this equivalent dose by a tissue-specific weighting factor for each organ or tissue type. This tissue- or organ-specific weighting factor accounts for the variations in the risk of cancer induction or other adverse effects for the specific organ. These products of equivalent dose and tissue weighting factor are then summed over all the irradiated organs to calculate the “effective dose.” (Note that effective dose is a calculated, not measured quantity.) The effective dose is, by definition, an estimate of the uniform, whole-body equivalent dose that would produce the same level of risk for adverse effects that results from the non-uniform partial body irradiation. The unit for the effective dose is also the sievert (Sv).
Quantities specific to CT – A number of special dose quantities have been developed to characterize the doses associated with CT. It is beyond the scope of this discussion to describe these unique dose descriptors. They include the Computed Tomography Dose Index, referred to as the CTDI, the “weighted” CTDI (CTDIW), the “volume” CTDI (CTDIVOL), the “multiple scan average dose” (MSAD), and the “dose-length product” (DLP).
Megavoltage X-rays are by far most common in radiotherapy for treatment of a wide range of cancers. Superficial and X-rays have application for the treatment of cancers at or close to the skin surface
The megavoltage linear accelerator has been the standard radiotherapy equipment for the past 20-30 years. Its production of x-rays is identical to that of lower-energy machines. However, the energy range of megavoltage units is quite broad—from 4 to 20 MeV. The depth of the maximum dose in this energy range is 1.5-3.5 cm. The dose to the skin is about 30%-40% of the maximum dose.
Most megavoltage units today also have electron-beam capabilities, usually in the energy range of about 5-20 MeV. In order to produce an electron beam, the tungsten target is moved away from the path of the beam. The original electron beam that was aimed at the tungsten target is now the electron beam used for treatment. Unlike that of photons, the electron skin dose is quite high, about 80%-95% of the maximum dose. A rule of thumb regarding the depth of penetration of electrons is that 80% of the dose is delivered at a depth (in cm) corresponding to one-third of the electron energy (in MeV). Thus, a 12-MeV beam will deliver 80% of the dose at a depth of 4 cm.
Radiation beam characteristics
Radiation therapy uses controlled high-energy rays to treat tumours and other diseases of the body. Radiation works by damaging the DNA inside cells making them unable to divide and reproduce. Abnormal cancer cells are more sensitive to radiation because they divide more quickly than normal cells. Over time, the abnormal cells die and the tumour shrinks.
The goal of radiation therapy is to maximize the dose to abnormal cells while minimizing exposure to normal cells. The effects of radiation are not immediate; the treatment benefit occurs over time. Typically, more aggressive tumours, whose cells divide rapidly, respond more quickly to radiation. Radiation therapy is painless and will not make the patient radioactive.
There are two ways to deliver radiation:
External beam radiation is delivered from outside the body by using a machine to aim high-energy rays (x-rays, gamma rays or photons) at the tumour.
Internal radiation (brachytherapy) is delivered from inside the body by surgically placing radioactive material, sealed in catheters or seeds, directly into the tumour.
It is crucial that the radiation dose is delivered only to the target. Shaping the beam to match the target minimizes exposure to normal tissue. The problem is that most tumours are irregularly shaped and most radiation beams are round. Beams can be shaped using treatment planning software and hardware.
Multiple beams are used generally that converge on the target region. This allows sparing of the normal tissues (skin, normal bowels and bladder, hip joints, rectum). The energy of the primary beam can be adjusted to create different dose-depth characteristics and the shape of the beams are custom designed for each patient based on their anatomy and cancer location and stage.
Rotating the beam 360 degrees around the patient enables very small beams with varying intensity to be aimed at the tumour from multiple angles. Unlike helical IMRT treatments or other forms of radiation therapy, with the radiation treatment being delivered to the patient can be modulated continuously throughout treatment. This means that higher doses of radiation are delivered to hit the tumour harder, and less radiation is delivered to surrounding healthy tissue.
There are four main types of beam modifiers:
1. Wedge filter (external, internal) – A physical or external wedge is an angled piece of lead or steel that is placed in the beam to produce a gradient in radiation intensity. Manual intervention is required to place physical wedges on the treatment unit’s collimator assembly.
The wedge angle is defined as the angle through which an isodose curve at given depth in water (usually 10 cm) is tilted at the central beam axis under the condition of normal beam incidence.
2. Shielding (custom blocks, multileaf collimators) – Early conventional irradiation megavoltage devices as cobalt unit and old linear accelerators have no independent movable collimator jaws, and they can determine only symmetric fields. If we want to block of a part of the field without changing the position of the isocentre, we have to use different kind of shielding.
The aims of shielding are:
a. To protect critical organs
b. Avoid unnecessary irradiation to surrounding normal tissue
c. Matching adjacent fields
The multileaf collimator was developed to replace the traditional blocks. MLCs are used on linear accelerators to provide conformal shaping of radiotherapy treatment beams. Specifically, conformal radiotherapy and Intensity Modulated Radiation Therapy (IMRT) can be delivered using MLC’s.The MLC has movable leaves, which can block some fraction of the radiation beam, typical MLCs have 52-160 leaves, arranged in pairs.
Radiotherapy and the linear accelerator
A linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator is used to treat all parts/organs of the body. It delivers high-energy x-rays to the region of the patient’s tumour. These x-ray treatments can be designed in such a way that they destroy the cancer cells while sparing the surrounding normal tissue. The LINAC is used to treat all body sites, using conventional techniques, Intensity-Modulated Radiation Therapy (IMRT), Image Guided Radiation Therapy (IGRT), Stereotactic Radiosurgery (SRS) and Stereotactic Body Radio Therapy (SBRT).
Ionising radiation is a type of energy released by atoms in the form of electromagnetic waves or particles.
People are exposed to natural sources of ionising radiation, such as in soil, water, vegetation, and in human-made sources, such as x-rays and medical devices. Every day, people inhale and ingest radionuclides from air, food and water.
Ionising radiation has many beneficial applications, including uses in medicine, industry, agriculture and research.
As the use of ionising radiation increases, so does the potential for health hazards if not properly used or contained.
Acute health effects such as skin burns or acute radiation syndrome can occur when doses of radiation exceed certain levels.
Low doses of ionizing radiation can increase the risk of longer term effects such as cancer.
Ionising radiation is a type of energy released by atoms that travels in the form of electromagnetic waves (gamma or X-rays) or particles (neutrons, beta or alpha). The spontaneous disintegration of atoms is called radioactivity, and the excess energy emitted is a form of ionizing radiation. Unstable elements which disintegrate and emit ionizing radiation are called radionuclides.
All radionuclides are uniquely identified by the type of radiation they emit, the energy of the radiation, and their half-life.
The activity — used as a measure of the amount of a radionuclide present — is expressed in a unit called the becquerel (Bq): one becquerel is one disintegration per second. The half-life is the time required for the activity of a radionuclide to decrease by decay to half of its initial value. The half-life of a radioactive element is the time that it takes for one half of its atoms to disintegrate. This can range from a mere fraction of a second to millions of years (e.g. iodine-131 has a half-life of 8 days while carbon-14 has a half-life of 5730 years).
Exposure to radiation can be internal or external.
Radiation damage to tissue and/or organs depends on the dose of radiation received, or the absorbed dose which is expressed in a unit called the gray (Gy). The potential damage from an absorbed dose depends on the type of radiation and the sensitivity of different tissues and organs.
Beyond certain thresholds, radiation can impair the functioning of tissues and/or organs and can produce acute effects such as skin redness, hair loss, radiation burns, or acute radiation syndrome. These effects are more severe at higher doses and higher dose rates. For instance, the dose threshold for acute radiation syndrome is about 1 Sv (1000 mSv).
If the dose is low or delivered over a long period of time (low dose rate), there is greater likelihood for damaged cells to successfully repair themselves. However, long-term effects may still occur if the cell damage is repaired but incorporates errors, transforming an irradiated cell that still retains its capacity for cell division. This transformation may lead to cancer after years or even decades have passed. Effects of this type will not always occur, but their likelihood is proportional to the radiation dose. This risk is higher for children and adolescents, as they are significantly more sensitive to radiation exposure than adults.
Epidemiological studies on populations exposed to radiation (for example atomic bomb survivors or radiotherapy patients) showed a significant increase of cancer risk at doses above 100 mSv.
Prenatal exposure to ionising radiation may induce brain damage in foetuses following an acute dose exceeding 100 mSv between weeks 8-15 of pregnancy and 200 mSv between weeks 16-25 of pregnancy. Before week 8 or after week 25 of pregnancy human studies have not shown radiation risk to foetal brain development. Epidemiological studies indicate that cancer risk after foetal exposure to radiation is similar to the risk after exposure in early childhood.
The Ionising Radiations Regulations 1999 (IRR99) are a statutory instrument which form the main legal requirements for the use and control of ionising radiation in the United Kingdom. The main aim of the regulations as defined by the official code of practice is to “establish a framework for ensuring that exposure to ionising radiation arising from work activities, whether man-made or natural radiation and from external radiation or internal radiation, is kept as low as reasonably practicable (ALARP) and does not exceed dose limits specified for individuals”
The regulations impose duties on employers to protect employees and anyone else from radiation arising from work with radioactive substances and other forms of ionising radiation.
The regulations are split into seven parts containing 41 regulations. The main concepts of the IRR99 are: general principles, procedures, and risk assessments; arrangements for management of radiation protection; control of areas; control of people; control of radioactive substances and equipment; accident preparedness.
In addition to requiring that radiation employers ensure that doses are kept as low as reasonably practicable (ALARP) the IRR99 also defines dose limits for certain classes of person. Dose limits do not apply to people undergoing a medical exposure or to those acting as ‘comforters and carers’ to such.
Dose limits are intended to reduce the risk of serious effects occurring, such as cancer, and are in place to protect the eyes, skin and extremities against other forms of damage.
Dose limits are set to protect workers and members of the public from the effects of ionising radiation. They are set at a level that balances the risk from exposure with the benefits that use of ionising radiation brings.
The film badge dosimeter, or film badge, is a personal dosimeter used for monitoring cumulative radiation dose due to ionizing radiation.
The badge consists of two parts: photographic film or dental X-ray film, and a holder. The film is removed and developed to measure exposure. The film badge is used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. It is essentially useless for measuring neutron radiation.
The film holder usually contains a number of filters that attenuate radiation, such that radiation types and energies can be differentiated by their effect when the film is developed.
To monitor gamma rays or x-rays, the filters are metal, usually aluminium or copper. To monitor beta particle emission, the filters use various densities of plastic. It is typical for a single badge to contain a series of filters of different thicknesses and of different materials; the precise choice may be determined by the environment to be monitored. The use of several different thicknesses allows an estimation of the energy/wavelength of the incident radiation.
Filters are usually placed on both the back and front of the holder, to ensure operation regardless of orientation. Additionally, the filters need to be sufficiently large (typically 5 mm or more) to minimize the effect of radiation incident at oblique angles causing exposure of the film under an adjacent filter. This places a minimum useful size on the holder and badge.
The badge is typically worn on the outside of clothing, around the chest or torso. This location monitors exposure of most vital organs and represents the bulk of body mass.
The dose measurement quantity, personal dose equivalent Hp(d), is defined by the International Commission on Radiological Protection (ICRP) as the dose equivalent in soft tissue at an appropriate depth, d, below a specified point on the human body. The specified point is usually given by the position where the individual’s dosimeter is worn.
The film badge is now not widely used, being largely replaced by such as the Thermoluminescent Dosimeter (TLD).
A thermoluminescent dosimeter, or TLD, is a type of radiation dosimeter. A TLD measures ionizing radiation exposure by measuring the intensity of visible light emitted from a crystal in the detector when the crystal is heated. The intensity of light emitted is dependent upon the radiation exposure. Materials exhibiting thermoluminescence in response to ionizing radiation include but are not limited to calcium fluoride, lithium fluoride, calcium sulphate, lithium borate, calcium borate, potassium bromide and feldspar.
A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material. Some of the atoms in the material that absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.
Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor.
Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The “glow curve” produced by this process is then related to the radiation exposure. The process can be repeated many times.
Methods of reducing radiation exposure
Minimise the time of exposure
Increase the distance from the source of the radiation
Use shielding. This will significantly reduce the risk of exposure but only if appropriately used and in proper working order
Make sure equipment is checked and calibrated regularly
Avoid unnecessary exposure
Education and training of staff