Radiation therapy
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Radiation therapy (or radiotherapy) is the medical use of ionizing radiation as part of cancer treatment to control malignant cells (not to be confused with radiology, the use of radiation in medical imaging and diagnosis). Although radiotherapy is often used as part of curative therapy, it is occasionally used as a palliative treatment, where cure is not possible and the aim is for symptomatic relief. Other rare uses are to wipe out the immune system prior to transplant to reduce the incidence of tissue rejection, called total body irradiation (TBI); to calm hyperactive muscles—such as might cause twitchy eyes—with mild superficial treatments; and to form scar tissue around a stent to reinforce the vascular wall.
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Application
Radiotherapy is commonly used for the treatment of tumors. It may be used as the primary therapy. It is also common to combine radiotherapy with surgery and/or chemotherapy and/or hormone therapy. The most common tumors treated with radiotherapy are breast cancer, prostate cancer, lung cancer, colorectal cancer, head & neck cancers, gynaecological tumors, bladder cancer and lymphoma, although the cancer's stage (progress) and invasion into lymph nodes, as well as and other health and (unfortunately) monetary factors affect which treatment will have the greatest possibility of success.
Radiation therapy is commonly applied just to the localised area involved with the tumor. Often the radiation fields also include the draining lymph nodes. It is possible but uncommon to give radiotherapy to the whole body, or entire skin surface.
In order to spare interstitial tissue (such as skin or organs which radiation must pass through in order to treat the tumor) several angles of exposure are utilized such that the radiation beams overlap on top of each other at the tumor, providing a much larger absorbed dose there than in the surrounding, healthy tissue.
In the past, before immunosuppressive drugs were developed, radiation therapy was also used to prevent unwanted immune reactions following organ transplantation or in autoimmune diseases. Total body irradiation may be given as a preparative regimen for an allogeneic bone marrow transplant.
Side effects
Although the actual treatment is painless, using external radiation (see below) to tackle tumors inevitably leads to side effects. The side effects can occur during treatment (acute side effects such as soreness and redness over the affected area; nausea and vomiting) or long after treatment has finished (late side effects reflecting permanent organ damage). Implanting radioactive sources has the usual side effects associated with invasive procedures. It has not been demonstrated that the radiation treatment in the actual doses may lead to the induction of secondary cancers.
Dosage
Radiation therapy, like drugs, has biological effects. It is therefore useful to distinguish the total dose from the fractionation schedule. Radiation therapy is usually given daily, the dose depends primarily on tumor type, but many other factors such as whether radiation is given alone or with chemotherapy, before or after surgery, the success of surgery and its findings and many other reasons that are considered by the treating doctor (known as a radiation oncologist). For Radical (curative) cases the typical dose for a solid epithelial tumor may range from 50 to 70 grays (Gy) or more, while lymphomas (white cell) tumors might receive doses closer to 20 to 40 Gy given in daily doses (a daily dose is a fraction); in adults these are typically 1.8 to 2 Gy per fraction. These small frequent doses allow healthy cells time to grow back, repairing damage inflicted by the radiation. In short, total dose can be given in daily fractions using external beam radiation or the total dose can be given via other methods such as implants that deliver radiation continuously over a given timeframe. Depending on the implant type, it may be given as a fraction (e.g. High Dose Rate HDR) over minutes or hours or as another example permanent seeds may be implanted (such as in the prostate) which slowly deliver radiation until the seeds become inactive. In Palliative cases a single dose of 6-10Gy may be given to painful superficial tumours i.e. a rib metases to relieve pain.
Fractionation Schedules
As mentioned above, the typical fractionation schedule is 1.8 to 2 Gy per fraction, with 1 fraction/day. The typical treatment schedule is 5 days per week (no weekends). However, there are alternative fractionation schedules that have been tried. One of the best-known was the CHART (Continuous Hyperfractionated Accelerated RadioTherapy) regimen for lung cancer, which used 2 or 3 smaller fractions per day in the treatment of lung cancer. Although reasonably successful, this imposed huge strains on the departments delivering the service, as it required multiple treatments everyday, including weekends. Twice a day treatments have been tried for other sites, such as head and neck cancers. A special case of twice a day radiotherapy is the concomitant boost regimen.
In some paediatric cancers, fractionation schedules tend to give 1.5 - 1.7 Gy/fraction. The reason for this is that fractionation effects the balance between acute and late toxicity, and with smaller fractions late effects are less likely to occur and are less severe. Obviously late effects are of more concern in paediatric patients than adults.
How it works
Radiation therapy works by damaging the DNA of cells. The damage is caused by an electromagnetic, electron or proton beam directly or indirectly ionizing the atoms which make up DNA chain. Indirect ionization happens as a result of the ionization of oxygen, forming free radicals, which then damage the DNA. In the most common forms of radiation therapy, most of the radiation effect is through free radicals. Because cells have mechanisms for repairing DNA breakage, where the DNA is broken on both strands of the DNA are the most significant in modifying cell characteristics. Because cancer cells generally are undifferentiated and stem cell-like, they reproduce more, and have an diminished ability to repair sub-lethal damage compared to most healthy differentiated cells. The DNA damage is inherited through cell division, accumulating damage to the cancer cells, causing them to die or reproduce more slowly. Proton radiotherapy works by sending protons with varying kinetic energy to precisely stop at the tumor. Being researched is antiproton radiotherapy which would require fewer treatments than proton radiotherapy.
Implications
Tumors don't repair the radiation damage as well as nonmalignant tissue.
Most cells, however, die only during a specific phase of cellular reproduction, which has many curious implications:
- Some slowly growing tumors (for example, prostate) may be treated best by not treating them at all, since the patient will likely die from other causes, such as old age, before the cancer kills.
- It is thought that tumors which outgrow their blood supply, causing a low-oxygen state known as hypoxia, are more resistant to the effects of radiation because they reproduce less frequently, and are not subject to indirect damage caused by free radicals produced by the ionisation of oxygen.
- Some brain tumors do not die at extremely high doses. It is an open subject as to the mechanism by which they survive, but perhaps they do not reproduce in the usual way.
Kinds of radiation therapy
Three main divisions of radiotherapy are external beam radiotherapy (XBRT) or teletherapy, brachytherapy or sealed source radiotherapy and unsealed source radiotherapy. The differences relate to the position of the radiation source; external is outside the body, while sealed and unsealed source radiotherapy has radioactive material delivered internally. Brachytherapy sealed sources are usually extracted later, while unsealed sources may be administered by injection or ingestion. Proton therapy is a special case of external beam radiotherapy where the particles are protons.
Roughly half of the 2500 worldwide radiotherapy clinics are in the US (as of 2001).
3-dimensional radiation therapy
3-dimensional radiation therapy (also called 3-dimensional conformal radiation therapy) is a procedure that uses a computer to create a 3-dimensional picture of the tumor. The patient first undergoes a CT scan in the treatment position, a process known as simulation. The images from the scan are transferred to a treatment planning computer, and the physician traces the outline of the tumor and normal organs on each slice of the CT scan. The treatment planning computer allows the physician to try different beam arrangements on the patient, a process known as virtual simulation. The treatment planning computer may show the beam's eye view (BEV), which is a visual depiction of the treatment field in relation to the tumor and the bony anatomy of the patient and normal organs. Using information from the BEV, physicians can design custom blocking of parts of the radiation beam in order to spare normal tissue as much as posible, which allows doctors to give the highest possible dose of radiation to the tumor.
Intensity-Modulated Radiation Therapy
Intensity-Modulated Radiation Therapy (IMRT) machines are a specialized case of 3D conformal therapy that allow for the modulation of certain intensities associated with a specific beam-angle configuration such that any radiosensitive organs that the beam passes through are subjected to a diminished dose.
Image Guided Radiation Therapy
Image Guided Radiation Therapy (IGRT) machines have a CT scanner integrated with the treatment system, or an X-Ray Tube and aSi-detector mounted on the gantry of the linear acellerator. The patient can be scanned and the tumour located in 3D space immediately before treatment. The ability to correct for movement and setup errors allows smaller margins to be used, sparing healthy tissue and escalating the tumour dose.
See also
External links
- American Society Therapeutic Radiology and Oncology – ASTRO: the official site for radiation oncologists (http://www.astro.org)
- The Radiation Therapy Oncology Group: an organisation for radiation oncology research (http://www.rtog.org/)de:Strahlentherapie
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