Radiation therapy

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.

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 metastasis to relieve pain.

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Fractionation Schedules

As mentioned above, the typical fractionation schedule (in the USA & Europe) is 1.8 to 2 Gy per fraction, with 1 fraction/day. The typical treatment schedule is 5 days per week (no weekends). This schedule is also common in the South of England, whereas in the North of the UK fraction sizes are quite commonly 2.67 - 2.75 Gy per fraction (treating daily M - F), with a lesser number of total fractions. The reason for this is mainly a resource issue within the NHS, with Clinical Oncology departments having a much greater workload than the private clinics in Europe & the USA. Both fractionation schedules are effective, individual clinician preference and availability of resource are the deciding factors. (Longer schedules are common where payment is made to the clinic according to the number of treatment fractions delivered).There are other alternative fractionation schedules that have been tried. One of the best-known was the CHART (Continuous Hyperfractionated Accelerated RadioTherapy) regimen for lung cancer, which uses 3 smaller fractions per day in the treatment of lung cancer. Although reasonably successful, this can impose strains on the departments delivering the service, as it required multiple treatments everyday, including weekends. With an international shortage of qualified Radiation Therapists this is often difficult to sustain. 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.

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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 pediatric patients than adults.

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