Brachytherapy

Brachytherapy

Brachytherapy (from the Greek brachy, meaning "short"), also known as sealed source radiotherapy or endocurietherapy, is a form of radiotherapy where a radioactive source is placed inside or next to the area requiring treatment. Brachytherapy is commonly used to treat localized prostate cancer, cervical cancer and cancers of the head and neck.

Types

Brachytherapy exists in numerous forms:

  • Mold brachytherapy. Superficial tumours can be treated using sealed sources placed close to the skin. Dosimetry is often performed with reference to the Manchester system; a rule-based approach designed to ensure that the dose to all parts of the target volume is within 10% of the prescription dose.
  • Strontium plaque, used for very superficial lesions less than 1 mm thick. The plaque is a hollow, thin silver casing that encloses a radioactive Strontium-90 powdered salt. The beta (electron) particles produced from Strontium's radioactive decay have a very shallow penetration. Typically the Sr90 plaque is placed on the bed of a resected pterygium. A stat dose of around 10-12 Gy is delivered by timing the contact. As the electrons only penetrate a few mm of air, radiation protection issues are slightly less but very different from other radiation sources. Cleaning the plaques that are placed on the eye sclera is required but must be gentle because the silver casing is thin and easily damaged. Strontium belongs to the same chemical class as Calcium, i.e., an alkaline earth metal, and so will co-locate in the bone if any strontium salt makes contact with the eye and is absorbed. Operators can prevent exposure to the beta rays by facing the applicator away from their bodies.
  • Interstitial brachytherapy. Here the sources are inserted into tissue. The first treatments of this kind used needles containing Radium-226, arranged according to the Manchester system, but modern methods tend to use Iridium-192 wire. Iridium wire can be arranged either using the Manchester or the Paris system; the latter was designed specifically to take advantage of the new nuclide. Prostate cancer treatment with Iodine-125 seeds is also classified as interstitial brachytherapy. This also includes the removal of small lung cancers through wedge resection followed by placement of a brachymesh device, consisting of absorbable suture containing Iodine-125 seeds to reduce the risk of recurrence. For details of the gamma emitters please see commonly used gamma emitting isotopes.
  • Intracavitary brachytherapy places the sources inside a pre-existing body cavity. The most common applications of this method are gynaecological in nature, although it can also be performed on the nasopharynx.
  • Intravascular brachytherapy places a catheter inside the vasculature through which sources are sent and returned. The most common application of this method is the treatment of coronary in-stent restenosis, although the therapy has also been investigated for use in the treatment of peripheral vasculature stenoses and also considered for the treatment of atrial fibrillation. Although several systems have been used successfully for intravascular brachytherapy, the only device currently available is the Novoste Beta-Cath System which uses beta-emitting sources of Sr/Y-90.
  • Electronic brachytherapy places a miniature low energy (<50kVp) x-ray tube source into a pre-positioned applicator within body/tumor cavities to rapidly deliver high doses to target tissues while maintaining low doses to distant non-target tissues.

Dosing

Brachytherapy can be applied using different dosings.

Low dose rate

Low dose rate (LDR) involves implanting radioactive material temporarily or permanently.

High dose rate

High dose rate uses a dose of 20cGy per minutes or above (ICRU 38). A high dose rate source (often iridium 192, Ir-192) is the material used. HDR is an alternative to LDR in cases where it can be administered without a large number of fractions.

Side effects

There are many side effects associated with brachytherapy treatment, most of these are urinary in nature and are short lived. However, between six and fifty percent of patients (depending upon the seriousness of the cancer) may become infertile or impotent following brachytherapy. Another side effect can occur as a consequence of migration of seeds out of the treatment region; it is therefore important to filter urine so as to recover the radioactive seeds.

Application

Brachytherapy can be applied manually, or remotely using machines. Historically, manual application resulted in doctors being exposed to excessive levels of radiation. Later restrictions on radiation exposure resulted in the development of remote afterloading, which delivers the radioactive materials from a safely contained source via hollow tubes. The motor to push the materials is engaged once staff are removed from the treatment room. Modern high dose rate therapies for relatively thin target volumes include the interstitial and intracavitary use of beta emitting isotopes as well as low energy x-rays to achieve high target doses with tissue sparing low non-target doses and reduced exposure levels to users.

See also

References

External links

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