Brachytherapy

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Overview

Brachytherapy for prostate cancer is administered using "seeds," small radioactive rods implanted directly into the tumour.

Brachytherapy, 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. Conversely, external beam radiotherapy, or teletherapy, is the application of radiation that has been externally produced by a linear accelerator. Brachytherapy is commonly used to treat localized prostate cancer[1] and cancers of the head and neck.[2]

Brachy is from a Greek word for "short", so brachytherapy roughly translated is short distance therapy.

Brachytherapy can be split into four main types:

  • 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.
  • Surface Applicator is usually called Strontium plaque therapy and is 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 to 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 holding the applicator to face 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. 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 with the sources inside the vasculature. 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.

Remote and Manual Afterloading Machines

Afterloading Machines as they are called are the machines that perform brachytherapy treatments.

Manual Afterloading Machines

In the early days of brachytherapy (ca. 1920), the only way to place the radioactive material into the hollow tubes or hollow body cavities was for someone to carry the source up to the patient's bedside (room or operating theater) in a safe, take it out and place it inside the hollow destination. By necessity, the staff member (usually the doctor) undertaking this received some radiation dose. This was manual loading.

In cases such as cervix brachytherapy where a Heyman capsule was used, radiation exposure from manual loading could be appreciable as all the sources had to be placed individually while the patient was anaesthetized on the operating bed. It was not long before the doctors who were exposed reasoned that their exposure could be lessened by placing metal tubes first, and then placing the radioactive sources inside metal tubes at a later time. The metal tubes allowed the development of standard sizing and strength sources so that source numbers could be calculated first, and then prepared to facilitate a single step procedure to manually afterload.

Manual afterloading machines could not be activated from outside the room, as the source had to be manually inserted. The source would have been prepared in a hot lab as a source train and inserted in a theater or ward. The source could not be unloaded for nursing visits.

Remote Afterloading Machines

Although manual afterloading reduced exposures, the guiding principle of radiation protection is to keep exposures as low as reasonably achievable (ALARA) given prevailing economic, political and societal factors. The move to reduce exposures even further led to the introduction of remote afterloading. This technique relies on the use of hollow tubes which are connected to a safe containing a small radioactive source welded to a wire that is driven out by a stepping motor to predetermined positions to deliver radiation dose.

These machines deliver their treatment remotely. A plan is produced that describes the patterns of the stepping motor (distance and dwell time). The motor is only engaged when all staff have left the shielded room that holds the patient for the duration of the treatment.

This means that the nurse or therapeutic radiographer that administers can leave the room (located either in theatre or ward) and start the treatment outside. Empty catheters are placed into the patient and the 'live' source is entered at a later date. This means that the non-active dummy guides can be repositioned and checked. In other words, the source is not placed into the guides until the positioning is acceptable. The machine then runs a pneumatic drive wire through the catheters and guide wires to check that there are no obstructions and the source can safely run through the course of it. After this the check has been performed the source leaves its secure safe and the treatment begins. The development of the remote afterloading machines is a benefit to the many radiation safety issues surrounding manual afterloading machines, but they are expensive and more prone to error.

Advantages of Remote over Manual

  • No radiation exposure in patient transit
  • No exposure to theater or ward staff

HDR brachytherapy

High Dose Rate (HDR) brachytherapy is a common brachytherapy method. Applicators in the form of catheters are arranged, usually according to the Manchester or Paris system on, or in the patient. A high dose rate source (often iridium 192, Ir-192) is then driven along the catheters on the end of a wire by a machine while the patient is isolated in a room. The source dwells in a preplanned position for a preset time before stepping forward along the catheter and repeating, to build up the required dose distribution. The advantage of this treatment over implanting radioactive sources directly is that there is lower staff exposure and the source can be more active due to low staff exposure, thus making treatment times quicker.

LDR brachytherapy

Just like High dose rate (HDR), Low dose rate (LDR) involves implanting radioactive material and can be implanted temporarily or permanently. LDR brachytherapy with a machine works in a similar way[3]. Another variant is the sources being in the form of active and inactive balls which are again, driven into the patient using a machine.

Side Effects

There are many side effects associated with brachytherapy treatment on the prostate, 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 impotent following brachytherapy. Another side effect can occur as a consequence of migration of seeds out of the prostate; it is therefore important to filter urine so as to recover the radioactive seeds.

References

  1. a Merrick GS, Butler WM, Wallner KE, Galbreath RW, Adamovich E (2005). "Monotherapeutic brachytherapy for clinically organ-confined prostate cancer". West Virginia Medical Journal. 101 (4): 168–171.
  2. a Mazeron JJ, Noel G, Simon JM, Racadot S, Jauffret E (2003). "Brachytherapy in head and neck cancers". Cancer Radiotherapy. 7 (1): 62–72.
  3. a Brachytherapy Descriptions of LDR and HDR

External links

  1. Brachytherapy in New Zealand
  2. The GEC ESTRO Handbook of Brachytherapy


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