Types of Brachytherapy Procedures
Surface
Surface applications are suitable for thin diseases of the skin. It is best suited for areas that are difficult to treat with superficial x-rays or electrons - notably those with irregular surface contours.
Intracavitary
Intracavitary treatments involve the placement of an applicator in a body cavity. The most common application of this is for gynaecological malignancies, such as endometrial, cervical or vaginal tumours. In these cases, the applicator is placed within the vagina and/or uterus and the radioactive source is placed within the applicator.
Interstitial
Interstitial treatments are the most invasive, and involve placing needles (manual loading) or tubes (afterloading) into the tumour mass and/or surrounding tissues. Interstitial treatments can be used in any accessible area of the body, including placement during open surgery. Treatments include prostate bracytherapy, either as temporary high dose rate treatment or permanent very low dose rate seeds.
Eye plaques
Eye plaques are a special type of surface treatment. They involve 103Pd or 125I seeds in an applicator that are designed to treat uveal melanoma, or alternatively 90Sr plaques for pterygium treatments. An ophthalmologist is required for the former treatment to detach the eyeball from the extraocular muscles. These treatments are successful due to the short range of the photons/electrons used in the sealed sources, preventing damage to other ocular structures.
Brachytherapy Dose Rates
The rate at which dose is delivered has important radiobiological effects. It is known that using a lower dose rate reduces the quadratic component of the linear quadratic equation, by allowing repair of DNA damage to occur during treatment delivery.
Dose rate is divided into:
- High Dose Rate (HDR), delivering > 12 Gy/hr
- Example: 192Ir (high dose rate) in remote afterloader
- Medium Dose Rate, delivering 2 – 12 Gy/hr
- Rarely used
- Low Dose Rate (LDR), delivering 0.4 - 2 Gy/ hr
- Examples: 137Cs, 192Ir (low dose rate) as wires/needles
- Very Low Dose Rate (VLDR), delivering < 0.4 Gy/hr
- Examples: 103Pd, 125I seeds
- Pulsed Dose Rate (PDR) attempts to simulate LDR treatment with a HDR source, by only applying the treatment for a short time every hour. It has been shown to be equivalent to low dose rate treatment, with the advantage of allowing nursing care and visitors between 'pulses'.
Advantages and Disadvantages of Different Dose Rates
High dose rates allow rapid delivery of a large dose, but lose the advantage of normal tissue repair seen with lower dose rates. They are able to overcome the proliferation of rapidly growing tumours. Radiation protection is less of a problem due to remote afterloading of the sealed source.
Low dose rates allow for tissue repair to occur during treatment, but require a long hospital stay (unless the implants are permanent) and may not be able to overcome rapid tumour proliferation. Radiation protection issues are important for these treatments as they are usually manually loaded.
Delivery Techniques
Manual Loading
Manual loading refers to the insertion of radioactive sealed sources into the patient by a staff member (usually the radiation oncologist). It is still used for low dose rate and very low dose rate applications (eg. seed insertion).
Remote Afterloading
Afterloading refers to the insertion of the sealed source into a pre-placed tube. Remote afterloading is afterloading performed by a machine while the therapy staff are outside of the bunker. Remote afterloading is a more recent technique, and is essential for high dose rate treatments. These treatments would pose unacceptable exposure to staff if they were required to remain in the room during insertion of the sealed source.
