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High Dose Rate (HDR) Brachytherapy
Brachytherapy or implant therapy is a form of radiotherapy in which small, encapsulated radioactive sources are placed directly into or near a tumor. "Brachy" is a Greek prefix meaning short. This refers to the fact that the treatment distance between the source and the tumor is very small. The history of brachytherapy goes back more than 100 years. At the turn of the century a Dr. Henri Danols of St. Louis Hospital in Paris used radium supplied by Marie and Pierre Curie to treat a cancer patient.
In order to appreciate the benefits of brachytherapy it is important to understand that successful radiotherapy is dependent on the ability to deliver as much radiation dose as possible to a cancer and as little as possible to normal tissues. A higher radiation dose will increase the chance of eradicating a tumor while reducing the dose to healthy organs will lower side effects. This process of increasing the tumor dose and reducing normal tissue dose is called conformal radiation therapy. Thus over the years there has been a much interest in making radiotherapy as conformal as possible.
By its nature, brachytherapy is inherently a more conformal type of treatment than external beam radiotherapy. External beam radiotherapy refers to x-ray therapy delivered by an accelerator. X-rays delivered by an accelerator must first pass through the patient body to reach the tumor. Thus the tumor is treated outside-in. However, in brachytherapy the patient is treated inside-out. The x-rays from the radioactive source are produced first inside the tumor and the tissues away from the tumor only receive the exit dose, which is signicantly less than the entrance dose.
However, while brachytherapy possessed some significant advantages there were also some disadvantages that prevented its wide spread use. Principle among the disadvantages was the fact that patients had to be admitted to the hospital for anywhere from 24 to 72 hours for treatment. This was because the sources used for brachytherapy contained small amounts of radioactivity. Hence, it took a significant amount of time to deliver the prescribed dose and thus the hospital admittance. This type of therapy is referred to as low-dose rate brachytherapy or LDR.
This has changed with the development of high-dose rate or HDR brachytherpy. In HDR the low activity source has been replaced with a high activity Iridium-192 source. The Iridium source is approximately 1000 times stronger than the older sources. However, the strength of the source also makes it hazardous for staff to handle. In order to deliver the Iridium source for treatment safely the HDR afterloader was developed. The afterloader is a device about the size and shape of a tall hydrant. The Iridium source, which is only about the size of a grass seed, is housed in a tungsten safe inside the afterloader when not in use. The afterloader is connected to a control unit and computer outside the treatment room via cables.
Before treatment an applicator is placed into or near a tumor. The applicator is typically a piece of plastic or metal with a hollow central channel. For example the applicator used in the treatment of endometrial cancer is a piece of plastic and is cylindrical in shape. After the applicator is placed in the patient it is attached to the afterloader via a guide tube or catheter. During treatment the radioactive source is driven remotely under computer control from the safe to the applicator. During this time staff use a television camera and monitor to observe the patient.
Because the x-ray fluence or output is so much higher with HDR the treatment takes only 10 to 20 minutes. Thus there is no need to admit a patient and the patient can be treated on an outpatient basis. Also since the treatment is much quicker it is a more comfortable procedure for the patient and there is less chance that the applicator will move during treatment and hence change the delivered dose. Thus you can be more confident that your delivered dose is the same as your planned dose. Furthermore, with the use of powerful planning computers it is possible to program an almost infinite number of different dwell positions or source locations and delivery times in an applicator. This allows the oncologist to optimize dose delivery or tailor the dose delivered for each particular patient to a degree not possible with LDR treatment.