Brachytherapy

Brachytherapy is the clinical procedure that inserts small encapsulated radioactive sources into the treatment (tumor) volume. Radioactive sources are continuously irradiating the treatment volume by an exponentially decaying dose rate. From each source, the therapeutic range is usually in the millimeter (mm) to centimeter (cm) range. Therefore, it provides a superior localization of dose to the tumor volume compared to the conventional external radiation therapy. There are two different methods in brachytherapy: an intracavitary approach where radioactive sources are implanted in body cavities in close proximity of the treatment volume, and an interstitial approach in which the sources are directly implanted in the treatment volume. Typically, the intracavi-tary implant is temporary. On the other hand, interstitial implant can be temporary or permanent. Most gynecologic tumors have been treated with brachytherapy using long-lived 137Ce sources.52 Recently, more brachytherapeutic applications to other anatomic sites have been introduced. These new applications include prostate seed implant for localized prostate cancers, intravascular irradiation to prevent coronary artery restenosis, and "mammo-site" breast irradiation as briefly discussed next.

Palladium-103 and 125I seeds are used for the prostate seed implant procedure.53,54 Accurate seed placement within the prostate, the most important task, is achieved by using transrectal ultrasonography, which provides accuracy and precision. The treatment procedure requires extensive treatment planning to localize each source and produce a combined plan for all sources. There are usually two steps to complete the treatment planning process for permanent seed implants: preplanning the target volume and intraoperative planning. Preplanning is performed with ultrasound images acquired before the surgery. Based on this plan, the strength of the seeds to be ordered, the number, and proposed seed placement may be preplanned. The premise of intraoperative planning is that preplanning is not necessary. All planning can be done in the operating room with real-time images. Advantages of the preplanning approach include: preevaluation of whether this treatment can be deliverable, more planning time to optimize the seed placement for ideal dose distribution, and minimization of ordering unnecessary seeds. The dosimetric goals of prostate cancer implant therapy are to deliver sufficient dose to the prostate and to spare the rectum, bladder, and other adjacent critical structures. These goals are achieved with optimizing the strength and the number of seeds.54

Intravascular brachytherapy has been introduced to treat coronary restenosis.55,56 After a percutaneous coronary transluminal angioplasty (PCTA) is performed to reopen the blockage within the coronary artery, a radioactive source (192Ir, 125I, 90Sr, or 32P) may be positioned in the area of the restenosis. Depending on the selection of radioactive source, the treat ment time varies from 3 to 25 minutes.55 Due to the presence of the radiation-delivering catheter, minimizing the treatment time to restore maximum blood flow is a factor for consideration of source selection as well as other factors; other factors include the dose distribution in inhomogeneous tissue (calcified plaque versus normal tissue), and the artery diameter.56

Mammo-site brachytherapy is considered as an alternative to a standard 5-week external radiation therapy course for breast cancers.57,58 This procedure shortens the treatment time to just 5 days. A specific device has been designed for this procedure, consisting of a hollow catheter to which an inflatable balloon is attached. It is temporarily implanted into the lumpectomy site. Beads of radioactive iridium are inserted into the catheter within the inflated balloon, which helps the catheter to be centered so that the dose can be uniformly delivered to the lumpectomy surface. The actual treatment takes typically two 15-minute sessions per day for 5 days. The catheter stays in place over the entire course of treatment. The preliminary clinical results with respect to local control and breast cosmesis are encouraging.58

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