Techniques for Delivering Radiation Therapy
Radiation therapy may be delivered externally or internally. External radiation delivers high-energy rays directly to the cancer from a machine outside the body. Internal radiation, or brachytherapy, is the implantation of a small amount of radioactive material (seeds) in or near the cancer. Radiation can also be delivered as an isotope into a vein, as in the use of radioactive iodine for the treatment of thyroid cancer.
External Beam Radiation Therapy
EBRT is given via machines called linear accelerators, which produce high-energy external radiation beams that penetrate the tissues and deliver the radiation dose deep in the areas where the cancer resides. These modern machines and other state-of-the-art techniques have enabled radiation oncologists to significantly reduce side effects while improving the ability to deliver radiation.
EBRT is typically delivered on an outpatient basis for approximately 6 to 8 weeks. EBRT begins with a planning session, or simulation, during which the radiation oncologist places marks on the body and takes measurements in order to line up the radiation beam in the correct position for each treatment. During treatment, the patient lies on a table and is treated with radiation from multiple directions. The actual area receiving radiation treatment may be large or small, depending on the features of the cancer. Radiation can be delivered specifically to an organ or encompass the surrounding area, including the lymph nodes.
Three-dimensional conformal radiation therapy (3D-CRT)
EBRT can be delivered more precisely by using a special computed tomography (CT) scan and a targeting computer. This capability is known as three-dimensional conformal radiation therapy, or 3D-CRT. The use of 3D-CRT appears to reduce the chance of injury to nearby body structures. Since 3D-CRT can better target the area of cancer, radiation oncologists are evaluating whether higher doses of radiation can be given safely and with greater cancer cures.
Intensity Modulated Radiation Therapy (IMRT)
IMRT is an advanced form of 3-D conformal radiation therapy that allows doctors to customize the radiation dose by modulating, or varying, the amount of radiation given to different parts of the area being treated. The radiation intensity is adjusted with the use of computer-controlled, moveable “leaves” which either block or allow the passage of radiation from the many beams that are aimed at the treatment area. The leaves are carefully adjusted according to the shape, size, and location of the tumor. As a result, more radiation can be delivered to the tumor cells while less is directed at the normal cells that are nearby.
An analogy for IMRT is a shower nozzle that shoots many different streams of water from different directions, except that each stream can be turned on or off, or set to deliver different intensities. This is unlike standard radiation techniques that allow only a constant flow of radiation from each beam.
Image-guided Radiation Therapy (IGRT)
IGRT is a new approach to delivering radiation therapy that allows for more accurate delivery of radiation to the target tissue. IGRT involves imaging during the course of radiation treatment. A computer compares images taken at the time of treatment to images taken during the planning phase. Through this process, IGRT is able to account for changes in the patient’s body or position that may shift the exact location of the cancer. This allows increased accuracy of very complex treatment approaches. It also provides documentation of the degree of accuracy. IGRT is used in conjunction with EBRT, 3D-CRT or IMRT.
Intraoperative radiation therapy (IORT)
Intraoperative radiation therapy (IORT) refers to radiation therapy that is administered directly to the area of the cancer during surgery. Potential benefits of IORT include increased doses of radiation delivered to the cancer and reduced exposure of normal tissue (normal tissue can be moved or shielded during the procedure). IORT has been used in the treatment of several types of cancer, and may be particularly useful for localized cancers that are difficult to remove completely or that have a high risk of local recurrence (recurrence near the original cancer site).
Two different approaches to delivering IORT are intraoperative electron beam radiation therapy (IOERT) and high dose rate brachytherapy (HDR-IORT).1 The choice of which approach to use depends on the nature of the cancer and the facilities available. Thanks to improvements in technology, such as the development of mobile IOERT units, treatment may be provided directly in the operating room. This avoids the need to transfer the patient from the operating room to the radiation oncology department during surgery.
Internal radiation is known by a number of names, including “brachytherapy,” “seeds,” or “implants.” These terms refer to treatment in which radioactive material is placed directly into or near the cancer. Brachytherapy may be used to provide an additional boost of radiation to an area also being treated with external radiation therapy, or may be used as the only type of radiation therapy.
Brachytherapy may be permanent or temporary. In permanent brachytherapy, radioactive seeds are inserted and permanently left in place. After the procedure, the patient will temporarily contain a small amount of radiation from the seeds, although this amount is not generally dangerous to most other people. Some physicians may advise patients to avoid close contact with young children or pregnant women for several weeks. Over time, the radioactivity diminishes.
In temporary brachytherapy, the radioactive material is inserted for a specified period of time and then removed before the patient goes home. Temporary brachytherapy may be administered at a lower dose over a longer period of time (low dose rate, or LDR) or a higher dose over a shorter period of time (high dose rate, or HDR). During LDR brachytherapy, patients are generally hospitalized for as long as the radioactive implants are in place (often two to three days). In contrast, it may be possible to receive HDR brachytherapy on an outpatient basis.
Brachytherapy has proven to be useful in the treatment of several different types of cancer, including prostate, cervix, uterus, vagina, head and neck, and breast.
Breast brachytherapy is an alternative to traditional EBRT for women who choose breast conservation rather than a mastectomy. Breast conservation therapy involves removing the tumor in a procedure called a lumpectomy, and is followed by radiation therapy to reduce the likelihood of recurrence. Traditional EBRT following a lumpectomy consists of five to six weeks of radiation treatment, five days per week. With breast brachytherapy, a site-specific, prescribed dose of radiation is administered during a five-day course of therapy. Because of the relatively short duration of the treatment course, breast brachytherapy is an attractive option for women who choose lumpectomy over mastectomy, but do not wish to undergo 6 weeks of EBRT.
The procedure for breast brachytherapy involves inserting a deflated balloon into the cavity where the tumor was removed. An applicator shaft, or catheter, connects the balloon to the outside of the breast. The balloon is filled with saline, and both the balloon and the catheter remain in place during the time the woman is undergoing treatment. The radiation therapy is performed on an outpatient basis in a five-day-long, twice-per-day sequence of treatments. During the treatment, a radioactive “seed” is inserted into the catheter within the balloon in an exact dose, minimizing radiation exposure to the rest of the breast, skin, ribs, lungs, and heart. No source of radiation remains in the patient’s body between treatments or after the final procedure. The catheter and balloon are removed after the final procedure. Conventional brachytherapy for breast conservation requires the insertion of 14 to 20 catheters per procedure and is much more complex than the breast brachytherapy procedure described here.
Stereotactic Radiosurgery (SRS)
Stereotactic Radiosurgery (SRS) allows non-invasive treatment of brain tumors, arteriovenous malformation, and other selected conditions. It is an outpatient treatment that delivers a high dose of radiation to a highly defined target. Treatments can be prescribed to deliver the total dose of radiation in a single treatment or in a fractionated manner over a course of several weeks.
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