Radiation therapy is not a common way to treat colon cancer, though it may be used in certain circumstances. Radiation therapy, often with chemotherapy, is frequently used in the adjuvant or neoadjuvant setting for the treatment of rectal cancers, whereas chemotherapy alone is more common for the adjuvant and neoadjuvant treatment of colon cancers.
Doctors who specialize in treating cancers with radiation are known as radiation oncologists. During radiation therapy, high-energy x-rays are used to kill cancer cells. In advanced stages of colon cancer, radiation therapy is often given instead of surgery when an operation cannot be performed. Radiation therapy is also commonly given in combination with chemotherapy. Chemotherapy drugs have the ability to kill cancer cells directly and help make radiation therapy more effective in killing cancer cells.
Radiation therapy for colon cancer is typically delivered by a machine that aims x-rays at the body (external beam radiation). External beam radiation therapy (EBRT) for colon cancer is given on an outpatient basis, 5 days a week, for approximately 5 to 6 weeks. If ERBT is used as palliative treatment for stage IV cancer, it may be given for a shorter time, one day to three 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. After the simulation session, the patient begins a program of daily treatments. During treatment, the patient lies on a couch and is treated with radiation from multiple directions to the pelvis. The radiation oncologist may perform a second planning session or simulation near the end of treatment to focus the radiation to the area where cancer cells are most likely to remain. The last 3-5 days of treatment may be directed at this area.
Side Effects of Radiation Treatment
Although patients do not feel anything while receiving radiation treatment, the effects of radiation gradually build up over time. Many patients become somewhat fatigued as treatment continues. Loose stools or diarrhea are also common and urination may become more frequent or uncomfortable. Some patients may experience loss of pubic hair or irritation of the skin. When radiation therapy is given in combination with 5-fluorouracil chemotherapy, diarrhea can be worse. In a small percentage of patients, an obstruction or blockage in the small bowel can occur, which may require hospitalization or even abdominal surgery to relieve. Radiation therapy can also cause chronic changes in bowel function, resulting in loose stools and, when severe, inflammation of the prostate.
Strategies to Improve Treatment
The progress that has been made in the treatment of colon cancer has resulted from improved development of radiation treatments and surgical techniques and participation in clinical trials. Future progress in the treatment of colon cancer will result from continued participation in appropriate clinical trials. Currently, there are several areas of active exploration to improve radiation treatment of colon cancer.
New Radiation Therapy Modalities: Some radiation oncology centers have special treatment equipment for certain circumstances. For small early cancers, a focused radiation beam can be aimed directly at the cancer in the colon. Intra-operative radiation therapy (IORT) refers to treatment in a specially equipped operating room where a single dose of radiation is given during surgery. The radiation doctor is able to see the area being treated directly and move sensitive normal structures, such as the small bowel, away from the radiation beam. IORT is usually administered when surgery is being performed for locally extensive cancer or stage II-IV cancer that has recurred in the pelvis. Some studies have shown good rates of control of recurrent tumors when surgery is combined with both IORT and traditional radiation therapy.1 IORT is not indicated in patients with multiple recurrent cancers due to the high frequency of nerve damage if many tumors are treated.
Newer Radiation Techniques: External beam radiation therapy (EBRT) can be delivered more precisely to cancer-containing areas by using a special CT scan and 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 normal body structures, such as the bladder or rectum. Since 3D-CRT can better target the area of cancer, radiation oncologists are evaluating whether higher doses of radiation can be given safely with greater potential for cancer cures.
Intensity Modulated Radiation Therapy (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.
Newer Radiation Machines: Most EBRT uses high energy x-rays to kill cancer cells. Some radiation oncology centers use different types of radiation generated by special machines. These different types of radiation, such as protons or neutrons, appear to kill more cancer cells with the same dose. Radiation therapy that combines protons or neutrons with conventional x-rays is being evaluated in clinical trials.
1 Pezner RD, Chu DZ, Wagman LD, et al. Resection with external beam and intraoperative radiotherapy for recurrent colon cancer. Archives of Surgery 1999;134:63-67.