What Is Radiation for Cancer?

A radiation therapist and a cancer patient at Keck Medicine of USC – Newport Beach Radiation Oncology and Imaging. (Photo/Ricardo Carrasco III)

A Keck Medicine of USC radiation oncologist explains what to know about radiation treatment for cancer.

Radiation oncology — the field of treating patients with radiation therapy or radiotherapy — is a cornerstone of cancer treatment. Elizabeth Zhang-Velten, MD, PhD, a radiation oncologist with the USC Norris Comprehensive Cancer Center, part of Keck Medicine of USC, explains what to know.

Dr. Zhang-Velten provides radiation oncology services at Keck Medicine of USC – Newport Beach Radiation Oncology and Imaging and Keck Medicine of USC – Buena Park Radiation Oncology.

What is the difference between radiology and radiation oncology?

Radiology and radiation oncology used to be parts of the same field, with radiation oncology falling under the category of therapeutic radiology. But today they are viewed as two distinct specialties. Radiology is the use and interpretation of scans such as X-rays, CT scans and MRIs to diagnose different disease processes. Meanwhile, radiation oncology — or radiation therapy — uses ionizing radiation as a treatment to kill cancer cells.

How does radiation therapy work?

Radiation therapy can be broken down into two types: external and internal. External radiation is the most common.

With external beam radiation therapy, rays of radiation — X-rays — are shot through the body. This is the same kind of radiation used in diagnostic X-rays and CT scans, only they are higher-energy and more precisely focused. I always tell people they won’t feel or see anything when the radiation beam is on. Also, you are not radioactive after external beam radiation therapy, so you don’t have to avoid contact with other people, just like you don’t have to avoid people after you get an X-ray.

External radiation first requires mapping for treatment planning, also called a CT simulation. In this preliminary session, the radiation oncologist will scan the patient’s body in a specific position with specialized devices to make the positioning reproducible for future treatments. The radiation oncologist will draw, slice by slice through the CT, the areas at risk for tumor spread (which we want to dose well with radiation) and, just as importantly, the critical normal tissue structures (to which we want to limit the radiation dose). This helps determine where and how the radiation beams should be applied. Then the patient will come back for the actual radiation therapy sessions, during which they are placed in the same position as before. The treatment machine, called a linear accelerator, can take its own real-time images to make sure that everything lines up on the day of treatment just like it did at the time of the CT simulation scan. The linear accelerator then treats the patient for typically a few minutes. The entire process could take 15 to 20 minutes per day, including positioning, real-time imaging for position verification and small millimetric shifts that may be applied by the radiation oncologist before the radiation beam is turned on. Some radiation therapy courses are as short as one day in total, though depending on the situation some patients are treated with weeks of radiation.

Internal radiation, called brachytherapy, is when a radiation source is placed inside the body to treat a targeted area. It’s used in situations when you want to deliver a high, targeted dose to a tumor, but you want to limit the exposure of neighboring structures to radiation. Take, for instance, a tumor deep in the pelvis. If you were to shoot a ray of external beam radiation therapy, you would need to shoot through the bowel, the bladder and the rectum to get to the tumor in the middle. Brachytherapy is an elegant way to dose the tumor because the radiation seeds are placed inside the tumor and/or areas at risk and deliver the dose from the inside instead.

There are two general types of brachytherapy. Patients typically think of brachytherapy seeds, otherwise known as low-dose-rate (LDR) brachytherapy. In LDR brachytherapy, radioactive seeds are placed under image-guidance into the tumor. The seeds undergo radioactive decay over the course of weeks and months, thereby dosing the tumor and areas at risk for microscopic cancer cells. Another modality of brachytherapy is high-dose-rate (HDR) brachytherapy in which hollow needles are placed through the tumor and areas at risk. Here, the radioactive seeds are not left in the patient but instead dwell in these hollow needles for several minutes before they are removed. The patient is therefore not radioactive after HDR brachytherapy.

Why is radiation an important tool to treat cancer?

Radiation therapy is a crucial component of oncologic care, particularly in organ preservation. One example is patients with anal cancer. Decades ago, patients with anal cancer would undergo surgery to remove the anus and any cancerous lymph nodes; however, they then would be permanently left with an ostomy bag. Today, radiation and chemotherapy are used in combination as the standard of care for locally advanced anal cancer, with the goal of cure without surgery. Another example is laryngeal cancer. Radiation alone has shown excellent outcomes in early-stage laryngeal cancer and showed less hoarseness and breathiness of the voice as compared to surgery. In many cases of locally advanced laryngeal cancer, chemotherapy and radiation together can be used to replace surgery, which would be a total laryngectomy.

Radiation also sometimes allows a surgeon to do a smaller surgery. A great example is breast cancer. A century ago, the standard of care was a mastectomy, no matter how big or small the tumor was. But times have changed with advances in mammograms and other breast cancer screening tools. Now, breast tumors are being detected earlier and earlier, so does everyone still need a mastectomy? Initially, the strategy of lumpectomy alone was attempted, but too many patients suffered recurrences after just a lumpectomy. A groundbreaking trial in the 1970s showed, however, that the addition of a five-week course of radiation after a lumpectomy made the breast cancer control rate equal to that of a mastectomy for early-stage breast cancer. Since then, more trials have shown that shorter radiation courses and even partial-breast radiation courses are safe and effective options depending on a patient’s risk factors.

Can radiation treat both early- and later-stage cancers?

Yes, radiation can sometimes play a role in both settings.

For some early-stage cancers that haven’t spread, one type of radiation treatment, called stereotactic body radiation therapy, can be used. It delivers a very precise, high dose of radiation to the tumor over a few sessions. A patient with early-stage lung cancer who is not able to get surgery might be a candidate for stereotactic body radiation therapy, for example.

Radiation can also be combined with chemotherapy to treat certain locally advanced tumors.

Finally, radiation therapy is also used in patients with metastatic (stage IV) cancer. Radiation can be given palliatively in the setting of widely metastatic cancer to help alleviate a patient’s symptoms. But, of note, radiation is also increasingly being used in patients with metastatic cancer to treat oligoprogressive lesions, which can be thought of as “troublemaker spots” that are not responding to current treatment. If, for instance, chemotherapy medications are for the most part helping to control a patient’s cancer, except for one or two problematic areas, you may not want to discontinue chemotherapy altogether, especially if you know that other spots are responding to it. Instead, you can use high-dose stereotactic body radiation therapy to address the troublemaker spots. The data for this approach is strongest for lung cancer.

In addition, if there are “sanctuary sites” in the body that chemotherapy can’t reach or penetrate well, we may use radiation to treat those sites instead. A common example is brain metastases — although as chemotherapy and other systemic therapies advance, alternative options are emerging for some patients.

When is a low dose of radiation more effective than a high dose, or vice versa?

It’s a case-by-case basis. Sometimes patients will receive a low daily dose per day of radiation for weeks, called conventional fractionation. This kind of “low and slow” approach is recommended for certain situations — for instance, if you’re treating an area with a lot of precious real estate. One example is the head and neck area. There is so much packed into this area like critical nerves, muscles for swallowing, the voice box and the mucous membranes, and so it would be standard to treat with a low daily dose with the goal of reducing short-term and long-term toxicities.

Another reason to give a lower daily dose in one patient and not in another is if a high dose could have negative effects based on the individual patient’s baseline symptoms. For instance, if a patient with early-stage prostate cancer is being treated with radiation, and the patient is already urinating frequently, a large daily dose of radiation (called hypofractionation or ultra-hypofractionation or stereotactic body radiation therapy) could push them over the edge and into urinary retention. That’s a patient I don’t want to wallop with a high daily dose of radiation, even if a higher daily dose might make treatment more convenient and could be well-tolerated by a different patient. Instead, I’ll talk to the patient about either starting to take some medications for a few months to first decrease the size of the prostate — or, if the patient is eager to start radiation right away, I can provide a low and slow, conventionally fractionated course of treatment.

Finally, there are some patients who can receive radiation for something other than cancer. For example, inflammation from disorders such as osteoarthritis and plantar fasciitis actually responds well to radiation therapy. Since the culprit cells, white blood cells, are very sensitive to radiation, very low radiation doses can have a beneficial effect. For patients who have bothersome pain and poor range of motion due to these inflammatory conditions and are not seeing improvement with conservative measures such as physical therapy and NSAIDs, radiation could be a good option and should be discussed with a radiation oncologist.

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