Radiation Therapy for Cancer of Unknown Primary

Radiation therapy uses high-energy rays or particles to destroy cancer cells or slow their rate of growth. The goal of radiation therapy may change based on the situation.

For some cancers that have not spread too far from where they started, it can be used alone or with other treatments such as surgery with the goal of trying to cure the cancer.

If cancer has spread extensively, radiation can be used to relieve symptoms such as pain, bleeding, trouble swallowing, intestinal blockage, compression of blood vessels or nerves by tumors, and problems caused by metastases to bones.

External beam radiation therapy

The most common way to deliver radiation to a cancer is to carefully focus a beam of radiation from a machine outside the body. This is known as external beam radiation. To reduce the risk of side effects, doctors carefully figure out the exact dose and aim the beam as accurately as they can to hit the target.

External beam radiation therapy is much like getting a diagnostic x-ray, but the radiation is more intense. The procedure itself is painless. Each treatment lasts only a few minutes, although the setup time – getting you into place for treatment – usually takes longer. The radiation is usually divided into many treatments over several days or weeks.

Internal radiation therapy (brachytherapy)

Another method of delivering radiation is called internal radiation, interstitial radiation, or brachytherapy. Instead of using radiation beams aimed from a large machine, a radioactive material is placed directly into, or as close as possible to, the cancer. This type of radiation travels a very short distance in the body. The material itself may be left in the body for only a short time, or it may be left there permanently.

Internal radiation therapy lets your doctor give a dose of radiation to a smaller area and in a shorter time than is possible with external radiation treatment.

Sometimes, both internal and external beam radiation therapies are used together.

Possible side effects of radiation therapy

Side effects can vary from patient to patient and depend on the radiation dose and the part of your body treated. Some of the possible side effects are listed below:

General side effects

  • Fatigue (feeling tired)
  • Loss of appetite
  • Low blood counts
  • Skin changes (like a sunburn at the area where the radiation beams enter the body)
  • Hair loss (at the site where the beams enter the body)

Side effects from radiation to the head and neck

Radiation therapy to the head and neck area often causes damage to the throat and salivary glands, which can result in:

  • Throat pain
  • Mouth sores
  • Trouble swallowing
  • Loss of taste
  • Hoarseness
  • Dry mouth,

Over the long term it can also lead to cavities in the teeth and thyroid problems (from damage to the thyroid gland). This might mean that you need pills to replace thyroid hormone.

Side effects from radiation to the chest

  • Trouble and pain swallowing from irritation of the esophagus (the tube that connects the throat to the esophagus
  • Lung irritation that can lead to cough and shortness of breath

Side effects from radiation to the abdomen

  • Nausea
  • Vomiting
  • Diarrhea
  • Poor appetite

Side effects from radiation to the pelvis

  • Bladder irritation, leading to symptoms like pain or burning with urination and feeling like you have to go often
  • Irritation of the rectum and anus, which can lead to diarrhea, bleeding, and pain
  • In women, vaginal irritation and discharge.

Most of these side effects go away after treatment ends, but some are long-term and may never go away completely.

If chemotherapy is given along with radiation, the side effects are often worse.

There are ways to relieve many of these side effects, so it’s important to discuss any changes you notice with your cancer care team.

For more information on radiation therapy, see the “ Radiation Therapy” section of our website, or our document Understanding Radiation Therapy: A Guide for Patients and Families.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: July 2, 2014 Last Revised: January 27, 2016

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