Merck Manual

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Radiation Therapy for Cancer


Robert Peter Gale

, MD, PhD, Imperial College London

Last full review/revision Jul 2018| Content last modified Jul 2018
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Radiation therapy can cure many cancers (see also Overview of Cancer Therapy), particularly those that are localized or that can be completely encompassed within the radiation field. Radiation therapy plus surgery (for head and neck, laryngeal, or uterine cancer) or combined with chemotherapy and surgery (for sarcomas or breast, esophageal, lung, or rectal cancers) improves cure rates and allows for more limited surgery as compared with traditional surgical resection. Radiation therapy may be given before surgery or chemotherapy (adjuvant therapy) or after surgery or chemotherapy (neoadjuvant therapy).

Radiation therapy can provide significant palliation when cure is not possible:

  • For brain tumors: Prolongs patient functioning and prevents neurologic complications

  • For cancers that compress the spinal cord: Prevents progression of neurologic deficits

  • For superior vena cava syndromes: Relieves venous obstruction

  • For painful bone lesions: Usually relieves symptoms

Radiation cannot destroy malignant cells without destroying some normal cells as well. Therefore, the risk to normal tissue must be weighed against the potential gain in treating cancer cells. The final outcome of a dose of radiation depends on numerous factors, including

  • Nature of the delivered radiation (mode, timing, volume, dose)

  • Properties of the tumor (cell-cycle phase, oxygenation, molecular properties, inherent sensitivity to radiation)

In general, cancer cells are selectively damaged because of their high metabolic and proliferative rates. Normal tissue repairs itself more effectively, resulting in greater net destruction of tumor.

Important considerations in the use of radiation therapy include the following:

  • Treatment timing (critical)

  • Dose fractionation (critical)

  • Normal tissue within or adjacent to the proposed radiation field

  • Target volume

  • Configuration of radiation beams

  • Dose distribution

  • Modality and energy most suited to the patient’s situation

Treatment is tailored to take advantage of the cellular kinetics of tumor growth with the aim of maximizing damage to the tumor while minimizing damage to normal tissues.

Radiation therapy sessions begin with the precise positioning of the patient. Foam casts or plastic masks are often constructed to ensure exact repositioning for serial treatments. Laser-guided sensors are used. Typical courses consist of large daily doses given over 3 wk for palliative treatment or smaller doses given once/day 5 days/wk for 6 to 8 wk for curative treatment.

Types of Radiation Therapy

There are several different types of radiation therapy, including

  • External beam radiation

  • Stereotactic radiation therapy

  • Brachytherapy

  • Systemic radioactive isotopes

External beam radiation

External beam radiation therapy can be done with

  • Photons (gamma radiation)

  • Electrons

  • Protons

Gamma radiation using a linear accelerator is the most common type of radiation therapy. The radiation dose to adjacent normal tissue can be limited by conformal technology, which reduces scatter at the field margins.

Electron beam radiation therapy has little tissue penetration and is best for skin or superficial cancers. Different energies of electrons are used based on the desired depth of penetration and type of tumor.

Proton therapy, although limited in availability, has advantages over gamma radiation therapy in that it deposits energy at a depth from the surface, whereas gamma radiation damages all tissues along the path of the beam. Proton beam therapy also can provide sharp margins that may result in less injury to immediately adjacent tissue and is thus particularly useful for tumors of the eye, the base of the brain, and the spine.

Stereotactic radiation therapy

Stereotactic radiation therapy is radiosurgery with precise stereotactic localization of a tumor to deliver a single high dose or multiple fractionated doses to a small intracranial or other target. It is frequently used to treat CNS metastases. Advantages include complete tumor killing where conventional surgery would not be possible and few adverse effects. Disadvantages include limitations involving the size of the area that can be treated and the potential danger to adjacent tissues because of the high dose of radiation. In addition, stereotactic radiation therapy cannot be used in all areas of the body. Patients must be immobilized and the target area kept completely still.


Brachytherapy involves placement of radioactive seeds into the tumor bed itself (eg, in the prostate or cervix). Typically, placement is guided by CT or ultrasonography. Brachytherapy achieves higher effective radiation doses over a longer period than could be accomplished by fractionated, external beam radiation therapy.

Systemic radioactive isotopes

Systemic radioactive isotopes can direct radiation to cancer in organs that have specific receptors for uptake of the isotope (ie, radioactive iodine for thyroid cancer) or when the radionuclide is attached to a monoclonal antibody (eg, iodine-131 plus tositumomab for non-Hodgkin lymphoma). Isotopes can also palliate bone metastases (ie, radiostrontium or radium for prostate cancer).

Other agents or strategies, such as neoadjuvant chemotherapy, can sensitize tumor tissue to radiation and increase efficacy.

Adverse Effects of Radiation Therapy

Radiation can damage any intervening normal tissue.

Acute adverse effects depend on the area receiving radiation and may include

  • Lethargy

  • Fatigue

  • Mucositis

  • Dermatologic manifestations (erythema, pruritus, desquamation)

  • Esophagitis

  • Pneumonitis

  • Hepatitis

  • GI symptoms (nausea, vomiting, diarrhea, tenesmus)

  • GU symptoms (frequency, urgency, dysuria)

  • Cytopenias

Early detection and management of these adverse effects is important not only for the patient’s comfort and quality of life but also to ensure continuous treatment; prolonged interruption can allow for tumor regrowth.

Late complications

Late complications can include cataracts, keratitis, and retina damage if the eye is in the treatment field. Additional late complications include hypopituitarism, xerostomia, hypothyroidism, pneumonitis, pericarditis, esophageal stricture, hepatitis, ulcers, gastritis, nephritis, sterility, muscle contractures, and arteriosclerotic heart disease depending upon the area treated.

Radiation that reaches normal tissue can lead to poor healing of the tissues if further procedures or surgery is necessary. For example, radiation to the head and neck impairs recovery from dental procedures (eg, restoration, extraction) and thus should be given only after necessary dental work has been done.

Radiation therapy can increase the risk of developing other cancers, particularly leukemias, sarcomas, and carcinomas of the thyroid or breast. Peak incidence occurs 5 to 20 yr after exposure and depends on the patient's age at the time of treatment. For example, chest radiation therapy for Hodgkin lymphoma in adolescent girls leads to a higher risk of breast cancer than does the same treatment in adult women.

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