Ionizing radiation kills living cells by creating breaks and cross-links in DNA. Radiation oncologists (specialists in radiation therapy) take advantage of the fact that normal cells are better able to repair damaged DNA than are malignant cells.
The goal of radiation therapy is to destroy abnormal cells without inflicting significant damage on nearby normal cells. However, due to relatively crude targeting, traditional methods of radiation therapy caused significant toxicity.
Modern technologies provide for better targeting of tumors, thereby dramatically reducing the incidence of unpleasant side effects.
Uses for radiation therapy range from definitive management of single tumors to alleviation of symptoms from metastatic disease (palliative care). Additionally, radiation therapy is valuable for shrinking certain tumors prior to surgery—thus reducing the extent of the surgical procedure—and for treating patients after surgery to reduce the risk of recurrence.
Finally, radiation may be ideal for treating patients who cannot tolerate surgery.
Modalities of Modern Radiation Therapy
Modalities of radiation therapy are classified into two general categories: those that are administered from outside the body (external beam therapy) and those that are administered from within the body (internal therapy).
External Beam Radiation Therapy
- Three-dimensional conformal therapy: Detailed images generated by CT or MRI are integrated with computer-controlled radiation beams directed at the target tumor. Short, daily treatments are administered over several weeks. Mesh face masks, body molds, and freckle-like tattoos may be used to localize tumors and help immobilize the target area.
- Four-dimensional therapy: When tumors are prone to movement (e.g., lung, liver, pancreas, breast), computer-assisted tracking of images during activation of the radiation beam augments three-dimensional conformal therapy.
- Intensity-modulated therapy: When critical structures (i.e., nerves or vessels) are adjacent to or surrounded by the tumor, the radiation beam is subdivided into modulated “beamlets” to spare nearby normal tissues. Individual treatments usually last longer—sometimes more than 30 minutes.
- Stereotactic radiosurgery: Useful for intracranial lesions (brain tumors, acoustic neuromas, etc.). Multiple radiation beams are directed at a single target, thus delivering a high dose of radiation to the tumor while sparing nearby structures. A rigid framework is temporarily attached to the patient’s skull to eliminate movement during treatment, which often lasts up to an hour.
- Stereotactic body therapy: A robotic arm spins around the patient to deliver radiation to the tumor from several different angles. Markers or a rigid framework are applied to the patient beforehand to localize the tumor and limit its motion. A useful modality for spinal tumors, certain lung cancers, and for patients who cannot tolerate surgery.
Internal Radiation Therapy
- Brachytherapy: A radiation source (radioactive seeds, pellets, catheters, balloons, etc.) is placed inside the body near the target tumor. Some forms are permanent, while others remain within the body for a short period of time. Brachytherapy is sometimes used in conjunction with surgery to reduce recurrence risk. Useful for cancers of the mouth, vagina, and cervix, and some sarcomas and prostate tumors. Patients may have to limit social contacts for up to a month, but radiation dissipates within six months or so, even with permanent implants.
- Systemic therapy: Radioactive isotopes are administered orally or intravenously in order to target specific tumor types. Some tumors (thyroid cancer and some bone tumors) exhibit an affinity for certain isotopes (iodine-131, strontium-89, samarium-153, etc.). The tumors will absorb these radioactive isotopes while nearby normal structures are spared. Radioimmunotherapy—the use of monoclonal antibodies attached to radioisotopes—is now being used to treat non-Hodgkin lymphoma.
(Adapted from Gerber D, Chan T. Recent advances in radiation therapy. Am Fam Phys 2008;78(11):1254-62)
Recent advances in radiation therapy have expanded the utility, improved the effectiveness, and decreased the complications of this important treatment modality.