hematology and oncology
Radiation Therapy for Cancer
Types of Radiation Therapy
There are several different types of radiation therapy, including External beam radiation Stereotactic radiation therapy Conformal radiation therapy Brachytherapy Systemic radionuclides External beam radiation therapy can be done with Photons (gamma radiation) Electrons Protons Gamma radiationusing 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 therapyhas 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 therapyhas 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 uses multiple, precisely focused beams with precise stereotactic localization of a tumor to deliver a single high dose or multiple fractionated doses to a small intracranial or other target. The beams are delivered from many different angles that all meet at the tumor, thus passing through many different areas of healthy tissue on the way to the tumor; this means the tumor receives a much higher dose of radiation than any of the surrounding healthy tissue. Stereotactic therapy is frequently used to treat metastases to the central nervous system. 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. In conformal radiation therapy, imaging technology allows the radiation beam to be shaped to conform to the dimensions of the tumor, allowing more precise targeting. 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 radionuclides 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. 