External Beam Radiation Therapy (EBRT)

A machine — most often a linear accelerator producing high-energy photon beams — aims radiation at the tumor from many directions. Where the beams cross, the dose adds up inside the tumor, while each individual beam passes through less healthy tissue. The radiation damages the DNA of cancer cells; cells whose DNA is damaged beyond repair stop dividing or die, which is why the effect accumulates over days to weeks. Before treatment starts, imaging scans (CT, MRI) are used to map the tumor and plan the arrangement of beams (3D conformal radiation therapy). In intensity-modulated radiation therapy (IMRT) the dose is split into many small beamlets whose strength can be individually adjusted, so selected parts of the tumor receive higher doses while nearby organs receive less. Image-guided radiation therapy (IGRT) adds imaging during the sessions themselves to correct for changes in position. Stereotactic techniques — radiosurgery (SRS) for small, well-defined tumors in the brain and central nervous system, and stereotactic body radiation therapy (SBRT) for small, isolated tumors elsewhere, often in the lung or liver — converge focused, high-energy beams so precisely that treatment can be completed in one or a few sessions.