negative pi meson radiotherapy, a form of radiotherapy using a beam of subatomic particles known as negative pi mesons, or pions, emitted by a proton linear accelerator. When the particles are beamed at a tumor, they cause the atomic nuclei of malignant cells to explode and scatter intensely radioactive subatomic particles through the tumor. Pion radiotherapy requires fewer rad and has a 60% greater biological effect than conventional x-radiation techniques. It also may have less effect on normal tissue near the tumor. Some locally advanced neoplasms, especially those of the prostate, are destroyed. Gliomas and advanced cancers of the head and neck also may be well controlled with pion radiotherapy. Moderate acute toxicity occurs with treatment, but chronic toxicity is minimal. See also pion.
