Biology of Brachytherapy, Particle Therapy, and Alternative Radiation Modalities

Most radiation therapy is delivered by linear accelerators capable of photon and electron irradiation. However, alternative modalities may have advantages in some clinical situations. Brachytherapy is radiation delivered by a radioactive source placed within or in close contact to the target, causing a rapid dose falloff with distance. Brachytherapy is classified as low dose rate (LDR) or high dose rate (HDR), and sealed or unsealed sources. Unsealed sources are free-floating radionuclides that can be injected into an anatomical space or administered systemically. Heavy charged particles include protons and heavy ions. These charged particles exhibit a high-dose Bragg peak, with little to no exit dose beyond the intended depth of treatment. Protons have an RBE approximating photons, however heavy ions have a much higher RBE. Neutron beam therapy has a very high RBE, allowing for a greater effect on radioresistant tumors but also greater normal tissue toxicity. Boron-containing drugs may react with neutrons to further increase radiation dose to tumor.