In radiation therapy, the aim of the treatment is to deliver a very high dose to the tumour, but as little dose as possible to the surrounding healthy tissue. In this case, we are not talking about low doses. Using beams of high-energy gamma rays from cobalt-60 or from accelerators, localized high dose of radiation is delivered to tumour and this radiation tend to kill tumour cells. The amount of radiation used in photon radiation therapy is measured in grays (Gy), and varies depending on the type and stage of cancer being treated.
For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy (locally), while lymphomas are treated with 20 to 40 Gy. Note that, this is not the effective dose. A gray is unit of absorbed dose, which is defined as the amount of energy deposited by ionizing radiation in a tumour.