Optimizing radiation for cancer immunotherapy

Various protocols are under investigation for the purpose of optimizing radiotherapy for cancer treatment. Due to recent advances in image guidance and radiation delivery, a new option is to treat tumors with a single intense dose of radiation, 30 Gy or more (1,2). Alternatively, radiation doses can be “fractionated” over multiple treatment periods. Radiotherapy kills tumor cells and their associated stromal and vascular cells, and in some instances can induce T cell-mediated immunity that is effective at killing tumors outside the radiated area, a phenomenon called the “abscopal effect” (3,4). Since complete tumor remission usually depends on an effective anti-tumor immune response, it is important to determine how different radiation regimens influence anti-tumor immune responses. Filatenkov and coworkers (5) investigated the effect of radiation delivery protocols on the numbers and properties of immune cells in the microenvironments of colon cancer cells in mice. Weakly immunogenic ectopic CT26 colon tumors grown in syngeneic Balb/c mice usually responded to a single 30 Gy dose of intense radiation with durable tumor remissions due to T cell-mediated tumor killing. Fractionated radiation regimens were not as effective at stimulating T cells responses or durable remissions. The authors conclude that an examination of the tumor immune response may be useful for optimizing radiation regimens applied to various tumors.