Intensity-modulated radiation therapy (IMRT) represents a significant technological advancement in the ability to deliver highly conformal radiation therapy. Thanks to increased availability, general clinical implementation has become progressively more common. However, there are several precautions worthy of comment regarding the clinical applications of IMRT.

In theory, the increased irradiated volume and leakage radiation that occasionally accompanies IMRT could contribute to unanticipated complications and safety concerns. The protracted delivery time of IMRT with the associated increased linac monitor units can result in photoactivation of elements within the linac collimator, thereby inadvertently increasing radiation exposure to patients and staff when high-energy photons are used. The increased volumes of normal tissue exposed to lower doses of radiation through IMRT theoretically could promote carcinogenesis and complications due to the bystander effect, low-dose hyper-radiosensitivity, and diminished repair of double strand DNA breaks at very low doses. Tumor control may be adversely affected by the lower radiation dose-rates of delivery sometimes associated with IMRT as well the occasionally seen low dose ?cold shoulder? on the dose-volume histograms. Unusual clinical reactions can appear as a result of the complex, unfamiliar dose-distributions occasionally generated by IMRT treatment planning. Here we discuss some of the precautions worthy of consideration when using IMRT and how these might be addressed in routine practice.

Key words: IMRT, Radiation dose-rate, Bystander effect, Low-dose hypersensitivity.