Stereotactic radiation therapy uses high energy x-ray beam to shrink or control the growth of a tumor or abnormal cells by either killing the cells directly or by disrupting the ability of the cells to grow.
SRS and SRT are very similar, but SRS delivers a large dose of radiation on a single day and SRT has a fractionated treatment schedule. This means that in a SRT treatment the patient will have treatments spanning multiple days. Although, the total dose in SRT may be larger than in SRS any single day will have a much smaller dose delivery. Both are used for the treatment of localized tumors in the cranium and more recently in the body cavity.
Due to the large dose delivered in SRS, it necessitates a very precise positioning of the patient. To accomplish this task a head ring is attached to the patient and a CT is taken. This enables the accurate positioning of the patient with respect to the head ring. The head ring can then be fixed in place in the therapy room, which allows for the delivery of radiation on the sub millimeter scale.
SRT has treatments on multiple days and it is not practical to attach a head ring for every treatment. Fortunately, there is less of a need to be very precise in the delivery of radiation. It is therefore possible to use a masking system that will allow patients to be repositioned on a daily basis.