Fractionation is a well-established principle of radiotherapy, and describes fractionating or dividing the dose of radiation into many smaller increments. This reduces the effect of the radiation on the surrounding normal tissues. It is most effective when there is a high a/p ratio, i.e. when there is a marked difference between the radiosensitivity of the pathological target tissue and that of the surrounding structures. In conventional radiotherapy this is important, because the lack of localization means that the radiation field includes a large volume of normal tissue. Philosophically, gamma knife radiosurgery uses exactly the opposite approach: it relies on imaging, matching the radiation fields to conform precisely to the target tissue, and spares neighboring structures in this way, the actual radiation dose being given as a single unfractionated treatment. Intuitively, the stereotactic approach may be advantageous when the a/p ratio is not that high, typically the case with benign tumors, and hence the role of SRS in treating acoustic neuromas, meningiomas and AVMs. Conversely, fractionation (either with or without stereotaxy) may be more useful treating flat "en-plaque" tumors, large lesions, and pathology adjacent to radiosensitive structures such as the optic chiasm, where, even with image guidance, there is concern about the radiation delivered to the surrounding area.
Fractionation carries with it the disadvantage of necessitating multiple treatments. When it is used with stereotaxy, relocatable frames are necessary. These are inherently less accurate than the single fixed frame used in SRS. The impact of these concerns in clinical practice, and the relative role of SRS and SRT, are yet to be ascertained.
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