Static field conformal stereotactic radiosurgery: physical techniques.

PURPOSE: Lesions in the head that are irregularly shaped or large present challenges for radiosurgical treatment by conventional techniques. Single, large circular fields may treat normal tissue volumes. Multiple shot or multiple isocenter treatment plans provide better conformation to the target than a single field, but may be difficult to plan and/or treat. As an alternative to these conventions, we are developing static field, conformal stereotactic radiosurgery. In this technique a finite number of fixed, shaped, linear accelerator fields are used to irradiate the target. METHODS AND MATERIALS: Computer simulations were performed for a four-path arc and fixed field techniques and evaluated with dose distributions and dose volume histograms. Beam geometries are defined with a 3-D treatment planning system with beam's eye view capabilities. Equipment for treatment delivery has been designed, including a head frame/support stand assembly and a method for manufacturing the required custom collimators.
RESULTS: Isodose distributions and dose volume histograms show that beam geometries with seven or more fields provide target dose distributions equivalent to the arc treatment, but with small increases in peripheral dose. Dose homogeneity across the target volume increases as the solid angle of irradiation is increased. For a hemispherical target, the four-path arc and shaped, static fields provide equal target coverage while the shaped fields treat a smaller high-dose volume.
CONCLUSION: Shaped, static fields are an alternative to single isocenter arc radiosurgery and result in smaller volumes at high dose. This smaller volume could translate into sparing for normal adjacent tissues that would otherwise be treated.