STEREOTAXIC RADIOTHERAPY OF IRREGULAR TARGETS - A COMPARISON BETWEEN STATIC CONFORMAL BEAMS AND NONCOPLANAR ARCS

Stereotactic radiotherapy using a linear accelerator is usually equate d with the technique of delivery using multiple non-coplanar arcs, whi ch achieves a spherical dose distribution. As the majority of intracra nial lesions are not spherical, a range of schematized tumour shapes w ere planned to assess the role of static conformal beams in the treatm ent of irregular lesions. A sphere and 2 ellipsoids, ranging from 20 t o 50 mm maximum diameter located intracranially were planned using 3, 4, and 6 non-coplanar static beams with conformal blocks and were comp ared with four 120-degrees non-coplanar arcs. Comparison of the plans was made by the relative sparing of normal tissue outside the target v olume using three-dimensional dose-volume distributions. Non-coplanar arcs spared more normal tissue at low isodoses and achieved the best h igh dose sparing for spherical targets. For the majority of irregular targets, 3 and 4 static beams spared more tissue at doses greater-than -or-equal-to 50% and greater-than-or-equal-to 80% than the arc techniq ue. For all irregular volumes, maximum sparing of normal tissue to iso doses greater-than-or-equal-to 50% and greater-than-or-equal-to 80% of the treatment isodose was obtained with 6 static conformal beams. We conclude that irregularly shaped tumours suitable for stereotactic rad iotherapy with a linear accelerator are better treated with conformal static non-coplanar beams rather than with the multiple arc technique.