AUTOMATIC-GENERATION OF BEAM APERTURES

In order to specify arbitrarily shaped beam apertures for three-dimens ional radiation treatment planning, aperture contours (or outlines) ar e often manually drawn using a beam's eye view display of the target v olume and nearby normal structures. This can be a very time consuming process, and can be impractical for multileaf collimation and computer -aided optimization of a large number of fields. A method has been dev eloped that allows automatic generation of aperture shapes that outlin e the target volume and may spare neighboring structures whenever desi red. Margins of user-specified sizes (positive or negative) around the target and normal structures are also incorporated. For a chosen beam orientation, a 3D surface of each anatomic structure of interest is f ormed and projected onto a plane at the beam's isocenter. The outlines of each projected object are detected by an edge following algorithm, and margins are added. The outlines of normal structures are combined with that of the target volume to obtain the final aperture shape. Th is is done by overlaying filled versions of the outlines in such a way that regions of the target overlapped by normal structures are cut aw ay, leaving only the target volume region to be irradiated. The remain ing target volume outline is again detected to produce an aperture con tour. Normal structures may split the aperture into several pieces, so this method detects any number of disjoint aperture contours. The res ults of the algorithm are illustrated with apertures generated for nas opharynx and prostate tumors, including sparing of normal tissues.