FIG. 4.

Electron equivalent field sizes. Examples of constructing rectangular fields whose output approximates that of the irregular field for: (a) a posterior cervical strip; (b) a posterior cervical strip plus submandibular nodes; and (c) an internal mammary chain. The irregular field is delineated by the irregularly shaped curve. The beam axes are delineated by orthogonal 10 cm line segments. The constructed rectangle is delineated by the four intersecting lines, the center delineated by “x.” The arcs are a distance 3 cm from the center of the rectangle (Ref. 45).