Collimator angle and jaw tracking on a C-arm

Collimator angle is rotated away from 0° to break up the tongue-and-groove effect and interleaf leakage, which otherwise line up along the same axis for the whole arc. For dual-arc plans, using orthogonal angles (0° and 90°) reduced homogeneity index and gradient measure across several organs compared with the usual small offsets.

Jaw tracking closes the jaws to the active leaf aperture as the arc runs, removing transmission through the closed leaves. Adding jaw tracking together with automatic collimator angle selection improved both target and normal-tissue metrics in non-coplanar cranial VMAT.

Where it matters most: single-isocentre multiple targets. With several targets on one isocentre, the leaf pairs open for one target while another target sits in the same row — the "island blocking" problem. Collimator angle is the main tool for arranging the targets so their rows do not collide, and it is a per-plan decision rather than a departmental default.

Splitting the field. For a large target, restricting each field to about 15 cm and splitting into two overlapping fields — four arcs total — raised modulation and conformity and improved OAR sparing.