Asymmetric field arc rotations.

Optimal treatment planning of target volume that surrounds a vital critical structure is often very difficult. Treatment techniques using moving beam therapy with fields asymmetric with respect to rotational axis of the collimator head allow treatment of such target volumes with minimal dose to critical structures. The availability of independent motion of the collimator jaws on new medical accelerators allows easy setting up of asymmetric treatment portals. Therefore, treatment techniques utilizing asymmetric field arc rotations with acceptable dose distributions have been possible.