Collision free regions are predetermined for one or more class solutions. Each class solution has defined limits for allowed field geometry variations. Collision free regions in planning can be defined as a set of allowed isocenter positions relative to a fixation device. The collision free regions may be visualized by a user to plan for field geometry and isocenter position tradeoffs. Collision free regions in delivery can be defined as a set of allowed couch support coordinates. The treatment fields in a radiation treatment plan can be checked against the collision free regions in delivery to determine whether they will pose any collision risks.