Principle of field in field technique. Representation of the field in field method (FiF) using the example of a neurocranium RT. The diagrams outlined in red show the initial MLC fields for one beam direction that are identical for FF and FFF planning. The upper row (a) shows the fields at 10MV that are necessary to achieve the planning objectives. The additional field serves to block off overdosed areas. The lower row (b) shows the required additional fields because of the significant drop in radial intensity at 11MV FFF. In this case ten additional fields are required to compensate the under-dosing in order to achieve the planning objectives.