A new dosimetric quantity, the lateral build-up ratio (LBR), has been introduced to calculate depth dose distribution for any shaped field. Factors to account for change in incident fluence with collimation are applied separately. The LBR data for a small circular field are used to extract radial spread of the pencil beam, σ(r), as a function of depth and energy. By using the relationship between LBR, σ(r), energy and depth, a formalism is developed to calculate dose per monitor unit for any shaped field. Criteria for lateral scatter equilibrium are also developed which are useful in clinical dosimetry.