(a) Perfect match
(b) With an overlap
(c) With a gap
Figure 1: Two adjacent abutting fields. The setup and machine errors cause dose variation where the fields meet. (a) If the two fields are matched perfectly, they produce a uniform dose distribution at the junction. (b) When the fields overlap, an overdose is seen in the dose profile. (c) When there is a gap, an underdose is produced.