The aims of this paper are to describe a method of splitting large intensit
y-modulated fields that cannot be delivered as a single field and to verify
the accuracy of our method. Some multileaf collimators may be operated in
the dynamic mode to deliver intensity-modulated radiation treatments (IMRT)
using the 'sliding window' technique. In this technique each pair of leave
s sweeps over the treatment field while the beam is on. However, there are
limitations on the width of the held that can be treated due to the limited
length of the leaves. For instance, the leaf length of the Varian MLC is 1
4.5 cm. Since each leaf pair must travel from the left boundary to the righ
t boundary of the beam aperture, the maximum width of the field aperture th
at can be accommodated in one sweep of leaves is also limited to 14.5 cm, i
n fact to a slightly smaller value. It has been shown that IMRT is more eff
icient when used to plan and deliver the large and boost fields simultaneou
sly. in such situations, the fields must be large enough to cover simultane
ously the volumes of the gross tumour, microscopic disease and electively t
reated regions. Such field sizes are often larger than 14.5 cm wide. In thi
s paper, we present a dynamic 'feathering' technique to split the large int
ensity-modulated fields into smaller fields. In this technique, the compone
nt beams overlap each other by a small amount, and the intensity in the ove
rlap region gradually decreases for one held component and increases for th
e other. The sum of intensities remains the same as for the original field.
This method eliminates the field matching problems associated with the con
ventional step 'break' for static fields. The splitting process is integrat
ed into the IMRT treatment procedure and the entire planning process is aut
omated. Comparison of dose distributions calculated and measured in a phant
om showed good agreement. Such a method can be applied to the 'step and sho
ot' technique as well. IMRT fields of widths up to 25 cm can be delivered b
y splitting only once, which is adequate for most treatments.