intensity-modulated radiotherapy: current status and issues

IMRT: current status and issues of interest ● IMRT CWG
887
Fig. 5. DMLC technique of delivering IMRT (also referred to as the sliding window method). Dotted lines indicate
positions of a leaf pair (x axis) as a function of beam-on time (y axis). As the beam is turned on (point a; see Fig. 6),
both leaves move, with different speeds, from left to right. The point P begins to receive radiation when the right leaf
edge moves pass over it (point b). It receives radiation until the left leaf blocks the beam (point c). By controlling the
movement of the leaves and therefore the “beam-on-time” duration between b and c, one can deliver any desired
intensity to point P, or any other point under this leaf pair.
trol. The radiation is turned on only when the MLC
leaves are stopped at each prescribed segment position.
This IMRT method has been referred to as step-and-shoot
or stop-and-shoot. The IMRT CWG recommends this
form of conventional MLC IMRT be referred to as segmental
MLC (SMLC). The leaf sequences can be determined
by methods such as the one suggested by Bortfeld
et al. (82) or Siochi (83). Another type of SMLC implementation
makes use of multiple-shaped field segments
to create the IMRT fields (15, 16). Most medical linear
accelerator manufacturers are now offering SMLC–I-
MRT capability, and thus, widespread implementation of
this form of IMRT is anticipated during the next several
years.
A third conventional MLC IMRT approach, called
intensity-modulated arc therapy, was described by Yu et
al. (84– 86). Instead of rotating a slit-field around the
patient as done with tomotherapy, intensity-modulated
arc therapy uses multiple irregular fields shaped with a
conventional MLC during gantry rotation. Intensity-modulated
arc therapy is planned as a sequence of static
fields, every 5–10° apart, but delivered with multiple
superimposing arcs. Within each arc, the MLC shape is
continuously changed as a function of gantry angle on the
basis of the results of optimization, such that the cumulative
intensity distribution leads to the desired dose
distribution. This IMRT approach was first implemented
for clinical use at the University of Maryland (87).
Physical modulator (compensating filter) IMRT
Several groups have reported on the use of a physical
modulator to deliver IMRT (88–93). Filters can be designed
using a 3D-RTP system to calculate the required
thicknesses (along ray lines using an effective attenuation
coefficient for the filter material and dose–ratio parameters
for effective depths) to generate the desired IMRT
fluence profile when the filter is placed in the radiation
beam. Stein et al. (89) reported on the use of IMRT
physical modulators fabricated using low–melting-point
alloy poured into foam molds, which are cut using a
computer-controlled cutter. In addition, Dubal et al. (94)
recently reported on an IMRT physical modulator approach
for the treatment of breast cancer. The physical
modulator IMRT method has some advantages, including
higher resolution in the direction normal to the leaf
motion, higher precision compared with some MLC approaches,
simpler quality assurance (QA), and no matchline
problems. On the other hand, it has a relatively
cumbersome and time-consuming manufacturing process
and one must enter the treatment room to change the filter
for each gantry orientation, thus increasing the time
allocated for patient treatment. This approach will likely