The 2D-ARRAY seven29 A new way of dosimetric verification of - PTW

2.2 IMRT delivery techniques
Several IMRT delivery techniques can be found in publications (see, e.g., [1]). Today, most newly
installed IMRT delivery systems use modulated beams with fixed beam directions. The fluence
modulations are created by the multileaf collimator (MLC), which runs in one of two special IMRTmodes:
• The multi-segment method approximates the continuously modulated optimized fluence of each
fixed field direction by a stepwise fluence distribution. This is done by combining a set of small
homogeneous field segments of different weights, which are shaped by the MLC. This method
is often also referred to as step-and-shoot delivery, because the radiation is only turned on
when the leaves have reached their prescribed positions for each segment.
• A technically more advanced method is the dynamic MLC technique (often also called sliding
window method). Here the fluence profile along the moving direction of a leaf pair is created by
sweeping the leaf pair with different openings over the field while the beam is on all the time.
Both methods result in similar final dose distributions, although the dynamic MLC approach can
generate a better approximation of the optimized fluence and allows a faster delivery. On the other
hand, the multi-segment method is easier to implement and needs a slightly lower number of
monitor units.
In contrast to conventional homogeneous fields, the number of monitor units for both methods as
given by the treatment planning system no longer has a simple relation to the delivered dose of
that field. Therefore, many groups feel it necessary to verify each single treatment field
dosimetrically before its clinical use.
2.3 IMRT and quality assurance
IMRT is a very complex treatment modality. Therefore, new quality assurance procedures must be
implemented throughout the complete clinical process [1]. Here we will only discuss some
technical aspects. Typically, the clinical implementation of IMRT passes through at least three
steps:
1. The installation and commissioning phase of the treatment planning and the delivery system
2. The clinical starting phase, where each treatment is tested by an individual verification
procedure, which typically is extremely time-consuming and not very standardized
3. The routine phase, where standardized QA procedures are used to guarantee safe and reliable
IMRT deliveries for a large number of patients
Most groups focus only on the first two phases. However, the third phase is by far the most critical
one for a stable IMRT routine delivery. The routine use of IMRT for large patient numbers is only
possible if standardized QA procedures have been carefully defined and easy-to-use measuring
equipment together with user-friendly analysis software is available.
For both MLC modulation methods, the delivered doses have a complex, non-intuitive relationship
to the number of monitor units. It is also impossible to predict the exact combination of field
segments or the leaf motion patterns. Therefore, all IMRT groups, which are using the MLC for the
creation of fluence modulations, must establish a precise and reliable method for the dosimetric
verification of IMRT plans. Especially during the starting period, each plan must be dosimetrically
verified, but dosimetric verification is also recommended after software upgrades or the expansion
to new tumor entities.
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