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

However, the plan-related approach also has several practical disadvantages, which make its
routine use difficult:
• Real 3D dose measurements are extremely time consuming and not available under clinical
conditions. A plan-related verification is therefore normally restricted to only a few 1D or 2D cuts
out of the 3D dose cube. Therefore delivery problems (e.g., misaligned leafs) outside the
measuring zone may not be detected.
• Depending on the used measuring system, the plan-related approach may be accompanied by
severe dosimetric problems. A typical example is the film calibration, which is not trivial for films,
which are oriented parallel to the beam entrance direction.
• Because all fields are treated at once, it is often difficult to locate the reason for a detected
dosimetric discrepancy.
• The phantom-preparation is time consuming.
3.3 The field-related approach
For the field-related approach, each single treatment
field is transferred separately to a verification phantom.
All treatment parameters are the same as for the real
patient plan, except the gantry angle, which is normally
set to 0° for all beams. The field-related approach
requires only a very simple rectangular phantom,
which is able to carry a dosimetric film or another
planar dosimeter at a plane perpendicular to the beam
entrance direction (see Fig. 4). Sometimes the
phantom has additional holes for ion chamber
measurements. PTW provides the Universal IMRT
Verification Phantom T40020 for the field-related
approach.
The field-related approach is often criticized, because it does not reflect the real treatment as well
as the plan-related approach. On the other hand, the field-related approach has a number of
advantages, which are especially valuable if IMRT is operated under routine conditions.
• The verification process covers the complete modulated area of each field. All potential delivery
problems are therefore safely detected.
• Dose measurements are always performed in a plane perpendicular to the central axis; hence,
dosimetric problems (e.g., film calibration) are less critical than for the plan-related approach.
• A detected dosimetric discrepancy can easily be traced back to its reason, e.g., an improperly
adjusted leaf.
• The preparation and setup of the phantom is easy and not very time-consuming.
• The field-related approach is perfectly adapted to the use of various electronic 2D measuring
devices such as the 2D-ARRAY seven29.
For groups which have either no access to film dosimetry or want to avoid it because of its
complexity and cost, the latter point is an important argument in favor of the field-related approach.
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Dosimetric
film
Phantom
Hole for ion
chamber
Figure 4: A verification phantom for the planrelated
approach.