Title: Multileaf Collimator (MLC) Leaf Intrusion an Optimum Value? Variable MLC Leaf Width?
1Multileaf Collimator (MLC) Leaf Intrusion an
Optimum Value?Variable MLC Leaf Width?
- Martin E Welch
- Presented at the AAPM meeting 2001
2Quality Assurance (QA)
- Conformal fields created by the use of an (MLC)
are only as accurate as the calibration of each
Leaf. As treatments are performed at all Gantry
angles the QA should be performed at the four
cardinal points. Thus taking into account any
Leaf droop or sag, and must be performed over the
range of Leaf movement. - The Light Field Test Shape shown here, is one of
three that allows this test to be performed and a
hard copy (film) to be taken and checked in the
same time as a standard 10x10 field. A Light
Field Template of these fields is used to check
the film.
3Effect of Leaf Calibration Errors at the Central
Axis
- The effect of Leaf calibration errors is easily
seen on these two circular fields. - The errors seen are only due to Leaf calibration
and not rotational displacement. - Although the calibration error will be constant
over the Leaf range of travel. The effective
error will be greater as the treatment field gets
smaller.
4Leaf Tip to Contour Angle
- The angle that the Leaf tip is presented to the
required contour defines the fit of the produced
isodose. - As the angle increases the effective control
decreases. - With a curved contour the problem is greater due
to the error changing per Leaf. - For most shapes the control of the required
isodose is established by the use of the Leaf tip
and edges. It is the ratio of these that
determines the effective control
5The Effect of Depth on the Scalloped edge.
- The Scallop effect or the dip and peak in the
isodose produced by the width of the Leaf, does
change with depth. - Figures a/b/c 6MV show results of a triangular
shape measured in water at depths of Dmax/
D10/D15. The effect is clearly visible. - Figures a2/b2 show the same field for 15Mv. To
minimise variables no correction was made for
penumbrae and the Leaves were not moved. The 80
isodose represents 95 had the Leaves been moved.
The effect may still be seen at D10 a typical
patient separation.
6The Effect of Depth on the Scalloped edge.
- The figure on the right shows the scallop effect
reducing as the field size increases. - Control of small fields such as head and neck
work are significantly affected by the Leaf width
and the scallop effect.
7The Phased Field Technique Royal Free Hospital.
- This technique has been in clinical use at this
hospital since 1993. It is the basis of the
Siemens HD270 MLC. - It is a method of artificially reducing the Leaf
width at any point in the field. - Two fields are normally used. One field centre
being shifted by 5.0mm. - There is no increase in treatment time as each
field is 50 of applied dose.
8Effect of Leaf Tip to Edge Ratio Applied to a
Clinical Field
- The collimator angle is selected for optimum
control. - Four Leaf Tip to Edge ratios are demonstrated.
- Effect on penumbrae is easily seen on this
clinical shape.
9Leaf Intrusion Values of 0 and 50 applied to a
shape in three field sizes
- The 11.0cm field demonstrates that 50 intrusion
does give the best control. - The 5.0cm field produces a poor fit for both
values. - The 3.0cm field suggests that the lesser evil is
50 intrusion
10A Demonstration of three Leaf Intrusion Values
applied to the same field
11This Easily Recognisible Shape contains many of
the problems encountered with its clinical
counter parts.
12Conclusions
- The ratio of Leaf Tip to Edge controlling the
isodose greatly affects the result. - If this ratio approaches 50 the field at this
point is larger than required. - An intrusion value of 25 gives best all round
control. - The Phased Field Technique applied with this
value of intrusion enhances the effective
control. - This same technique reduces the interleaf leakage
by a factor equal to the number of fields
employed ie two fields equals a 50 reduction.