Multileaf Collimator (MLC) Leaf Intrusion an Optimum Value? Variable MLC Leaf Width?

1
Multileaf Collimator (MLC) Leaf Intrusion an
Optimum Value?Variable MLC Leaf Width?

• Martin E Welch
• Presented at the AAPM meeting 2001

2
Quality 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.

3
Effect 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.

4
Leaf 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

5
The 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.

6
The 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.

7
The 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.

8
Effect 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.

9
Leaf 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

10
A Demonstration of three Leaf Intrusion Values
applied to the same field
11
This Easily Recognisible Shape contains many of
the problems encountered with its clinical
counter parts.
12
Conclusions

• 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.