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.