1.) Penumbra: Comparison between MLC and Jaws

I have compared the field penumbra of MLC fields to the penumbra of conventional fields of the same size.

Setup:

• All measurements were done with a PTW MP3 waterphantom.
• SSD was 90 cm, Gantry 0 deg, Collimator 0 deg.
• A PTW 31002 thimble chamber (0.125 cm3) was positioned with its center at Isocenter, at a water depth of 10 cm.
• The chamber was mounted crossplane. Due to the almost square chamber volume (5.5 mm diameter, length 6.5 mm), the chamber stem effect was neglected.
• Scans were made inplane and crossplane, scan resolution was 1 mm, measuring time 0.5 s per point.
• The Clinac was set up to beam on at a doserate of 500 with dose servo off.
• Measurements were done for the two photon energies 6MV and 15MV.
• Jaws were set at the nominal field sizes of 4*4, 10*10 and 20*20 cm for "Jaw-fields".
• For "MLC-fields" the jaws were set at the recommended settings of 4.4*5.6, 10.4*11.6 and 20.4*21.6. The larger value is always crossplane, the direction of leaf movement.
• Closed leaves of MLC-fields were closed off-axis, otherwise the inplane scans would run through the leaf abuttment region.
• Penumbra was defined by 80% - 20%.
• All values in the tables are given in mm.
• PTW Coordinate system: when seen in BEV, Gantry on top, +x points to the Gantry (inplane), +y to the right (crossplane). See Figure 1 for orientation of the waterphantom.

Figure 1: Coordinate Systems (PTW Mephysto measuring Software)

Coordinate System Mapping

Penumbra	caused by
-x	X1-jaw, MLC Carriage B
+x	X2-jaw, MLC Carriage A
-y	Y1-jaw
+y	Y2-jaw

Results:

Discussion:

When the curved face of the leaves defines the field edge, the penumbra should be somewhat larger than with jaws. This can be seen in the tables, where the difference MLC-penumbra minus Jaw penumbra is larger for +y/-y edges. The increase in penumbra lies between 0.55 and 0.99 mm. The inplane penumbra for MLC-fields is always slightly smaller than for Jaw-fields.

2.) Leaf Transmission

For determination of the pure interleaf-transmission (between adjacent leaves) the leaves should not be shielded by the carriage guideplate (which gives some additional shielding, see below). Two MLC shapes define the setup for this test, one shape for each carriage.

Figure 2: Test pattern for carriage B transmission test

Setup for Carriage B Transmission Test:

• The jaws are set to X1 = 10 cm, X2 = 0 cm, Y1 = Y2 = 5 cm (dashed lines in Figure 2). This sets up an asymmetric field of 10*10 cm. The scanline for the waterphantom will be 5 cm off axis, through the center of the field defined by the jaws.
• The other parameters are the same as for the penumbra tests above (SSD, chamber, etc.).
• An inplane scan is made at a depth of 10 cm (Figure 3)

Figure 3: Inplane scan with closed MLC, 15X, Carriage B

• This gives the point of maximum transmission. The center of the scan shows the gap between leaves 20B and 21B. So the maximum in this scan lies between leaves 22B and 23B. That is the point where the chamber is positioned for the following dose measurements.
• Dose is measured as an average over 3 to 5 100MU integrations.
• Then the MLC is retracted, and again the dose determined. With these two readings, the leaf transmission can be calculated (see table below).
• Tests were made for both energies and both carriages (For carriage A, just mirror everything).

Results:

Discussion:

When leaf 1 is set at the same value as leaves 2-40 (-2cm), which corresponds more to the true clinical situation, the closed field is also shielded by the carriage guideplate, and the X15 transmission is even less (1.77% for X15). The corresponding MLC shape is shown in Figure 4.

Figure 4: Closed MLC where Carriage guideplate gives some additional shielding (compare leaf 1 here to leaf 1 in Fig. 2)

In the above table, the difference between the two carriages is not significant. Scans were also performed at the longest possible field of 40 cm, to "see" all leaves. Scan resolution was 0.1 mm to 0.4 mm, to catch all details (Figure 5 and 6).

Figure 5: Full scan for carriage A, 15X

Figure 6: Full scan for carriage B, 15X

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Last modified: Oct 9, 1998 HK