Fig.1: The expert group, consisting of Mr. Papauschek, Mr. Papauschek and Mr. Papauschek, discusses the problem.	Fig.2: A Gantry mounted stage (G-Stage) shall be designed for the PTW 2D-ARRAY. This is a simple task for the expert team.
Fig.3: After a few days, the G-Stage is mounted to the collimator head.	Fig.4: The mechanical stability of the assembly has high priority. An aluminum ring is used for docking.
Fig.5: This avoids Varian's Accessory Mount, which is less ideal. The ring is mounted directly to the collimator head.	Fig.6: The ARRAY surface is at SSD100 if 1cm of Solid Water is placed under the ARRAY.
Fig.7: To mount the stage, the Gantry has to be rotated down and 4 screws removed.	Fig.8: The set of screws on the right is used instead of the original set (left).
Fig.9: The whole procedure takes only a few minutes. The plastic cover of the collimator may stay in place.	Fig.10: Four more screws and two plastic rails hold the ARRAY in place when the Gantry rotates.
Fig.11: These screws are tightened only gently.	Fig.12: There is no mechanical play in the assembly, only elastic deformation.
Fig.13: Due to weight, the stage hangs down a little at Gantry 270°, after aligning the ARRAY at gantry 0°.	Fig.14: The chambers are 5x5mm. The displacement is less than 1mm.

The G-Stage is used for all linac checks according to the Standard ONORM S5290 that require beam performance measurements at nonzero Gantry angles.

Port image taken with aS500 of the G-stage with mounted 2D-ARRAY. Note the four aluminum cylinders which are several cm behind the ARRAY.

For constancy checks, the MultiCheck software can be used. Fig.15 shows a test field of 26x26cm at Gantry 0°. With respect to the reference measurement (not shown), the overall field size is a little smaller, indicated by the light blue shading in the area "Deviation from Reference".

Fig.15: A 26x26cm field at Gantry 0°.	Fig.16: The same field at Gantry 180°.

Fig.16 shows the same field after repeating the measurement at Gantry 180°. The margin zone area is now light red, indicating higher dose compared to the reference. This could be due to some play in the jaws. If the jaws open up a little, field size increases and the margin zone chambers receive more dose. Note that in the Margin zone area, 0.1% is set to maximum color. The effect is so small that it could probably not be detected with film.

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