Clearance Below the Phantom
When the structure set with couch is used, care should be taken that the height of the phantom above the couch corresponds to the true value on the linac. In our case, on the linac couch we measured 38.5 mm of air gap below the phantom cylinder1 with the four adjustable feet of the rotation unit fully extended, which lifts the phantom to maximum height.
However, in the Eclipse phantom as imported from the PTW CD, the distance between couctop and rotation unit is 70 mm, which is nearly twice as much:
If left uncorrected, the consequence would be that in Eclipse isocenter shadowing by the couch would start at gantry 132°,
whereas during measurement of a VMAT treatment, it would start at 128° because the phantom in reality is at a lower height:
Dose differences could result. We therefore corrected the couch position in the Eclipse structure set by moving the couch up a little, so that the TPS geometry agrees with reality.
Notes
1The measured value of 38.5 mm applies to the Standard top and was determinded after rotating the Unit to 180° (with the unit at 0°, we measured 38.8 mm). With the SRS top, we measured a couchtop distance of 115 mm. Adding both clearance values to the respective top radii of 160 and 85 mm gives isocenter heights which are not identical (STD: 38.5 + 160 = 198.5 mm, SRS: 115 + 85 = 200 mm). The reason for the 1.5 mm discrepancy is still unclear.
Remember that levelling the phantom via the four adjustable feet also slightly affects clearance.