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The accuracy of CBCT alignment and couch shift process is of fundamental importance in the accuracy of delivered dose in lmage Guided Radiation Therapy (IGRT). Item 710-039, the 73mm Cube, has one (1) Central axis tungsten marker and two (2) offset tungsten markers for the testing and verification of predetermined measurable couch shifts.This phantom can be used to test the accuracy of CBCT alignment and couch shift in a simple and efficient manner. Images can be transferred to the treatment planning system to check coincidence of treatment planning system to couch shifts.
General Operation for Daily CBCT Shift Verification
Place the phantom cube on the CT couch. Align the phantom utilizing the positioning lasers on the CT machine and the laser alignment markings on the phantom. An axial CT scan of the phantom is acquired. The reference images are imported into the TPS and a simple plan is generated where the tungsten marker is aligned to the isocenter described by the TPS.
Normally one would place the phantom on the Linac couch in a known offset position from isocenter with the use of inscribed markings on the phantom. Then a CBCT scan is acquired in the offset position and the therapist aligns the phantom as one would align the patient using tools on the OBI workstation. The necessary couch shift is applied to move the phantom to the isocenter. After the couch shift is performed, the user can verify the location of the isocenter after the shift and document the deviation from the true isocenter. This test will ensure the CBCT alignment process is performing as intended within the tolerance levels established by the physicist.
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