Program Information
Off Axis MV-KV Alignment: Can We Assume?
W Duggar*, A Markovich , C Yang , University of Mississippi Med. Center, Jackson, MS
Presentations
WE-RAM3-GePD-J(B)-6 (Wednesday, August 2, 2017) 10:30 AM - 11:00 AM Room: Joint Imaging-Therapy ePoster Lounge - B
Purpose: Coincidence of isocenters has been a key component of radiation therapy since its inception. In the current age of IGRT and stereotactic treatments, this coincidence has become especially important for the kV imaging and MV treatment isocenters. With a traditional Winston-Lutz test, modified for imaging, this coincidence is confirmed at isocenter and then assumed consistent away from isocenter. Due to the fact that many targets are away from central axis, perhaps this coincidence should be confirmed.
Methods: On a Varian Trilogy LINAC of IMRT level tolerance per TG-142, a small cube with a 5 mm BB at the center was scanned on the CT simulator for reference data. Three different Winston-Lutz reminiscent treatment plans were created which included MV fields from all cardinal gantry and collimator angles plus the two extreme couch kick positions with the field sizes defined by Jaws in the G-T direction and MLC in the left-right direction. One plan was isocentrically aligned at BB center, the 2nd 3 cm to the left of BB center, and the 3rd 5 cm to the left of BB center. The cube was positioned with a 1 cm offset in each direction before acquisition and registration of a CBCT to the reference image. Shifts were performed and then the MV treatment fields were ported and then exported to the Pipspro software by Standard imaging for analysis. The process was repeated thrice (24 MV measurements) for each isocentric position relative to the BB. A one-way ANOVA test was utilized to compare the three populations of MV-kV discrepancies.
Results: No statistically significant difference between the populations was identified nor did any measurement exceed tolerance.
Conclusion: For IMRT level tolerance and a well-calibrated machine, kV-MV coincidence at isocenter may be assumed away from isocenter.
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