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Patient Set Up Errors That OSMS and Small Field of View CBCT Cannot Catch with Varian Exact Couch Top

K Chu*, V Potari , M DeLuca , R Rostock , M Bailey , Western Connecticut Health Network, Danbury Hospital, Danbury, CT


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: When performing VMAT delivery with the Varian Exact® couch, we demonstrate the importance of 1) using large Field of View (FOV) CBCT for head and neck treatments; 2) setting table lateral equal 0cm, before the patient is placed on the table for pelvic treatments with immobilization from mid-thigh to feet; 3) setting lateral SSD by moving the table laterally. These therapy verification techniques have been lost with the use of modern CBCT and Optical Surface Monitor Systems (OSMS), which cannot detect these errors.

Methods: The Varian Exact Couch consists of a table top as well as two thick carbon fiber bars. These structures should be included in VMAT plans with full gantry rotation. Both bars are placed “IN� for planning. Therapists are asked to do large FOV CBCT. Physics reviews patient imaging after the first treatment, and notes the couch position in the CBCT versus the plan.

Results: Review of large FOV CBCT shows the support bar may be up to 3cm off laterally. For time efficiency, therapist often chose the small FOV or leave the bars set to “out�; in these cases, the bars cannot be seen as they are outside the CBCT FOV. OSMS typically do not include the table.By reviewing initial patient imaging, it was discovered that the placement of H&N board sits in the middle of the table rather than over the end. In this case, dosimetry was asked to include the Exact® couch with Bars “IN�. The H&N board overhangs the end of the table only for brain cases.

Conclusion: Physics should review initial patient imaging to compare the table structures position to the plan. Effective implementation of TG-176 recommendations to improve patient set up to the treatment plan relies on active communication and collaboration between Physics and Therapy.


Setup Errors, Cone-beam CT, Immobilization


Not Applicable / None Entered.

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