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Predicting Treatment Couch Coordinates to Reduce First Fraction Overrides

Scott Lacey

S Lacey*, J Antolak, Mayo Clinic, Rochester, MN

MO-B-Salon EF-2 Monday 10:00:00 AM - 12:00:00 PM Room: Salon EF

Purpose: To develop a tool for predicting treatment couch coordinates based on CT simulation data and demonstrate its effectiveness at reducing first fraction couch coordinate overrides.

Methods: A spreadsheet was developed to calculate treatment couch coordinates based on measurements of two diverging wires embedded in the couch top of the CT simulator. One wire is parallel to the longitudinal axis of the couch, whereas the second wire remains parallel to the couch surface but is angled approximately 5 degrees to the longitudinal axis. Lateral and vertical coordinates are determined from the differences in X and Y coordinates between isocenter and the first wire. The lateral distance between the two wires measured in the axial plane containing isocenter is used to determine the longitudinal coordinate. The calculations assume standardized indexing of immobilization at simulation and treatment. Calculated couch coordinates were compared to actual treatment coordinates recorded for 140 patients. The number of first fraction couch overrides was counted and compared to the number that would have occurred if default couch coordinate values had been used.

Results: For the 153 unique isocenters that were treated, only 12 (8%) required a couch override for the first fraction. In comparison, if default couch coordinates were used then 150 (98%) would have a couch override for the first fraction. Of the 12 overrides, 7 were expected due to an intentional change in indexing.

Conclusion: The described technique for calculating treatment couch coordinates has been implemented for nearly all patients at multiple radiotherapy centers. A significant reduction in first fraction couch coordinate overrides has been demonstrated. In addition to adding a quality control check at the first treatment, the elimination of the routine first fraction couch override is expected to increase patient safety by combating override desensitization.

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