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Four gold seed markers (1 x 3 mm) were placed in the prostate (under sagittal ultrasound guidance transrectally without local or general anesthesia) 3 or more days prior to CT simulation. Patients were simulated supine without contrast and with ankles braced and a foam ring handholder (2.5 mm CT slice thickness). Skin marks were placed in the simulator suite and not changed over the course of treatment (no offline setup corrections were used).
For all 40 patients and 1019 fractions, the prostate was localized by ultrasound (US; SonArray, Varian Medical Systems) in lateral (left/right), vertical (anterior/posterior), and longitudinal (superior/inferior) dimensions. Values were recorded, but the patient was not shifted. Next, seed marker (SM) kV X-ray (ExacTrac X-ray 6D, BrainLAB AG) measurements of lateral, vertical, and longitudinal offsets were made. The patient was then shifted (based on SM coordinates) and treated.
Assuming initial setup to skin marks as the origin, the mean suggested US vs. SM unidimensional shifts (for all dimensions) were hypothesized to be similar and within 1 mm of the origin. The 3D distance discrepancy (Dus vs. sm) between suggested simultaneous SM and US shift points was calculated. We hypothesized a mean discrepancy >5 mm as clinically significant.