Dwell position inaccuracy in the Varian GammaMed HDR ring applicator.

Varian has issued two Product Notification Letters warning of known inaccuracies in dwell positions for their GammaMed HDR ring applicator. This inaccuracy was measured for two sets of applicators. Autoexposed radiographs were taken of the HDR source at different dwell positions and analyzed per Varian recommendations using tools within the BrachyVision treatment planning program. Comparison between programmed and actual dwell positions showed the actual positions shifted distally by an average of 0.34 cm (0.17 cm-0.59 cm) across all positions in all rings. A correction method was developed and tested. During planning, the tip of the ring was extrapolated distally beyond its actual position in the patient image set and a proximal offset of the same distance was applied to the dwell positions. A global shift of 0.3 mm corrected all but the most proximal actual dwell position to within +2 mm of the planned position.