A small tolerance for catheter displacement in high-dose rate prostate brachytherapy is necessary and feasible.

PURPOSE We examined catheter displacement in patients treated with fractionated high-dose rate (HDR) brachytherapy boost for prostate cancer and the impact this had on tumor control probability (TCP). These data were used to make conclusions on an acceptable amount of displacement. METHODS AND MATERIALS The last 20 patients treated with HDR brachytherapy boost for prostate cancer at our center in 2007 were replanned using simulated interstitial catheter displacements of 3, 6, 9, and 12 mm with originally planned dwell times. The computer-modeled dose-volume histograms for the clinical target volumes were exported and used to calculate the TCP of plans with displaced needles relative to the original plan. Actual catheter displacements were also measured before and after manual adjustment in all patients treated in 2007. RESULTS In the 20 patients who were replanned for caudal catheter displacements of 3, 6, 9, and 12 mm, the median relative TCP was 0.998, 0.964, 0.797, and 0.265, respectively (p < 0.01 when all medians were compared). All patients replanned with a 3-mm displacement, compared with only 75% with a 6-mm displacement, had a relative TCP greater than 0.950. In the 91 patients treated in 2007, before adjustment, 82.3% of fractions had a displacement greater than 3 mm compared with 12.2% of fractions after adjustment. CONCLUSIONS Catheter displacement in HDR brachytherapy significantly compromises the TCP. The tolerance for these movements should be small (< or =3 mm). Correcting these displacements to within acceptable limits is feasible.