SU-H4-GePD-T-06: Evaluation of Dose Uncertainty for in Trans-Rectal Ultrasound (TRUS) -Based High-Dose-Rate Prostate Brachytherapy

Purpose: In TRUS-based high-dose-rate prostate brachytherapy, the accurate identification of needle-tip position is critical. However, needle-tip identification can be subject to large uncertainty and errors because of ultrasound image quality and imaging artifacts. Purpose of this study is to evaluate the dose uncertainty from the positon error and try to find a solution. Methods: Needle-tip position calculated from the physically measured residual needle length is a helpful guidance. But it can also lead to a systematic error if the tip position of reference needle(s) was not identified correctly, and additional uncertainties will happen when needles bend significantly. To evaluate the dosimetric impact, we analyzed 40 cases treated recently. Three set of data were used in the calculation: positions identified directly from the ultrasound (data1), needle-tip positions calculated using the residual length (data2), and average of the two plus a random uncertainty between 0–5 mm (data3). Treatment plans were generated using data2 and then recalculated for other two data sets. Results: The average tip position difference between data1 and data2 was 3.6 mm while maximum values can be over 10 mm. The average prostate V100% for original plans was 97.3%, urethra V125% was 0.31 cc, rectum V75% was 0.24 cc and bladder V75% was 0.13 cc. The prostate V100% dropped to 96.8% after recalculated using data1, the number further dropped to 95.4% using data3. New plans were generated to cover a region of prostate plus 2 mm expansion along needle direction. The average prostate V100% was 97.6% while it only drops to 97.3% and 97.1% by using data 1 and data3 in this approach. Conclusion: The needle-tip position errors can have a considerable dose impact on prostate dose coverage. A small margin along the needle direction can significantly reduce the dose coverage uncertainty while still maintain low dose level to critical structures.