Customized Needle Guides for Inserting Non-Parallel Needle Arrangements in Prostate HDR Brachytherapy: A Phantom Study

Purpose: To evaluate the use of customized guides for inserting optimized, non-parallel arrangements of high-dose-rate (HDR) brachytherapy needles into prostate gelatin phantoms. Materials and Methods: Four prostate gelatin phantoms were constructed with prostate, urethra, bladder, rectum, penile bulb, and pubic arch anatomical structures. The target volume was the prostate, and the other structures were organs at risk (OAR). The anatomical structures were digitized, and a non-parallel needle arrangement was optimized for each phantom using Needle Planning by Integer Program (NPIP). The optimized needle arrangements contained 13-15 needles, which is less than the standard number currently used in our clinic. None of the optimized needles punctured any non-prostate structures. Inverse Planning by Integer Program (IPIP) was used to compute a dose plan for each optimized needle arrangement that conformed to the guidelines set in the Radiation Therapy Oncology Group (RTOG) 0321 dosimetric protocol. Customized needle guides were digitally created for each phantom to implant the corresponding optimized needle arrangement. The needle guides were 3D-printed in ABS plastic using a uPrint Plus 3D printer (StrataSys). The needle guides were mechanically registered to the phantoms. The needles were inserted into each phantom by a researcher without any clinical brachytherapy experience and without the assistance of trans-rectal ultrasound (TRUS) guidance. The phantoms and implanted needle arrangements were digitized, and another dose plan was computed for each implant. Compliance with the RTOG 0321 protocol, puncturing of OAR, and placement error were evaluated. Results: Three of the needle arrangements were implanted without puncturing any OAR. Due to placement error, one implanted needle arrangement punctured the penile bulb and urethra. We considered this case a failure, and we did not post-process it further. The successfully implanted needle arrangements met all the dose requirements in the RTOG 0321 protocol. The RMS placement error between the dwell positions in the planned and implanted needle arrangements was 3.6 mm, 4.5 mm, and 2.3 mm for the three cases. The majority of the error was along the needle insertion direction (i.e. the z-direction), which is most likely because templates do not restrict motion along this direction, and therefore is where error is the most user-dependent, or because the anatomy was digitized from 3 mm thick CT slices taken in the zdirection, and therefore registration in this direction is the most uncertain. Conclusions: Customized needle guides are a promising method for inserting optimized, non-parallel, brachytherapy needle arrangements. Although NPIP, IPIP, and 3D printers were used to construct the guides in this study, other planning and construction methods could be used without changing our fundamental workflow structure. However, the templatepatient registration and needle insertion workflow must still be improved to reduce placement error. This includes incorporating real-time TRUS guidance to monitor progress and avoid puncturing OAR, which we propose as future work.