Purpose: Inverse planning is gaining increased popularity in high-dose-rate (HDR) brachytherapy due to the superior target volume coverage and normal tissue sparing achievable with inverse optimization. However, inverse planning often results in large variation in needle dwell times between adjacent dwell positions, which may increase the plan’s susceptibility to interfractional needle displacement. This study is aimed to: 1) propose a robust optimization technique that increases dwell time uniformity between neighboring dwell positions; and 2) demonstrate the robustness of generated plans to needle displacement.
Methods: Nine gynecological HDR cases using the Syed-Neblett template were investigated. Clinical robust plans were created in Oncentra® Brachy using the robust planning technique, which involved forward graphical optimization followed by a manual post-hoc uniformity correction. Inverse plans were retrospectively generated using the hybrid inverse planning optimization (HIPO) module. Systematic or random needle displacements were induced on six needles adjacent to the bladder and rectum whose motion tend to induce needle displacement. Then plan doses were recalculated and dose-volume histogram (DVH) metrics, including the high-risk clinical target volume (HR-CTV, n=9) and gross tumor volume (GTV, n=4) D90, V100, V150, as well as bladder and rectum D2cc, were compared against those at baseline.
Results: Comparable target and organ-at-risk (OAR) DVH metrics were observed in the HIPO and robust plans at baseline. Both systematic and random needle displacement decreased dose to the target and OARs. However, with systematic needle displacement, the robust plans provided 4.1% improvement in GTV D90, as compared to the HIPO plans (P<0.05). Similarly, the robust plans also showed 3.1% improvement (P=0.05) in GTV D90 with random needle displacement.
Conclusion: A robust planning technique for HDR brachytherapy is proposed. Compared to inverse planning, it offered more robust GTV coverage when interfractional needle displacement occurs. Further studies are needed to evaluate its clinical benefits to gynecological patients.