Impact of Inverse Optimization Settings on Plan Quality In High Dose Rate Prostate Brachytherapy
Abstract
Purpose
This study quantifies how two inverse optimization-based planning parameters: dwell time deviation constraint (DTDC) and active step size (AS) affect target coverage, dose heterogeneity, and organ‑at‑risk (OAR) doses in prostate high dose rate (HDR) brachytherapy planning. Characterizing sensitivity to DTDC and AS is essential for improving plan reproducibility while maintaining consistent target coverage and meeting OAR constraints.
Methods
Five prostate HDR monotherapy cases (prescription 13.5 Gy) were replanned using inverse optimization-based planning technique while keeping all clinical optimization objectives and structures constant. A baseline plan used default setting (DTDC=0.2, AS=1mm). Parameter sweeps were performed one-at‑a‑time: DTDC = 0.1, 0.3, 0.4, 0.5, 0.8 (AS fixed at 1mm), and AS = 2–5mm with step size 1 (DTDC fixed at 0.2). Dose volume histograms were exported and analyzed with an in-house Python script. Target metrics included prostate D90 (%Rx), V100 (%), and V150 (%). OAR endpoints included bladder/rectum/urethra D2cc and D0.1cc (Gy).
Results
Cohort mean metrics (n=5) showed sensitivity to both parameters relative to the default plan (DTDC=0.2, AS=1mm). Increasing DTDC to 0.8 reduced bladder D2cc from 7.86 to 6.69 Gy and urethra D2cc from 9.14 to 7.86 Gy but decreased prostate coverage (D90: 110.4%Rx to 102.7%Rx; V100: 98.1% to 91.9%). Increasing AS from 1mm to 2–5mm reduced rectum D2cc from 7.58 to 7.02–7.15 Gy and prostate V150 from 31.6% to 23.5–25.3%, with modest reductions in D90 (105.8–106.8%Rx) and V100 (95.8–97.2%).
Conclusion
DTDC and active step size meaningfully alter target coverage, dose heterogeneity, and OAR dose in prostate HDR planning. Higher DTDC improved bladder/urethra sparing at the expense of coverage, while AS≥2mm consistently reduced V150 and rectal dose with comparatively small losses in D90/V100. By tuning DTDC and AS, users can tailor target coverage and OAR doses to institution‑specific planning standards, improving plan quality and reproducibility.