Purpose: Simultaneous integrated boost (SIB) to a Dominant Intra-prostatic Lesion (DIL) presents unique planning challenges over more traditional prostate radiotherapy, particularly since the boost dose is well in excess of known dose limits for critical organs-at-risk (OARs). The purpose of this work was to develop and validate a Knowledge-Based Planning (KBP) approach for prostate+DIL radiotherapy despite the high degree of DIL target variability, DIL/prostate prescription dose ratios, and low sample size.
Methods: Using a commercial KBP system (RapidPlan, Varian Medical Systems), a DVH estimation model was generated using 28 previously-treated prostate+DIL plans, all using VMAT technique with a prescription dose ranging from 56-78 Gy to the prostate and 61.2-89.7 Gy to the DIL in 20-39 fractions. DIL/prostate prescription ratios ranged from 1.04-1.57 in the available sample. The resultant prostate+DIL KBP routine was open-loop validated with seven independent plans, using a single optimization sequence without manual intervention. Validation plans were normalized to match prostate PTV coverage (V100%≥95%). Per institutional standards for plan quality evaluation, target D_max was evaluated as well as OAR dose-volume histogram metrics: rectum V90% and V75%, bladder V90% and V75% bladder, and penile bulb Dmean. To establish plan quality equivalence, the absolute difference between KBP and clinical dose metrics (|ΔAₓ|=A(KBP)-A(CP)) were quantified. To assess plan complexity, total monitor units (MUs) were compared.
Results: D_max within the DIL target was lower for 5/7 KBP plans, with overall |ΔDmax| range of 0.4-5.4%. For OARs, rectum |ΔV90%| range was [0.1-1.1%]; rectum |ΔV75%| range=[0.3-2.3%]; bladder |ΔV90%| range=[0.2-2.2%]; bladder |ΔV75%| range=[0.2-1.3%]. The penile bulb |∆Dmean| range was [0.3-9.5%] with one KBP cases having nearly 10% lower Dmean. MUs were lower for all KBP plans, with an average MU reduction of -7%[-15%,0%].
Conclusion: KBP for prostate+DIL radiotherapy is a feasible autoplanning option in radiotherapy with equivalent plan quality observed in this study.
Funding Support, Disclosures, and Conflict of Interest: Dr. Kevin Moore: Consulting fees and honoraria from Varian Medical Systems.