1064 - Assessment of Image Guidance and Adaptive Planning on the ART of SABR Phase II Trial for Near "Margin-Less" Adaptive Radiation Therapy of Primary Localized Prostate Cancer

Purpose/Objective(s):

Ultra-hypofractionated stereotactic ablative body radiotherapy (SABR) is a method that aims to deliver prostate radiation with greatly reduced margins to avoid unnecessary toxicity, reduce number of treatments, and improve cost effectiveness. Daily online Adaptive Radiotherapy (ART) driven by Varian’s Ethos system offers an opportunity for margin reduction and accommodation of inter-fraction motion by fractional target and organ at risk (OAR) contouring. This study investigates the efficacy of online adaptive treatment on the 2-fraction SABR trial using a Hypersight capable Varian Ethos system.

Materials/Methods:

Men treated with adaptive SABR on protocol for low- to intermediate-risk prostate adenocarcinoma (ECOG = 2) with 27 Gy in two fractions 72-hours apart were included. Each patient had a hydrogel spacer placed with optional carbon fiducial markers =1 day prior to treatment. PTV margins were defined as a CTV expansion of 2mm anteriorly & laterally, and 1mm posteriorly and online ART was delivered on a Varian Ethos platform equipped with Hypersight. OARs and targets were recontoured based on daily CBCT imaging, and dose re-calculated to decide whether to treat with the scheduled or adapted plan. Target coverage was assessed with D95% (%) and V98% (%) metrics for the PTV between scheduled and adapted plans, and bladder, large & small bowel, and rectum metrics were considered for OAR analysis. The time from planning CBCT acquisition to first verification CBCT (t1) and from first to second verification scans (t2) were recorded, alongside total fraction treatment time (T). Shifts applied following first and second verification scans were recorded in vertical (vrt), longitudinal (lng), and latitudinal (lat) directions.

Results:

Adapted plans were chosen for 87% of fractions, with an overall mean improvement in PTV coverage from fractions one to two of 3.7% to 6.5% for D95% (%), and 4.7% to 7.7% for V98% (%). For adapted cases OAR metrics met constraints but increased on average between both fractions as a trade-off for target coverage: for the small bowel D0.03cc < 24 Gy increased by 0.7% from first to second fraction, while large bowel D0.03cc < 27 Gy increased by 9.6%. For patients that did not require mid-treatment intervention, the average t1, t2, and T times were 18.8, 7.0, and 31.6 mins respectively, with median vrt, lng, lat shifts of 0.11, 0.04, 0.02 mm for first verification, and 0.01, 0.01, 0.00 mm for second verification. For patients that required bladder or rectum intervention, the average total fraction time was 37.3 mins.

Conclusion:

These preliminary data show that a 2-1 mm margin prostate ART SBRT treatment can successfully increase target coverage with minimal mid-treatment shifts, and demonstrate the feasibility of the Varian Ethos system for 2-fraction, reduced margin treatment capable of managing inter- and intra-fractional motion. Further work will focus on evaluating the clinical outcomes of this treatment.