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Dosimetric Comparison of Biologically-Guided Radiotherapy and X-Ray-Guided Stereotactic Ablative Radiotherapy for Oligometastatic Prostate Cancer

WT Hrinivich1*, R Phillips1 , AJ Da Silva2 , N Radwan1 , MA Gorin3 , SP Rowe4 , KJ Pienta3 , MG Pomper4 , J Wong1 , P Tran1 , K Wang1 , (1) Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, (2) RefleXion Medical, Inc., Hayward CA, (3) Department of Urology, Johns Hopkins University School of Medicine, Baltimore, MD, (4) Department of Radiology, Johns Hopkins University School of Medicine, Baltimore, MD


(Saturday, 3/30/2019)  

Room: Exhibit Hall

Purpose: Prostate-specific membrane antigen (PSMA)-targeted PET tracers have improved early detection of oligometastatic (OM) prostate cancer. Evidence suggests that stereotactic ablative radiotherapy (SABR) for OM prostate cancer may improve clinical outcomes, but localization of multiple small lesions for treatment is challenging with conventional X-ray-based imaging systems, limiting deliverable doses. Biologically-guided radiotherapy (BgRT) is being developed to utilize strong contrast of PET signal for real-time intra-fraction tracking of multiple targets. This study investigates PSMA-directed BgRT using a cohort from our Phase II randomized trial of SABR to men with recurrent hormone sensitive OM prostate cancer.

Methods: Fifteen patients from our trial treated with SABR and imaged with PSMA PET-CT were used to compare clinical SABR plans versus BgRT plans generated from a prototype treatment planning system (TPS). The TPS models a 6MV linear accelerator mounted on a ring gantry that includes a PET detector and high-speed binary multi-leaf collimator (MLC). The PET signal is used as fiducial to indicate target location. Photon fluence is thus optimized as a function of PET activity within a user-defined “firing zone�. Clinical SABR and BgRT plans were compared in terms of maximum PTV dose (Dmax), mean dose to proximal organs at risk (DOAR), conformity index, gradient index, and the correlation of Dmax with maximum PET activity.

Results: Clinical SABR and BgRT plans resulted in mean±standard deviation Dmax of 128±11% and 150±13% (p<0.001), DOAR of 11±8% and 10±8% (p=0.02), conformity indices of 0.74±0.08 and 0.72±0.08 (p=0.4), and gradient indices of 4.47±0.63 and 5.40±0.83 (p=0.003), respectively. Dmax and maximum PET activity had correlation coefficients of -0.15 (p=0.6) and 0.59 (p=0.02) for clinical SABR and BgRT.

Conclusion: BgRT plans resulted in increased PTV dose and decreased OAR dose while incorporating underlying PET activity to enable real-time biological guidance, demonstrating the feasibility of PSMA-directed BgRT for OM prostate cancer.

Funding Support, Disclosures, and Conflict of Interest: This work was supported in part by RefleXion Medical.


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