Potential impact of 68Ga-PSMA-11 PET/CT on prostate cancer definitive radiation therapy planning

Background: Standard-of-care imaging for initial staging of prostate cancer (PCa) underestimates disease burden. Prostate specific membrane antigen (PSMA) positron emission tomography/ computed tomography (PET/CT) detects PCa metastasis with superior accuracy with potential impact definitive radiation therapy (RT) planning for non-metastatic PCa. Objectives: i) To determine how often definitive PCa RT planning based on standard target volumes cover 68Ga-PSMA-11 PET/CT defined disease, and ii) To assess the potential impact of 68Ga-PSMA-11 PET/CT on definitive PCa RT planning. Patients and Methods: This is a post-hoc analysis of an intention to treat population of 73 patients with localized PCa without prior local therapy who underwent 68Ga-PSMA PET/CT for initial staging as part of an Investigational New Drug trial. 11/73 were intermediate-risk (15%), 33/73 were high-risk (45%), 22/73 were very high risk (30%), and 7/73 were N1 (9.5%). Clinical target volumes (CTVs) that included the prostate, seminal vesicles, and pelvic lymph nodes (LNs) using Radiation Therapy Oncology Group (RTOG) consensus guidelines were contoured on the CT portion of the PET/CT by a radiation oncologist blinded to the PET findings. 68Ga-PSMA-11 PET/CT images were analyzed by a nuclear medicine physician. PSMA-positive lesions not covered by planning volumes based on the CTVs were considered to have a major potential impact on treatment planning. Results: All patients had PSMA-positive primary prostate lesion(s). 25/73 (34%) and 7/73 (9.5%) had PSMA-positive pelvic nodal and distant metastases, respectively. The sites of nodal metastases in decreasing order of frequency were external iliac (20.5%), common iliac (13.5%), internal iliac (12.5%) obturator (12.5%), perirectal (4%), abdominal (4%), upper-diaphragm (4%), and presacral (1.5%). The median size of the nodal lesions was 6 mm (range 4-24 mm). RT planning based on the CTVs covered 69/73 (94.5%) of primary disease and 20/25 (80%) of pelvic nodal disease, on a per-patient analysis. Conclusion:68Ga-PSMA-11 PET/CT had a major impact on intended definitive PCa RT planning in 12/73 of patients (16.5%) when RT fields covered the prostate, seminal vesicles and the pelvic LNs, and in 25/66 of patients (37%) when RT fields covered only the prostate and seminal vesicles (without pelvic LNs).

Journal of nuclear medicine : official publication, Society of Nuclear Medicine. 2018 Apr 13 [Epub ahead of print]

Jérémie Calais, Amar U Kishan, Minsong Cao, Wolfgang P Fendler, Matthias Eiber, Ken Herrmann, Francesco Ceci, Robert E Reiter, Rettig B Matthew, John V Hegde, Narek Shaverdian, Christopher R King, Michael L Steinberg, Johannes Czernin, Nicholas G Nickols

Department of Molecular and Medical Pharmacology, UCLA, United States., Department of Radiation Oncology, UCLA, United States., Ahmanson Translational Imaging Division, Department of Molecular and Medical Pharmacology, UCLA., Department of Urology, UCLA, United States., Department of Radiation Oncology, VA Greater Los Angeles Healthcare System, United States.