Pilot Study of a Novel Ring Gantry-Based PET/CT Linear Accelerator in Patients with Prostate Cancer Receiving [18F]-DCFPyL for PSMA PET Imaging.

Abstract

The RefleXion X1® system is a hybrid PET imaging-radiotherapy system that uses real-time positron emissions from a PET tracer to deliver biologically guided radiotherapy (BgRT). This study (NCT05470699) evaluated the hypothesis that the X1 PET imaging subsystem would be able to detect [18F]-DCFPyL PSMA PET signal sufficient to generate a deliverable BgRT plan in patients with prostate cancer. Patients with prostate cancer scheduled for a diagnostic [18F]-DCFPyL PSMA PET scan as part of standard of care were eligible. Upon completion of the diagnostic PSMA PET scan, images were transferred to the radiotherapy planning system for target identification and contouring. If at least one PET avid tumor lesion was identified, the patient was then scanned on the X1 unit. BgRT planning was performed on each X1 scanned patient. The target lesion volume, activity concentration (AC) and normalized target signal (NTS) were acquired. Successful and deliverable BgRT plans required that the target AC was ≥ 5 kBq/ml and NTS ≥ 2.7. Twenty-six patients underwent [18F]-DCFPyL PET scans (13 with rising PSA after surgery or radiotherapy, 6 with known metastases and 7 with newly diagnosed high-risk prostate cancer). Median (range) PSA was 3.40 (0.04-122). In 16 patients a PET avid tumor was identified and contoured for planning (4 lymph nodes, 5 bone, 6 prostate gland, and 1 prostate bed). In 13 patients the target lesion was visualized on the X1 PET scan, while in 3 patients the target lesion was too close to the bladder to be clearly visualized. BgRT planning was feasible and met standard of care published SBRT organ dose constraints in 8 patients (3 prostate gland, 3 bone, 2 lymph nodes). BgRT planning was not feasible in 8 patients due to insufficient AC, low NTS or proximity of the target lesion to the PET avid bladder. The accompanying table compares median (range) target volume, AC and NTS for feasible versus not feasible plans. This is the first study to investigate the feasibility of using [18F]-DCFPyL PET imaging for BgRT plan generation on the X1 system in patients with prostate cancer. Lesions that are relevant to radiotherapy of prostate cancer can be visualized including lymph node and bone metastases. A dedicated BgRT workflow with PSMA PET imaging on the X1 at 60 minutes post injection will result in higher target AC and will optimize BgRT planning. PET avid lesions < 1 cm or close to the bladder may make BgRT planning challenging. [18F]-DCFPyL-guided BgRT is technically feasible using the RefleXion X1. BgRT using targeted PET radiopharmaceuticals to biologically guide external beam radiotherapy represents a promising new dimension in radiation oncology and warrants further investigation.