Abstract
P521
Introduction: The RefleXion X1® system is a hybrid PET imaging-radiotherapy system designed to deliver biology-guided radiotherapy (BgRT), a novel radiotherapy delivery modality that uses real-time positron emissions from a PET tracer to deliver radiotherapy beamlets. The purpose of this study was to evaluate the performance of the PET imaging subsystem for detection of signal as a foundation to evaluate [18F]-DCFPyL-based BgRT treatment delivery in patients with prostate cancer.
Methods: Patients with prostate cancer scheduled for a diagnostic [18F]-DCFPyL PSMA PET scan as part of standard of care were eligible for this study. Upon completion of the diagnostic PSMA PET scan on a non-RefleXion PET system, images were transferred to the radiotherapy planning system for target identification and delineation. 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.
Results: Twenty-one patients underwent from 9/2/2022 to 12/22/2022 (12 with rising PSA after surgery or radiotherapy, 6 with known metastatic disease and 3 with newly diagnosed high-risk prostate cancer). Median (range) PSA was 2.30 (0.04-122). A PET avid tumor was identified in 15 patients (4 in lymph nodes, 5 in bone, 4 in the prostate gland, and 1 in the prostate bed) who were then scanned on the RefleXion X1 system. BgRT planning was feasible and met standard of care stereotactic body radiotherapy (SBRT) organ dose constraints in 7 patients (Figure 1). BgRT planning was not feasible in one patient due to proximity of the target lesion to the bladder and in seven patients due to insufficient activity in the target volume. For patients where BgRT planning was successful, median (range) target volume, AC and NTS were 3.4 cc (0.5-50.9 ), 14.2 kBq/ml (5.5-48.0) and 14.0 (3.9-33.9), respectively, compared to 0.7 cc (0.1-2.5), 1.9 kBq/ml (1.1-13.1) and 2.7 (2.0-3.3), respectively, in patients where BgRT planning was not feasible. Given the distance to travel from the diagnostic PET scanner to the X1 unit, the median (range) time from [18F]-DCFPyL injection to X1 scan was 165 (142-218) minutes. Reducing this interval would have resulted in higher target lesion AC and NTS, and potentially successful BgRT plans in a greater number of patients.
Conclusions: This is the first study to prospectively investigate the feasibility of using PET imaging for BgRT plan generation on the RefleXion X1 system in patients with prostate cancer. Lesions that are relevant to radiotherapy of prostate cancer can be well visualized in various locations including lymph nodes and bones, the most common sites of metastasis. [18F]-DCFPyL PET signal intensity (AC) and contrast (NTS) of the lesions are generally superior to those imaged with FDG. 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 that are close to the urinary bladder may make BgRT planning challenging. [18F]-DCFPyL-guided BgRT is technically feasible using RefleXion X1. BgRT using targeted PET radiopharmaceuticals to biologically guide external beam radiotherapy represents a promising new dimension in nuclear oncology and warrants further investigation.
This study was supported by funding from RefleXion, Inc., Hayward, CA 94545
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