18F-NaF PET/CT-directed dose escalation in stereotactic body radiotherapy for spine oligometastases from prostate cancer

Abstract

Purpose: To investigate the technical feasibility of SBRT dose painting using 18F-NaF positron emission tomography (PET) scans guidance in patients with spine oligometastases from prostate cancer. Materials/methods: Six patients with 15 spine oligometastatic lesions from prostate cancer who had18F-NaF PET/CT scan prior to treatment were retrospectively included. GTVreg was delineated according to the regular tumor boundary shown on PET and/or CT images; and GTVMATV was contoured based on a net metabolically active tumor volume (MATV) defined by 60% of the SUVmax values on18F-NaF PET images. The PTVs (PTVreg and PTVMATV) were defined as respective GTVs (plus involved entire vertebral body for PTVreg) with a 3-mm isotropic expansion margin. Three 1- fraction SBRT plans using VMAT technique along with 10 MV flattened filter free (FFF) beams (Plan24Gy, Plan24-27Gy, and Plan24-30Gy) were generated for each patient. All plans included a dose of 24 Gy prescribed to PTVreg. The Plan24-27Gy and Plan24-30Gy also included a simultaneous boost dose of 27 Gy or 30 Gy prescribed to the PTVMATV, respectively. The feasibility of18F-NaF PET-guided SBRT dose escalation was evaluated by its ability to achieve 100% of the prescription dose to cover at least 90% of the PTV volume while adhering to organ-atrisk (OAR) dose constraints. Results: In all 33 SBRT plans generated, the planning objectives and dose constraints were met without exception. Plan24-27Gy and Plan24-30Gy had a significantly higher dose in PTVMATV than Plan24Gy (p < 0.05), respectively, while maintaining a similar OAR sparing profile. Conclusion: Using VMAT with FFF beams to incorporate a simultaneous18F-NaF PET-guided radiation boost dose up to 30 Gy into a SBRT plan is technically feasible without violating normal tissue tolerances. The relationship between local control and normal tissue toxicity during18F-NaF PET-guided dose escalation in SBRT should be validated in clinical trials.