SU-C-BRA-01: 18F-NaF PET/CT-Directed Dose Escalation in Stereotactic Body Radiotherapy for Spine Oligometastases From Prostate Cancer

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. Methods: As a proof of concept, six patients with 14 spine oligometastatic lesions from prostate cancer who had ¹⁸F-NaF PET/CT scan prior to treatment were retrospectively included. GTV_reg was delineated according to the regular tumor boundary shown on PET and/or CT images; and GTV_MATV was contoured based on a net metabolically active tumor volume (MATV) defined by 60% of the SUV_max values on ¹⁸F-NaF PET images. The PTVs (PTV_reg and PTV_MATV) were defined as respective GTVs (plus involved entire vertebral body for PTV_reg) with a 3-mm isotropic expansion margin. Three 1-fraction SBRT plans using VMAT technique along with 10 MV FFF beams (Plan_24Gy, Plan_24–27Gy, and Plan_24–30Gy) were generated for each patient. All plans included a dose of 24 Gy prescribed to PTV_reg. The Plan_24–27Gy and Plan_24–30Gy also included a simultaneous boost dose of 27 Gy or 30 Gy prescribed to the PTV_MATV, respectively. The feasibility of 18F-NaF PET-guided SBRT dose escalation was evaluated by its ability to achieve the prescription dose objectives while adhering to organ-at-risk (OAR) dose constraints. The normal tissue complication probabilities (NTCP) calculated by radiological models were also compared between the plans. Results: In all 33 SBRT plans generated, the planning objectives and dose constraints were met without exception. Plan_24–27Gy and Plan_24–30Gy had a significantly higher dose in PTV_MATV than Plan_24Gy (p < 0.05), respectively, while maintaining a similar OAR sparing profile and NTCP values. Conclusion: Using VMAT with FFF beams to incorporate a simultaneous ¹⁸F-NaF PET-guided radiation boost dose up to 30 Gy into a SBRT plan is technically feasible. The relationship between local control and normal tissue toxicity in SBRT dose painting should be validated in clinical trials