Comparison of Proton Versus Photon SBRT for Treatment of Spinal Metastases Using Variable RBE Models

Area of Science:

• Radiation Oncology
• Medical Physics
• Oncology

Background:

• Proton stereotactic body radiation therapy (SBRT) for spinal metastasis is under investigation due to concerns about spinal cord injury.
• The relative biological effectiveness (RBE) of protons is variable and influenced by multiple factors, challenging clinical application.
• Current clinical practice often uses a fixed RBE of 1.1, which may not accurately reflect the biological impact of proton therapy.

Purpose of the Study:

• To compare the dosimetric plan quality of intensity-modulated proton therapy (IMPT) versus photon robotic SBRT for spinal metastases.
• To evaluate the impact of variable RBE models on dose distribution and organ-at-risk (OAR) sparing in proton SBRT.
• To assess the potential for improved target coverage and OAR sparing with proton therapy in complex spinal metastasis cases.

Main Methods:

• A dosimetric analysis was performed on nine patients with complex spinal metastases treated with photon SBRT.
• Intensity-modulated proton therapy (IMPT) plans were generated at 30 Gy(RBE) and dose-escalated to 45 Gy(RBE) using a fixed RBE of 1.1.
• Variable RBE models (Carabe, McNamara, Wedenberg) were applied to calculate RBE-weighted dose (D_RBE) distributions, considering LET and dose per fraction.

Main Results:

• IMPT significantly improved planning target volume (PTV) coverage compared to photon SBRT at both 30 Gy(RBE) and 45 Gy(RBE) prescriptions.
• IMPT resulted in significantly lower maximum spinal cord doses compared to photon SBRT.
• Variable RBE models indicated a significant difference in spinal cord expansion dose compared to fixed RBE models, highlighting the importance of RBE variability.

Conclusions:

• Proton SBRT, particularly IMPT, offers potential advantages in target coverage and OAR sparing for spinal metastases compared to photon SBRT.
• The use of variable RBE models is essential for accurate assessment of proton therapy dose, as fixed RBE can underestimate OAR dose.
• Further prospective studies are warranted to confirm these dosimetric findings and establish the clinical efficacy and safety of proton SBRT for spinal metastases.