Purpose: Stereotactic body radiotherapy is now the standard of care for management of early stage lung cancer; however, for exceptionally large or ultra-central tumors a less aggressive hypo-fractionated approach (60Gy, 15 fractions) is available. We sought to identify patients previously treated with hypo-fractionated volumetric arc photon radiotherapy (VMAT) that may benefit from proton therapy and identify what organ-at-risk (OAR) dose reductions could be statistically significant.
Methods: Ten non-small cell lung cancer patients with either large (PTV largest diameter > 5cm) and/or ultra-cental (7 patients) treatment volumes treated with hypo-fractionated (60Gy, 15 fractions) VMAT were re-planned using 3 to 5 field pencil beam scanning (PBS) proton therapy using the same fractionation. The iGTV was robustly optimized (multi-field) using 3.5% range uncertainty and 5mm isocenter shifts. The PTV was nominally optimized and field-specific targets were generated for spot placement. Planning was performed on the average CT and forward calculated on the exhale and inhale phase CTs to check coverage stability. Statistical analysis was completed using a 2-sided paired t-test for each OAR.
Results: The spinal cord (D5cc), esophagus (D0.03cc, D5cc), trachea (D10cc), bronchus (D10cc), and Lungs-iGTV (mean) dose differences were all found to be statistically significant (p<0.05) showing improvement with protons. All patients also had reduced mean doses to heart, great vessels and ipsilateral lung. Five patients had not met the chest wall constraint for the clinical plan due to PTV overlap and the proton plan was able to significantly (p=0.03) reduce the dose (D50.00cc) to the chest wall for these patients (average reduction 3.3Gy). The proton plans were able to meet conformity index and gradient measure metrics and coverage remained stable on phase CTs.
Conclusion: PBS proton therapy could improve dose metrics for patients whose anatomy is unfavorable to meet hypo-fractionated planning constraints using VMAT, particularly those near the chest wall.