Author: David Chighvinadze, Indra J. Das, Jeremy D. Donaghue, Kevin Stephans, Gregory Videtic, Sugandima Weragoda, Ping Xia 👨🔬
Affiliation: Cleveland Clinic, Cleveland Clinic Sandusky, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Cleveland Clinic Fairview 🌍
Purpose:: Evaluate the impact of lung density variations on plan quality and delivery parameters in lung SBRT Plans.
Methods: For a hypothetical case with a real patient anatomy, three GTVs with volumes 0.6cm3, 3.7cm3,13.4cm3 (diameters 1cm, 2cm, 3cm respectively), were created and density overridden to 1g/cm3. Corresponding Lung-GTV densities were overridden to 0.05, 0.1, 0.2, 0.3g/cm3 typically found in lungs. PTVs were generated as a 5mm expansion from corresponding GTVs. SBRT plans were created with 6 MV FFF and 10 FFF beams, resulting in 12 plans using Pinnacle treatment planning system (TPS). All were normalized to achieve 95% of PTV receiving prescription (Rx) dose of 54 Gy in three fractions. Rx iso-dose line (IDL) (normalized to maximum pixel dose of plan), conformal index (CI=100%Rx volume/PTV), gradient index (R50=50% of Rx dose volume/PTV), total monitor units (MUs)/fraction were recorded for each plan.
Results: As density increased IDL increased from 59.9±3.1% to 75.6±0.5% for 6FFF and from 62.7±1.4% to 74.1±0.3% for 10FFF independent of GTV size. The plan uniformity increased with density and CIs approached 1 for all 6FFF plans except for very low density of 0.05 g/cm3. This was more pronounced for 10FFF at 0.05g/cm3, indicating that 10FFF is not best suited for this density. With increasing density, R50 decreased and approached 4 (from 7.7±1.2 to 3.9±0.3 for 6FFF and from 8.1±2.1 to 4.4±0.6 for 10FFF). For smallest GTV, R50 for 10FFF were worse than for 6FFF (7.7 vs. 8.1). With increasing density, MUs/fraction decreased from 6063.3±242.5 to 4167±173 for 6FFF and from 5034.7±164.6 to 3583.7±118.3 for 10FFF.
Conclusion: Lung density variations can affect SBRT plan quality and increase MUs/fraction. For very low lung density, 6FFF beam is recommended. For higher density, 10FFF beam can achieve similar plan conformity, and could reduce treatment time for breath-hold treatments.