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Lung Density Threshold Analysis for Clinical Acceptability When Using Homogeneous Dose Calculation for Lung SBRT Patients

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M Sands

M Sands*, M Petronek , T Zhuang , A Magnelli , P Qi , K Stephans , G Videtic , S Balik , The Cleveland Clinic Foundation, Cleveland, OH


SU-I-GPD-T-596 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Previous studies have shown that a homogenous dose calculation in SBRT lung planning results in a statistically significant difference in prescription dose coverage to the plan tumor volume (PTV) for patients with low lung density. This study aims to define a lung density threshold at which homogenous tissue dose calculation methods result in clinically unacceptable plans.

Methods: A group of 24 patients previously treated with lung SBRT were selected and a plan was developed using two coplanar VMAT arcs prescribed to 60 Gy in 3 fractions. The dose calculation included a density override to equal to 1 g/cm³ for all internal organs. The resulting clinically acceptable plan met several organ at risk (OAR) dose limits and the following planning goals: PTV V60Gy=95%, PTV V54Gy=99%, ITV V60Gy=100%. Monitor units (MU) were held constant and the plan was recalculated with heterogeneous tissue densities and a lung density override that varied from 0.10-0.45 g/cm³ in increments of 0.05 g/cm³ covering the original lung density spread of the patient group.

Results: A Pearson correlation value was calculated between the lung density and the PTV V60Gy resulting in a strong correlation value equal to 0.72. Additionally, a moderate correlation value of 0.47 was observed between the PTV V54Gy and lung density. Greater than 50% of patients deviated from planning goals at lung densities of 0.20 g/cm³ for PTV V60Gy, 0.15 g/cm³ for PTV V54Gy, and 0.10 g/cm³ ITV V60Gy.

Conclusion: Patients with lung densities <0.2 g/cm³ should be considered in a warning zone and are at moderate risk of losing coverage to PTV V60Gy coverage; while, patients with lung densities ≤0.15 g/cm³ are at high risk of losing coverage to PTV V60Gy, PTV V54Gy, and ITV V60Gy and homogenous dose calculations should be avoided.

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