Dosimetric Challenges of Small Lung Lesions in Low-Density Tissue Treated with Stereotactic Body Radiation Therapy

Background/Objectives: Stereotactic body radiation therapy (SBRT) is widely used for small lung tumors, but the physics of electron transport in low-density lungs remains incompletely understood. This study quantifies the effect of lung density on dosimetry for small lesions. Methods: To study the dosimetric parameters a pseudo patient option was chosen. A lung SBRT patient with a central lesion was modeled in the Eclipse treatment planning system using the AAA algorithm. Three target sizes (1.0, 1.5, and 2.0 cm) were planned with lung densities overridden from 0.1 to 1.0 g/cm³. Standard SBRT constraints were applied, and dosimetry indices (CI, HI, GI), maximum dose, and MU/Gy were recorded to see the pattern. Results: Dose-volume histograms (DVHs) showed marked dependence on both lesion size and lung density. Lower densities produced higher maximum doses (up to 135% at 0.1 g/cm³), steeper DVH tails, and significantly increased MU/Gy. Conformity was achievable in all cases, but at the cost of degraded homogeneity and gradient indices. At higher density (1.0 g/cm³), maximum dose values fell to 108-110% which is typical in non-lung cases. Conclusions: SBRT planning in low-density lungs requires substantially higher MU and results in greater dose spillage despite acceptable conformity. These findings highlight the importance of considering density effects when comparing clinical outcomes across institutions and selecting optimal plans, where minimizing MU/Gy may reduce unnecessary dose burden.