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Updated: Dec 7, 2025

Evaluating Regional Pulmonary Deposition using Patient-Specific 3D Printed Lung Models
Published on: November 11, 2020
An analytical expression for R50% dependent on PTV surface area and volume: a lung SBRT comparison
Dharmin D Desai1, E L Johnson2, Ivan L Cordrey1
1Department of Radiation Oncology, CHI Memorial Hospital, Chattanooga, TN, USA.
Abstract:
In stereotactic body radiation therapy (SBRT), R50% is a common metric for intermediate dose spill and is defined in RTOG 0915 as the ratio of 50% isodose cloud volume (IDC50%) to the planning target volume (PTV). By coupling sound physical principles with the basic definition of intermediate dose spill, we derive an exact analytical expression for R50% for the case of a spherical volume. This expression for R50% depends on three quantities: the surface area of PTV (SAPTV ), the volume of PTV (VPTV ), and the dose gradient Δr. Validity of our analytical expression for R50% was confirmed via direct comparison to peer-reviewed, multi-institutional, diverse clinical data. The comparison of our R50% values computed from our analytical expression to the clinical data yielded an average percent difference of 3.8 ± 4.5%.
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