A geometry‐based scaling model for stereotactic treatment volume definition in stereotactic centralized ablative radiation therapy

Abstract Background Stereotactic Centralized Ablative Radiation Therapy (SCART) is a spatially fractionated radiotherapy strategy designed for large or bulky tumors by delivering an intensified ablative dose to a centrally located subvolume while allowing controlled dose reduction toward the tumor periphery. However, clinical implementation of SCART critically depends on reproducible definition of the stereotactic treatment volume (STV), which currently lacks a standardized geometric framework. Purpose To develop and preliminarily evaluate a geometry‐based scaling model for STV definition based on internal dose fall‐off characteristics under stereotactic volumetric modulated arc therapy (VMAT) delivery. Methods A phantom‐based stereotactic VMAT planning study was performed using idealized cylindrical STVs with radii of 1–3 cm and axial lengths of 3–10 cm. VMAT plans were generated using prescription doses of 15–24 Gy per fraction over three fractions. Internal dose fall‐off was quantified using the equivalent radius ( r 15 ) of the 15 Gy isodose surface, defined as the radius of a volume‐equivalent cylindrical isodose distribution. The predicted STV radius ( d ) was subsequently modeled from outer reference target geometry using dose‐specific linear regression analysis. Results The extracted 15 Gy isodose surface demonstrated reproducible geometry‐associated attenuation behavior across the investigated stereotactic VMAT configurations. The equivalent radius ( r 15 ) increased monotonically with STV size, ranging from approximately 2.0–2.1 cm to 4.1–4.3 cm at 15 Gy × 3 and from approximately 3.0–3.3 cm to 6.3–6.6 cm at 24 Gy × 3 as STV radius increased from 1 to 3 cm. Variation associated with axial target length generally remained within approximately 0.2–0.4 cm for a given prescription level. Increasing arc number produced only modest effects on dose attenuation behavior, with a median inter‐configuration difference in r 15 of approximately 0.12 cm. Dose‐specific regression models demonstrated excellent agreement with phantom‐derived measurements, with R 2 values ranging from 0.991 to 0.999. Conclusion This study establishes a geometry‐based STV scaling framework for SCART based on reproducible internal dose fall‐off characteristics under stereotactic VMAT delivery. The proposed dose‐specific regression model provides a practical and physically interpretable approach for inward STV scaling from measurable gross tumor volume geometry and may support more reproducible implementation of SCART planning.