KEY POINTS
- The study addressed the absence of a standardized method for defining the central stereotactic treatment volume in stereotactic centralized ablative radiation therapy, a spatially fractionated strategy that delivers an ablative dose to the centre of bulky tumors.
- Investigators generated stereotactic volumetric-modulated arc therapy plans in a homogeneous LUCY phantom using idealized cylindrical treatment volumes with radii of 1, 2, or 3 cm and axial lengths of 3, 5, or 10 cm.
- Plans were created on Trilogy and Halcyon platforms using two to four coplanar arcs and prescriptions of 15, 18, 21, or 24 Gy per fraction for three fractions. The radial extent of the 15 Gy isodose was converted into an equivalent cylindrical radius and used to derive the scaling model.
- As treatment-volume radius increased from 1 to 3 cm, the equivalent 15 Gy radius increased from approximately 2.0–2.1 cm to 4.1–4.3 cm for the 15 Gy × 3 regimen and from 3.0–3.3 cm to 6.3–6.6 cm for 24 Gy × 3.
- Axial target length changed the equivalent radius by approximately 0.2–0.4 cm at a given prescription level. Increasing the number of arcs had only a modest influence, with a median difference between configurations of approximately 0.12 cm.
- Dose-specific linear models predicted central treatment-volume radius with R² values of 0.991–0.999. Root mean square errors ranged from 0.014 to 0.038 cm, and mean absolute errors ranged from 0.011 to 0.031 cm within the phantom dataset.
- In an illustrative liver case with a gross tumor volume of 271.8 cm³ and axial length of 7.86 cm, the model predicted a central treatment-volume radius of 1.19 cm and an inward contraction of 2.13 cm. This was a planning demonstration rather than clinical validation.
- The model was derived from regular cylinders in a homogeneous phantom and did not account for irregular tumors, tissue heterogeneity, respiratory motion, non-coplanar delivery, or proximity to organs at risk. The 15 Gy isodose also extended longitudinally beyond the modeled volume by approximately 14–50% in some configurations.
CLINICAL TAKEAWAY
The framework could make central target definition in SCART more reproducible and reduce subjective planner-dependent scaling. It should currently be treated only as an initial planning reference: the model has not been prospectively validated in heterogeneous patient anatomy or linked to clinical outcomes.