Stereotactic centralized ablative radiation therapy: a framework for ultra-heterogeneous radiotherapy of bulky tumors.

Bulky tumors remain difficult to treat because their size, irregular geometry, proximity to organs at risk, and heterogeneous microenvironment often conflict with standard radiotherapy objectives. Conventional fractionated radiotherapy prioritizes relatively homogeneous target coverage, whereas stereotactic body radiotherapy (SBRT) is most effective when ablative dose can be delivered safely to the entire lesion. Large or anatomically constrained tumors frequently fit neither model. We propose Stereotactic Centralized Ablative Radiation Therapy (SCART) as a reproducible technical framework for ultra-heterogeneous radiotherapy (UHRT) in selected bulky tumors. SCART defines a central SCART treatment volume (STV) within the gross tumor volume (GTV) and prescribes treatment using a core-to-periphery dose ratio (Rcp). Stereotactic apertures are restricted to the STV at each beam angle or control point, concentrating ablative dose centrally, creating a steep dose gradient through a transitional tumor volume (TTV), and maintaining a constrained low-dose rim at the tumor boundary. In this way, SCART shifts the planning objective from uniform whole-tumor coverage to safe maximal core irradiation while preserving peripheral and peritumoral normal-tissue constraints. The approach is motivated by the spatial organization of bulky tumors, in which hypoxic, radioresistant, and immunosuppressive features are often enriched centrally while the periphery remains more vascular and immunologically active. Early clinical experience suggests technical feasibility in carefully selected patients, but the framework remains hypothesis-generating. Prospective studies are needed to define patient selection, dose ranges, reporting metrics, toxicity, response patterns, and rational combinations with systemic therapy.