2681 - The "Goldilocks" Window: A Mechanistic LPL Model Resolving the Oxygen Paradox of the FLASH Effect

Purpose/Objective(s):

Hypothesis: We hypothesize that the differential sparing of FLASH radiotherapy is driven by the specific oxygen dynamics of the stem cell niche—a "Goldilocks" zone of physiologic hypoxia (1–5 mmHg)—where radiolytic oxygen depletion effectively suppresses damage fixation, unlike in effectively anoxic or normoxic tumor microenvironments.

Our objective is to resolve the persistent paradox between bulk vascular oxygenation measurements and the "Oxygen Depletion Hypothesis" of FLASH. We developed a mechanistic extension of the Lethal and Potentially Lethal (LPL) model to identify the specific baseline oxygen window where sparing is maximized, thereby explaining normal tissue sparing without empirical dose-rate adjustments.

Materials/Methods:

We introduced an explicit precursor lesion population into the LPL formalism. The fate of these precursors is governed by competing oxygen-dependent chemical fixation versus enzymatic restitution. Fixation kinetics are coupled to a time-varying nuclear oxygen tension, pO₂, which depletes radiolytically (G ˜ 0.42 mmHg/Gy) and recovers via diffusion toward a baseline pO_2,0. Uniquely, this framework distinguishes bulk vascular oxygenation from the lower nuclear oxygen levels found in metabolically shielded stem cell niches (modeled baseline pO_2,0 ˜ 3.0 mmHg) versus anoxic tumor cores (pO_2,0 ˜ 0.2 mmHg) or well-oxygenated tissues (pO_2,0 = 30 mmHg). The model was constrained to recover standard LPL behavior in the low dose-rate limit.

Results:

The model identifies a "Goldilocks" window of sensitivity. For normal tissues residing in the physiologic hypoxia range (1—5 mmHg), FLASH pulses (40 Gy/s) drive nuclear down the steep portion of the oxygen enhancement curve, suppressing damage fixation relative to conventional delivery (0.05 Gy/s). At 10 Gy, the sparing metric ? = ln(SF_FLASH/SF_CONV) reached ˜ 0.33. Crucially, the model predicts negligible sparing for tissues outside this window: perivascular/well-oxygenated tissues remain saturated, while perinecrotic tumors remain at the OER floor (see Table). Sparing diminishes as delivery time approaches the oxygen recovery constant (˜ 50 ms), quantitatively linking pulse structure to biological outcome.

Conclusion:

By resolving the niche-to-nucleus oxygen gradient, this mechanistic framework explains why FLASH spares normal tissues (regulated physiologic hypoxia) while maintaining control of deeply hypoxic tumors. This target-level oxygen framing helps reconcile the tissue dependence of the FLASH effect and provides constraints for designing clinically relevant delivery parameters.

* ? at 10 Gy total dose, FLASH = 40 Gy/s, CONV = 0.05 Gy/s.

Baseline Condition	Nuclear pO2,0 (mmHg)	FLASH Sparing (?)*	Biological Outcome
Perinecrotic Tumor	~0.2	~0.05 (Negligible)	No Sparing (OER Floor)
Stem Cell Niche	~3.0	~0.33 (High)	Maximal Sparing
Perivascular Zone	~30.0	~0.00 (None)	No Sparing (OER Saturation)