A tumor spheroid contains three populations: proliferating cells at the surface, quiescent cells in the interior, and necrotic cells at the core. The quiescent cells aren't dead — they're dormant, sitting in a low-nutrient zone where resources are insufficient for division but sufficient for survival. They wait.
Leschiera et al. applied pulsed electric fields to tumor spheroids at different intensities and tracked the response with a hybrid computational model validated against experiment. Cell death and the release of damage-associated molecular patterns (DAMPs) scale monotonically with pulse intensity — stronger pulses kill more cells. But spheroid fate doesn't track this monotonic curve. The quiescent cells play a dual role: some die from the treatment, freeing space and resources; others survive and resume proliferation into the newly empty space.
The treatment kills the active tumor and inadvertently feeds the dormant one. At intermediate intensities, enough proliferating cells die to open space and release nutrients, but enough quiescent cells survive to exploit the opening. The spheroid regrows — not from the cells that were already dividing, but from the cells that were waiting for room.
This matters for electroporation-based cancer therapy because it identifies the hidden competitor. The therapeutic target is the visible, proliferating tumor. The actual threat is the invisible, quiescent interior that the therapy itself promotes by clearing its neighbors. Designing treatment without accounting for quiescent cells optimizes for killing the wrong population.
The immunogenic angle offers a possible escape: electroporation triggers immunogenic cell death, releasing DAMPs that can stimulate immune response. If the immune system can recognize and attack the awakened quiescent cells, the dual role reverses from liability to asset. But that requires the immune response to outpace the regrowth — a race between immune recognition and quiescent re-entry into the cell cycle.
The sleeper was never the problem until the treatment woke it up.