Multimodal PET Defines a 'Goldilocks' Thermal Window for Focused Ultrasound Ablation and Immunotherapy Combinations

Thermally ablative focused ultrasound (T-FUS) offers a noninvasive, spatially precise strategy for local tumor destruction, with the added potential to remodel tumor architecture and immune dynamics in ways that influence downstream therapeutic delivery and efficacy. Despite promising preclinical and clinical findings, the T-FUS parameters that best balance tumor debulking with preservation of local biologic, e.g. immunotherapy, penetrance remain unclear. Thermal dose, defined by the relationship between tissue heating, exposure duration, and biological effect, is likely a critical determinant of this balance. Excessive thermal dose may eliminate the vascular and stromal features needed to support immunotherapy access, whereas insufficient thermal dose may fail to achieve meaningful cytoreduction. Here, we deploy multimodal PET, contrast-enhanced ultrasound, and tissue profiling to define a "Goldilocks Zone" for T-FUS that balances bulk tumor destruction with immunotherapy delivery.

Subtotal T-FUS was applied to 4T1 tumors using three thermal dose regimens resolved by in silico modeling. Ablation was quantified by H&E and TTC staining. Post-ablative perfusion and microvascular coverage were assessed by contrast-enhanced ultrasound and immunofluorescence, respectively. Tumor oxygenation was measured by intravenous hypoxyprobe labeling. After T-FUS, mice underwent dynamic [¹⁸F]-FDG PET and immunoPET with a model tumor-targeted antibody, [⁸⁹Zr]-αCD47, to relate cytoreduction to antibody penetrance. ImmunoPET findings were further evaluated by ex vivo biodistribution analysis.

In silico modeling established three T-FUS regimens that generated distinct thermal dose profiles and were deployed in vivo in a solid breast tumor model. Histopathology, perfusion imaging, and hypoxia analysis revealed dose-dependent and dose-divergent biological effects that informed a candidate Goldilocks thermal window. Low thermal dose produced measurable but limited tumor debulking, whereas high thermal dose caused disproportionate functional perfusion collapse. An intermediate thermal dose achieved robust partial ablation, broad hypoxia relief, and preservation of residual tumor physiology sufficient to support antibody access. Dynamic [¹⁸F]-FDG PET confirmed a marked reduction in metabolically active tumor burden after Goldilocks T-FUS. Serial [⁸⁹Zr]-αCD47 immunoPET showed that bulk antibody signal was maintained after ablation, and integration of immunoPET with matched [¹⁸F]-FDG PET revealed approximately 3-fold enrichment of antibody exposure within the residual viable tumor compartment of ablated tumors. These findings demonstrate that appropriately tuned thermal ablation can debulk tumor while preserving, and potentially concentrating, immunotherapy access within the remaining targetable tumor niche.

This study identifies thermal dose as a critical consideration for T-FUS immunotherapy combinations and establishes a PET-informed framework for balancing cytoreduction with therapeutic delivery. Rather than functioning solely as a local debulking modality, we demonstrate that T-FUS can be tuned to yield a post-ablation tumor state that remains accessible to large biologics. These findings provide timely, translationally relevant guidance for tailoring T-FUS regimens to achieve local tumor destruction while preserving an immunotherapy-permissive niche for combination treatment.