Targeting spatial patterns of CAFs and tumor cells for precision stroma-targeted radionuclide therapy

Stroma-targeted imaging, particularly fibroblast activation protein inhibitor (FAPI) PET-CT, has demonstrated remarkable diagnostic success across multiple cancer types. In contrast, stroma-targeted radionuclide therapy using ¹⁷⁷Lu-labeled compounds showed limited clinical efficacy (response rate: 10%-20%), despite achieving sufficient macroscopic accumulation within tumor lesions. We therefore hypothesize that the microscopic spatial organization between cancer-associated fibroblasts (CAFs) and tumor cells critically influences intratumoral distribution and radiation dose deposition of ¹⁷⁷Lu at the microscale. The primary objective of this study is to investigate how spatial pattern affects ¹⁷⁷Lu distribution and dose deposition, and to experimentally validate the differential therapeutic outcomes across different spatial patterns.Using medullary thyroid carcinoma (MTC) as a representative model, we performed quantitative histopathological analysis of 279 tissue sections from 15 patients through co-registration, automated image segmentation, and spatial distribution profiling. This identified two recurrent spatial patterns: a "surrounding" type, where CAFs form a continuous ring around tumor clusters, and an "infiltrative" type characterized by a reticular CAF network with dispersed tumor nests. These patterns were consistently observed across primary tumors, lymph node metastases, and invasive foci.Monte Carlo simulations revealed highly heterogeneous ¹⁷⁷Lu dose distributions across spatial patterns. While infiltrative lesions showed effective dose delivery, surrounding lesions exhibited dose deposition primarily confined to CAF-rich peripheries. We established biologically relevant 3D bioprinted models mimicking both patterns using 6% GelMA 30 hydrogel, which matches human thyroid mechanical properties (10-20 kPa). Immunohistochemical and immunofluorescence analyses confirmed biological fidelity. Autoradiography with [¹⁷⁷Lu]Lu-FS-86 demonstrated distinct uptake patterns - concentrated in CAF rings (surrounding) versus homogeneous distribution (infiltrative) - validating functional relevance. These models confirmed that the infiltrative pattern allowed homogeneous dose distribution of 177 Lu and significant cell death, whereas the surrounding type exhibited limited penetration and limited response. Importantly, we observed consistent therapeutic outcomes across multiple cancer types including lung, liver, breast, and medullary thyroid carcinomas, indicating that stroma-targeted radionuclide efficacy is spatial pattern-dependent rather than tumor type-dependent. The establishment of this spatial classification system provides a framework for identifying optimal stroma-targeting radionuclides based on distinct spatial patterns, demonstrating significant potential for clinical translation.