Adaptive, Clinically Guided Multimodal Therapy with Supportive Drug Sensitivity Testing in a Dog with Hepatic Neuroendocrine Carcinoma: A Case Report.

Hepatic neuroendocrine carcinoma (NEC) in dogs is a rare malignancy with limited therapeutic guidance and no established standard of care. This report describes an adaptive, clinically guided multimodal treatment approach in a dog with advanced hepatic NEC with regional lymph node involvement. Sequential systemic therapies-including doxorubicin, mitoxantrone with lomustine and prednisolone, and subsequently toceranib-were administered based on clinical response assessment using imaging (VCOG RECIST), hematologic monitoring, and quality-of-life evaluation. Ex vivo drug sensitivity testing (DST) was performed to provide functional reference information but was interpreted as supportive rather than predictive. Notably, discordance was observed between strong in vitro sensitivity to doxorubicin and early clinical progression, underscoring the limitations of monoculture-based assays in recapitulating in vivo tumor biology. Sustained stable disease was observed following transition to toceranib with continued adjunct immunomodulatory therapy; however, the independent contribution of each treatment component cannot be determined. This case highlights the feasibility of iterative treatment refinement in rare canine malignancies and emphasizes that DST findings should be integrated cautiously within a broader clinical decision-making framework.