Assessment of Clinical and Immunological Factors of Local Recurrence in Canine Soft Tissue Sarcoma.

Canine soft tissue sarcomas (STSs) are locally invasive tumours, making local recurrence a major clinical challenge. Time to recurrence (TTR) and associated immune factors remain poorly characterized. This study aimed to characterize the immune microenvironment of canine STS and identify factors associated with TTR. Medical records from 27 dogs with STS that recurred after surgery were reviewed. Histopathological, transcriptomic, and immunological analyses were performed in a tissue-available subcohort of 20 dogs. Immune-related gene expression was characterized using the NanoString canine IO panel. Tumour-infiltrating immune cells and tertiary lymphoid structures (TLSs) were assessed immunohistochemically using CD3, CD20, and FoxP3. Associations with TTR were evaluated using Cox proportional hazards models and log-rank tests. Median TTR was 358 days. Age and tumour location were associated with TTR in univariable analyses of the full cohort, but neither age nor the overall location effect remained significant when analysed together. Higher NOD2 expression and Cluster 1 assignment were associated with shorter TTR across exploratory multivariable models. Cluster 1 tumours exhibited abundant lymphocyte infiltration and enrichment of immunosuppressive pathways, whereas Cluster 2 tumours demonstrated lower immune infiltration and proinflammatory signatures. TLS-positive tumours had greater lymphocyte infiltration, with FoxP3+ cells accumulated within TLSs. Paired recurrent tumours contained significantly more large TLSs than primary tumours. Higher NOD2 expression and an immunologically active but suppressed tumour phenotype were associated with shorter TTR, whereas the association with anatomical location was model-dependent. Regulatory T cells and TLSs may contribute to the immune landscape associated with local recurrence and warrant further investigation as potential therapeutic targets.