Hypertrophic Osteopathy Associated With Primary Pulmonary Histiocytic Sarcoma in a Dog.

Hypertrophic osteopathy (HO) is a paraneoplastic or parainflammatory syndrome characterised by painful periosteal new bone formation and soft tissue swelling, most commonly associated with thoracic neoplasia. The pathogenesis of HO remains incompletely understood and may involve neurovascular mechanisms, altered pulmonary filtration of megakaryocytes and platelets and increased release of growth factors such as vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF). This report describes the first documented case of HO associated with primary pulmonary histiocytic sarcoma (PHS) in a dog. A 7-year-old male Golden Retriever was referred for evaluation of diffuse pain, limb swelling and lameness. Diagnostic imaging revealed bilateral periosteal proliferation affecting the forelimbs and a pulmonary mass within the right caudal lung lobe. Histopathology and immunohistochemistry following lung lobectomy and lymphadenectomy confirmed a diagnosis of PHS with nodal metastasis. Surgical excision resulted in complete clinical resolution of pain despite persistent radiographic evidence of HO. Subsequent disease progression occurred despite multiple chemotherapy protocols, including lomustine, vinorelbine, and metronomic chemotherapy; however, the dog remained consistently pain-free throughout follow-up. Overall survival time was 450 days. This case expands the spectrum of neoplasms associated with HO and further supports the hypothesis that tumour location, rather than histotype, plays a central role in the development of this syndrome. Moreover, the sustained absence of pain despite persistent periosteal lesions and metastatic progression suggests that tumour burden and tumour microenvironment may significantly influence clinical manifestations of HO.