Occult osseous metastasis of a colonic adenocarcinoma visualized with technetium tc 99m hydroxymethylene diphosphate scintigraphy in a horse.

A 5-year-old Arabian horse was admitted with a 5-day history of undulant pyrexia of unknown origin, inappetence, obtundation, and acute collapse. Physical examination results were unremarkable except for a grade II/VI left-sided systolic cardiac murmur and abdominal splinting. Mild chronic inflammatory changes were evident on clinicopathologic evaluation. Echocardiography revealed moderate aortic insufficiency. A solitary soft tissue opacity was found on thoracic radiography but not on ultrasonography. Palliative treatment was ineffective. Nuclear scintigraphy with WBC labeled with technetium Tc 99m hexamethylpropyleneamine oxime did not identify abnormalities, but a second nuclear scan with technetium Tc 99m hydroxymethylene diphosphate identified polyostotic disease. Examination of a biopsy specimen from an affected rib revealed disseminated adenocarcinoma. The horse was euthanatized. Necropsy and histologic examination revealed a colonic adenocarcinoma with osseous metaplasia that had disseminated to multiple parenchymal organs, muscle, and bone.