Description and Practical Application of the Physiologic Distribution of 3’-Deoxy-3’-[18F]Fluorothymidine in Companion Animals

Access to positron emission tomography (PET), and more recently PET combined with computed tomography (PET/CT), is increasing in veterinary medicine. This molecular imaging technology allows clinicians to map biological functions within patients based on the distribution and selective uptake of specialized positron-emitting radiopharmaceuticals. Although most clinical studies utilize 2-deoxy-2-[¹⁸F]fluoro-Dglucose (¹⁸FDG), a versatile but relatively nonspecific tracer that interrogates the energy metabolism of tissues, there is a growing need to establish reference values for alternative or adjunct tracers in veterinary species. Among these is 3’-deoxy-3’- [¹⁸F]fluorothymidine (¹⁸FLT), a thymidine analog that selectively accumulates in proliferating tissues. In the present work, ¹⁸FLT distribution in clinically healthy adult dogs and young adult cats was imaged using a state-of-the-art PET/CT scanner to define normal uptake levels within numerous tissues, including major parenchymal organs, bone marrow, and other sites of increased radiopharmaceutical uptake. The marrow signal was subsequently segmented into separate skeletal regions, and used to quantitatively define the adult marrow distribution pattern in the dog. Marrow activity is concentrated in the vertebral column (particularly within the thoracic and lumbar regions), sternum, ribs, and proximal aspects of the appendicular skeleton in the adult dog. Feline marrow distribution is similar; however, considerable uptake within more distal appendicular structures suggests that age-related marrow conversion is ongoing in 3-year-old cats. Outside the marrow compartment, physiologic uptake was observed within the urinary and biliary systems, intestinal tract, and variably within lymphoid structures. Prominent uptake within the hepatic parenchyma was also observed in cats, but not dogs, at the times imaged in this study. The details of normal canine and feline ¹⁸FLT biodistribution included in this dissertation may be used to inform lesion interpretation in dogs and cats with suspected disease. Likewise, quantitative details of adult marrow distribution in dogs may be used by clinicians to guide the selection of marrow sampling sites or inform tissue-sparing efforts during radiotherapeutic planning in canine patients.