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  • 12-year-old male castrated Labrador Retriever with recent onset of anxious behavior, restlessness, and inappropriate urination that has slowly progressively worsened.
  • Fifteen months prior to presentation, definitive radiation therapy (20 fractions of 2.5 Gy) was completed for a right sided front-olfactory extra-axial mass, presumed to be a meningioma.
  • Previous brain MRI performed 5 months post-radiation therapy (11 months prior to this presentation), revealed partial response to therapy with mild decrease in tumor size and improved perilesional edema.

 

Sagittal, dorsal, and transverse FDG PET-MRI fusion images of the brain.

T1w post-contrast BRAVO Stealth sagittal, dorsal, and transverse images of the brain.

T2w dorsal fat saturated FRFSE, and T2w FLAIR sagittal images of the brain

  • 18F-FDG-PET brain MRI performed; 4.7mCi FDG administered IV. -Immediately caudal to the cribriform plate, along the right olfactory bulb and frontal lobe, is a large, well-defined, hypermetabolic, FDG avid (SUVmax = 4.16) mass that is broad-based, round, heterogeneously T2w and FLAIR hyperintense, T1w isointense, and strongly heterogeneously contrast enhancing with a thin contrast enhancing rim.
  • The mass bulges along the cribriform plate with ill-defined lysis and focal rostral extension into the caudal nasal cavity; similar to previous. The mass measures mildly larger than previous at 2.2cm W x 2.6cm H x 3.2cm L (previously 1.79cm W x 2.76cm H x 2.74cm L).
  • Moderate FLAIR hyperintensity circumferentially surrounds the mass, mildly progressive from previous.
  • CSF within the subarachnoid space along the cerebrum is mildly increased in volume diffusely with widening of the sulci, and the lateral ventricles are mild to moderately dilated, similar to previous.
  • The interthalamic adhesion is small at 4.2mm H (7.0mm initially, 4.5mm 5 month post-radiation therapy).
  • Occipital lobe gray matter was slightly less FDG avid (SUVmax = 4.15) than frontal lobe gray matter (SUVmax = 5.18). White matter FDG avidity was less than gray matter (SUVmax = 3.68).
  • FDG-avid right fronto-olfactory extra-axial mass, mildly progressive in size and perilesional edema from previous, with similar focal cribriform plate lysis and intranasal extension, most consistent with meningioma.
  • Alternative, less likely differential diagnoses were esthesioneuroblastoma or histiocytic sarcoma. -Progressive cerebral atrophy with relative occipital lobe hypometabolism, most consistent with cognitive dysfunction. Secondary hydrocephalus ex vacuo.

The majority of canine brain tumors are diagnosed and treated presumptively via characteristic MRI features without ante-mortem histopathologic diagnosis due to limitations of owner finances and increased patient morbidity. Based on imaging features in this case, a presumptive diagnosis of meningioma was made; unfortunately tumor biopsy was not performed. 18F-Fluorodeoxyglucose (FDG) positron emission tomography (PET) is a non-invasive, highly sensitive, advanced imaging technique for quantifying biologic function of tissue by using radioactive tracers (radiopharmaceuticals) to target specific mechanisms in the body. FDG-PET provides metabolic information about brain tumor glucose metabolism that is complementary to standard structural imaging with CT and MRI.

PET plays a key role in human neuro-oncologic imaging, and is a valuable tool in the diagnosis, differentiation, grading, and prognostication of brain tumors in people. In this case, the primary goal was to determine if clinical signs were secondary to tumor progression or other underlying disease process such as newly identified neoplasia, cerebrovascular accident, cognitive dysfunction, or inflammatory/infectious etiologies, and to assess tumor glucose metabolism and metabolic activity with FDG-PET MRI. The tumor was FDG-avid in this case, with similar tumor glucose metabolism to the cortical gray matter (Tumor:Normal cortical gray matter SUV ratio = 0.8 [frontal lobe], and 1.0 [occipital lobe])). Mild tumor hypermetabolism relative to cortical gray matter may indicate a lower grade meningioma, or decreased activity secondary to previous radiation therapy.

Given the minimally progressive mass and perilesional edema, coupled with progressive cerebral atrophy and hypometabolism of the occipital lobes, clinical signs were thought to be predominantly attributed to progressive canine cognitive dysfunction. The dog was discharged with instructions to continue previously prescribed oral steroids and anticonvulsant medications. FDG-PET remains relatively novel in veterinary medicine and reports of canine brain tumor imaging characteristics are lacking. Further evaluation of FDG-PET characteristics and glucose metabolism of canine brain tumors is required to determine the utility of FDG-PET in the diagnosis, prognostication, and development of targeted therapeutic approaches in canine patients.

1. Sturges BK, Dickinson PJ, Bollen AW, et al. Magnetic resonance imaging and histological classification of intracranial meningiomas in 112 dogs. J Vet Intern Med. 2008;22(3):586-595. doi:10.1111/j.1939-1676.2008.00042.

2. Noh D, Choi S, Choi H et al. Evaluation of interthalamic adhesion size as an indicator of brain atrophy in dogs with and without cognitive dysfunction. Vet Radiol Ultrasound. 2017 Sep;58(5):581-587. doi:10.1111/vru.12528.

3. Kang BT, Son YD, Lee SR, et al. FDG uptake of normal canine brain assessed by high-resolution research tomography-positron emission tomography and 7 T-magnetic resonance imaging. J Vet Med Sci. 2012 Oct;74(10):1261-7.

4. Slot KM, Verbaan D, Buis DR, et al. Prediction of Meningioma WHO Grade Using PET Findings: A Systematic Review and Meta-Analysis. J Neuroimaging. 2020 Nov 2.

5. Brown RK, Bohnen NI, Wong KK, et al. Brain PET in suspected dementia: patterns of altered FDG metabolism. Radiographics. 2014 May-June;34(3):684-702. https://doi.org/10.1148/rg.343135065