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Background: Thyroid tumor is a common endocrine tumor that accounts for up to 3.8% of all tumors in dogs. Most of them are malignant and usually nonfunctional in dogs. 18F-fluorodeoxyglucose positron emission tomography (FDG-PET) is an imaging modality that detects intracellular accumulation of radioactive deoxyglucose administered in the body and is used in combination with computed tomography to provide functional information with exact anatomical localization. It is used in human medicine to detect residual or recurrent head and neck neoplasm after treatments, such as surgical resection. This report describes the first case of diagnosing recurrent thyroid carcinoma (TC) through FDG-PET in a dog. Case: A 9-year-old castrated male Maltese dog presented with a palpable mobile mass in the right ventral cervical region. Radiography and ultrasonography (US) showed a radiopaque mass adjacent to the trachea, and the right thyroid gland was enlarged on computed tomography. The surgically excised mass was encapsulated and measured to be 2.3 × 1.0 × 3.4 cm (width x length x height) in size. Histopathologically, the mass was diagnosed as differentiated follicular TC, and gross and vascular invasions were observed. To prevent recurrence, postoperative carboplatin chemotherapy was performed for 5 months. Two months after completion of chemotherapy, a nodule of approximately 7 mm in diameter was detected in the thyroidectomy bed by US. FDG-PET scanning was performed as an effective means of evaluating the malignancy, local recurrence, and metastasis of differentiated follicular TC. The nodule had the dimensions of 2.8 × 5.9 × 8.6 mm, a maximum standardized uptake value (SUV) of 8.49, and a mean SUV of 5.6. The results of FDG-PET suggested the recurrence of TC; therefore, the second chemotherapy protocol using toceranib was applied for 16 months. After initiation of the second chemotherapy, follow-up examinations were conducted approximately every 4 months. On the 134th day, although the nodule was not palpated, its size was observed to have increased to 5.0 × 3.8 × 13.6 mm on cervical US on the 232nd day, showing heterogeneous and hypoechoic parenchyma. On the 405th day, the tumor was enlarged to a size of 13.4 × 12.9 × 22 mm and identified as a lobular, amorphous shape, and its heterogeneity was increased. Moreover, two pulmonary nodules with well-defined margins were found on radiography in the left caudal lung lobe (9 × 10 mm and 12 × 12 mm [width × length]); thus, lung metastasis was suspected. On the 536th day, anorexia and lethargy occurred, and the dog was lost to follow-up. Discussion: In the present case, local recurrence of TC was suspected based on cervical US. Although US was useful as a screening tool, additional examinations were necessary for evaluating local invasiveness, malignancy, and nodal/distant metastasis. FDG-PET can detect recurrence at an early stage because it can sense increased tumor metabolism through physiologic absorption of FDG, even before the beginning of anatomic change in the lesion. Therefore, FDG-PET can assist in treatment planning and provide better prognosis. In humans, focal FDG uptake and a high maximum SUV in the thyroid gland on FDG-PET were associated with a higher risk of cancer. Because there was no evidence of neoplasia except the thyroid lesion during the FDG-PET examination, the tumor showed an increasingly malignant pattern of the thyroid gland on US during the follow-up period, and the metastatic pulmonary nodules were identified on the 650th day after the thyroidectomy. Therefore, the present case was diagnosed as recurrent TC. This report describes the use of FDG-PET for diagnosing local recurrence of TC, pointing to FDG-PET as a potential strategy to evaluate loco-regional recurrence and distant metastasis of TC. Keywords: canine, FDG-PET, follicular thyroid carcinoma, metastasis, tumor, cancer.

Background: Myelitis is the inflammation of the spinal cord parenchyma alone, whereas meningitis is the inflammation of the meninges. Steroid-responsive meningitis-arteritis (SRMA) is a meningomyelitis in which the major lesions involve the meninges, not the spinal cord parenchyma, and respond well to glucocorticoid treatment. However, myelitis in dogs has rarely been reported, and myelitis with a good response to glucocorticoid treatment without relapse has not been reported. This report describes 5 cases of steroid-responsive myelitis (SRM) in dogs.Cases: Case 1. A 8-year-old intact female Cocker Spaniel presented with progressive nonambulatory paraplegia. Whole spinal parenchymal lesions were identified using magnetic resonance imaging (MRI) scan. Mononuclear pleocytosis with increased total protein levels was the only abnormal finding on cerebrospinal fluid (CSF) analysis. Prednisolone (PDS) was administered followed by dose tapering according to therapeutic response. Cyclosporine was administered until the termination of PDS. Since then, no recurrence of neurological symptoms has been observed. Follow-up MRI and CSF analysis revealed resolution of previously observed abnormal findings. Case 2. A 2-year-old intact female Maltese presented with non-progressive paraparesis. A spinal parenchymal lesion in the lumbosacral region was observed on MRI. PDS was administered and slowly tapered at approximately 3-week intervals. No recurrence of neurological symptoms was observed after the treatment. Case 3. A 6-year-old intact female Miniature Pinscher presented with neck pain, along with leukocytosis and neutrophilia. Cervical spinal parenchyma lesions were revealed through MRI. Increased total protein concentration with mixed cell pleocytosis was observed on CSF analysis. Immunomodulatory therapy, similar to that in case 2, was initiated. A second MRI and CSF analysis revealed an improvement in the previously observed abnormalities. Case 4. A 2-year-old, intact female Toy Poodle presented with acute paraplegia and back pain. Lesions were observed in the spinal parenchyma at the T12-L3 levels on MRI. The treatment was conducted as in case 2. During treatment, neurological symptoms, including paraplegia and back pain, were not observed. Follow-up MRI revealed improvement in the spinal lesion. Case 5. A 6-month-old, castrated male Standard Poodle presented with progressive paraparesis. On MRI, lesions were observed in the T11-T13 regions. Immunomodulation therapy, similar to that in case 2, was initiated. No recurrence of neurological symptoms was observed after treatment initiation. Discussion: SRM is similar to SRMA in terms of good steroid-responsiveness and noninfectious inflammation etiology; however, it does not exactly satisfy the diagnostic criteria for SRMA, nor does it progress similarly. The characteristics of SRM that do not satisfy the diagnostic criteria of SRMA include the absence of fever, C-reactive protein elevation, hyperglobulinemia, and relapse, and the presence of spinal parenchymal lesions without parenchymal or meningeal enhancement on MRI. It is also a seemingly different from spinal cord-only meningoencephalomyelitis of unknown origin due to its better treatment response and prognosis. However, the dogs in the present report with SRM satisfied the diagnostic criteria for transverse myelitis in human patients. Therefore, SRM, including good steroid responsiveness and good prognosis without relapse, may represent a novel type of meningomyelitis. Keywords: canine, inflammatory spinal cord disease, myelitis, spinal cord, steroid.