Diagnostic challenge in veterinary pathology: Disseminated tumor in a young dog

Clinical History, Laboratory Results, and Gross Findings

A 1-year-old male German Shepherd dog was brought to a veterinary clinic for a mass in the oral cavity and regional lymphadenopathy. The lesion was treated as an abscess, with administration of antibiotics and anti-inflammatory drugs. In the following days, the general condition of the dog progressively worsened; he refused to eat and manifested respiratory distress. Thoracic radiographs indicated the presence of nodules disseminated throughout the lung. The differential diagnoses of fungal pneumonia and neoplasm were proposed by the clinician. In a short time, the patient died and was submitted to the Teaching Veterinary Hospital of the University of Milan for necropsy.

Gross examination revealed a 6 × 4 × 2 cm, ulcerated, white, firm mass on the mucosal surface of the right upper lip, infiltrating the soft tissues, but without maxillary bone infiltration (Fig. 1). Submandibular lymph nodes were bilaterally severely increased in size (up to 6 cm in diameter). Multiple nodules ranging from 0.5 to 4 cm in diameter were found in the left mandibular bone, thoracic wall, lung, heart, mediastinum, liver, stomach, pancreas, right kidney, mesentery, and adrenal glands (Fig. 2).

Figures 1–3.

Dog. Figure 1. Right upper lip, mucosal aspect. Primary tumoral mass with focal ulceration. Figure 2. Lung. Multiple metastatic nodules disseminated throughout pulmonary parenchyma. Figure 3. Lung, metastatic nodule. Cytological imprint showing loosely cohesive groups of small round cells with scant bluish, occasionally vacuolated cytoplasm. May-Grunwald Giemsa.

Cytological imprints made from the lip mass, an enlarged submandibular lymph node, and a pulmonary nodule were all consistent with an undifferentiated malignant round cell tumor (Fig. 3).

A diagnosis of disseminated tumor was thus made. The lip mass was considered the primary tumor because of its size and severe enlargement of the regional lymph nodes.

Microscopic Findings

Histology of the mass on the lip and nodules collected from multiple sites revealed similar microscopic features, consisting of an unencapsulated, poorly demarcated, infiltrative, highly cellular neoplasm. Neoplastic cells were arranged in solid areas and cords supported by scant fibrovascular stroma (Fig. 4). Cells were round, 10 to 15 µm in diameter, with distinct cell borders, high nucleus-to-cytoplasm ratio, scant pale eosinophilic cytoplasm, round central nuclei, finely granular chromatin, and indistinct nucleoli. Large intratumoral necrotic areas and multifocal metastatic emboli were present at the periphery of the primary tumor. Short bundles of spindle-shaped cells were occasionally found in the disseminated nodules.

Figures 4–6.

Dog. Figure 4. Stomach, metastatic nodule. Neoplastic cells are organized in solid areas and nests separated by thin fibrovascular septa. Inset: neoplastic cells are small, round to slightly spindled, with scant pale cytoplasm. Hematoxylin and eosin (HE). Figures 5–6. Lung, metastatic nodule. Neoplastic cells have strong cytoplasmic expression of desmin (Fig. 5), and only occasional expression of vimentin (Fig. 6). Immunohistochemistry, DAB chromogen.

Differential Diagnoses

Based on the histological features, a first diagnosis of “small round blue cell tumor” was made, and lymphoma, rhabdomyosarcoma, peripheral primitive neuroectodermal tumor, and Merkel cell tumor were among the most likely diagnoses.

Further Investigations and Diagnosis

To identify the cell type of origin of the tumor, the following immunohistochemical panel of antibodies was applied: vimentin (clone 3B4, Dako), desmin (clone Y66, Abcam), CD3 (Dako), CD20 (Thermo Fisher Scientific), and NSE (clone BBS/NC/VI-H14, Dako). The tumor was negative for CD3, CD20, and NSE. Vimentin was weakly expressed in the cytoplasm of less than 20% of neoplastic cells, while desmin was strongly expressed in the cytoplasm of over 95% of neoplastic cells, indicating a muscular differentiation of the tumor (Figs. 5 and 6).

Based on the young age of the dog, the histological features of the tumor, and the immunohistochemical results, a diagnosis of solid alveolar rhabdomyosarcoma was made.

Discussion

The case reported herein represented a diagnostic challenge from both the clinical and pathological points of view. Based on the young age of the affected dog, the clinician initially considered an inflammatory condition (abscess) as the most likely diagnosis for the mass located on the upper lip. However, pulmonary nodules detected by thoracic radiographs suggested a fungal or neoplastic disease. At necropsy, the presence of multiple masses in several organs indicated a disseminated tumor. Cytology performed during the necropsy confirmed the neoplastic nature of the lesions, and a provisional diagnosis of undifferentiated round cell tumor was made.

Histologically, based on the solid growth pattern of the neoplasm and the round cell morphology of neoplastic cells, a diagnosis of “small round blue cell tumor” was made, which as in humans can include lymphoma and neuroectodermal tumors. However, after examination of the neoplastic nodules collected from different organs, occasional short bundles of spindle-shaped cells were found, which led us to include rhabdomyosarcoma among the differential diagnoses.

Immunohistochemistry with a panel of antibodies was negative for CD3, CD20, and NSE, ruling out lymphoma and neuroendocrine tumors, and was weakly positive for vimentin and strongly positive for desmin, identifying the neoplasm as a rhabdomyosarcoma. Rhabdomyosarcomas are reported to consistently express desmin and variably express vimentin, depending on the degree of differentiation of the neoplastic cells. ³

Rhabdomyosarcoma is a rare malignant neoplasm of humans and animals, typically occurring in young patients. ¹ In animals, the neoplasm is more commonly reported in dogs, ¹ but other species like cats, ⁵ horses, ² cattle, ⁷ pigs, ⁹ sheep, ⁶ budgerigars, ⁴ and sea lions ¹⁰ are also represented. Histologically, rhabdomyosarcoma is subclassified as embryonal, alveolar, botryoid, and pleomorphic. ¹ Botryoid rhabdomyosarcoma of the urinary bladder is most frequently diagnosed, followed by head and neck regions, in which sites the embryonal type is more commonly reported than the alveolar. ¹ Alveolar rhabdomyosarcoma is classically characterized by neoplastic cells lining fibrous septa associated with “floating” degenerated cells in the center of the clear alveolar structures, but a solid variant characterized by dense sheets of so-called “small blue round cells” is also described. ^1,3 Rhabdomyosarcomas in dogs are usually positive for vimentin, desmin, muscle actin, and sarcomeric actin. ¹ Myogenin and MyoD1 can also be useful to confirm desmin-negative canine rhabdomyosarcomas. ⁸ The present case further highlights the importance of considering rhabdomyosarcoma in the differential diagnoses for head and neck masses in young dogs.