What's Your Diagnosis? Dog With a Laryngeal Mass

An 8-year, 3-month-old female, entire Boxer dog presented to the Queen Mother Hospital for Animals (QMHA), Royal Veterinary College (RVC) for evaluation of increased upper airway noise and dysphagia. A laryngeal mass had been identified by the referring veterinarian 5 months previously while the dog was undergoing general anesthetic for removal of oral masses (confirmed as peripheral odontogenic fibromas). At this time, no clinical signs were associated with the mass. However, in the weeks prior to presentation, the dog became unable to tolerate solid food and started showing respiratory symptoms relating to the mass and was subsequently referred for evaluation. CT examination revealed a 35 × 28 × 39 mm soft tissue mass occupying the right side of the rima glottidis, displacing the laryngeal lumen. The mass extended from the level of the rostral margin of the tympanic bulla to the level of C1 and dorsally into the caudal aspect of the nasopharynx. Direct aspirate preparations (Figures 1 and 2) from the laryngeal mass were air-dried, fixed, and stained using modified Wright stain (Hema-tek 3000, Siemens).

Incisional biopsy samples obtained at the same time as the aspirates were fixed in formalin, routinely processed, and stained with H&E.

Sections had a densely cellular pleomorphic malignant neoplasm, composed of diffuse sheets of irregularly polygonal to elongate cells of two populations. The more prominent population had large amounts of vacuolated cytoplasm with lower numbers of smaller round cells with moderate amounts of finely granular eosinophilic cytoplasm. Most cells had a single irregularly round nucleus with one prominent large nucleolus. Anisocytosis and anisokaryosis of the larger cells were marked, with frequent multinucleated giant cells and 57 bizarre mitotic figures in 2.37 mm² (Figure 3). Atypia of the smaller cell population was mild. The histological interpretation was an atypical malignant neoplasm with differentials including rhabdomyoma/sarcoma, oncocytoma, granular cell tumor, amelanotic melanoma, or histiocytic sarcoma.

Histochemical staining revealed strong cytoplasmic positivity for periodic acid-Schiff (PAS) in the small cells but negativity in the larger cells, diffuse strong membranous, and cytoplasmic positivity for vimentin in approximately 95% of cells (Figure 4), diffuse moderate cytoplasmic positivity for desmin in approximately 80% of cells, and negativity for toluidine blue, phosphotungstic acid hematoxylin (PTAH), cytokeratin, ionized calcium-binding adaptor molecule 1 (Iba-1), melan-A, smooth muscle actin (α-sma), neuron-specific enolase (NSE), and S100 (Table 1).

Based on these staining characteristics and cellular pleomorphism, a diagnosis of rhabdomyosarcoma was made.

Laryngeal rhabdomyosarcoma is mainly reported in adult dogs, being locally invasive but rarely metastatic. This local invasion makes complete excision difficult, with tumor recurrence often resulting in euthanasia [1, 2].

Without immunohistochemical (IHC) staining characteristics, diagnosis can be difficult due to cellular pleomorphism or the presence of multiple distinct cellular populations as demonstrated in this case [3, 4]. The range of cellular morphologies is suspected to resemble various developmental phenotypes of the myocyte, from round immature myoblasts with variable amounts of pink cytoplasm to more mature multinucleated myotubular cells [1]. The description of the immature myoblasts appears to correlate with the small pink cells described in the current case; however, the large, highly granular to vacuolated cells often exhibiting marked cellular pleomorphism made the cytological diagnosis highly challenging.

Rhabdomyosarcomas can be differentiated histologically into four subclasses, including alveolar, botryoid, embryonal, and rarely pleomorphic rhabdomyosarcoma, allowing prediction of biological behavior. Alveolar RMS are further classified into classic and solid types, with embryonal separated into rhabdomyoblastic (dominated by round to polygonal cells with variable pink cytoplasm), myotubular (predominance of multinucleated PTAH positive “strap-cells”), and spindled which is rare in veterinary species [1, 3].

IHC characteristics are often variable, depending on the degree of cellular differentiation and the variability of myogenic regulating factor expression. Vimentin is the first intermediate filament expressed at the myoblast stage, with further differentiation resulting in desmin expression. Desmin can be useful, even in undifferentiated and highly pleomorphic RMS. Expression of this marker with concurrent lack of cytokeratin and PAS expression would exclude a diagnosis of oncocytoma [5]. Ideally, for further confirmation of skeletal muscle origin, MyoD1 and myogenin expression, representing early embryonal transcription factors, could have been considered, although expression may be variable depending on the degree of differentiation [1]. Unfortunately, these markers were not available at our institution. Negative smooth muscle actin (SMA) staining excluded smooth muscle origin. Despite the highly granular morphology and PAS positivity, a granular cell tumor was excluded based on S100 and NSE negativity [5]. Findings were also not compatible with histiocytic sarcoma (Iba-1 negativity), amelanotic melanoma (melan-A and S100 negativity), or anaplastic carcinoma (cytokeratin negativity).

The current case ultimately demonstrates an uncommon neoplasm, which provided a diagnostic challenge given the degree of cellular pleomorphism, ultimately requiring immunohistochemistry for diagnosis.

The authors declare no conflicts of interest.