Case of the month #9
by Oriol Jornet Rius (DipECVCP)
A 9 month-old intact female dog was presented due to the acute appearance of a mass under the jaw. The dog had no other clinical complains and physical examination was unremarkable apart from the presence of a subcutaneous mass in the submandibular region. The mass was 1*1.5*1 cm in sized, irregularly shaped, and hard in consistency. R-rays and ultrasound (Figure 1) of the mass were performed. Ultrasound revealed an irregular, lobulated, encapsulated, and of heterogeneous echotexture soft tissue mass at the level of the submandibular region.
Figure 1. Ultrasound of the submandibular mass. Please note the irregular, lobulated, encapsulated, and of heterogeneous echotexture soft tissue nodular lesion (red arrow).
Fine-needle aspirates of the mass were taken and sent for cytological evaluation (see figure 2).
What’s your diagnosis?
Figure 2A-C. Digital scans of fine-needle aspirates from a subcutaneous submandibular mass in a dog. Diff-Quik, Romanowsky stain. A. 10x; B. 40x; C. 100x.
Two digital cytology scans/slides were examined. Both slides demonstrated high nucleated cellularity with excellent diagnostic quality. On a mildly basophilic and granular background, with a low to moderate amount of peripheral blood, and occasional lysed cells and chromatin streaming material, a main population of monomorphic round to ovoid cells was noted.
Round cells were individually distributed and were large in size raging between 16-20 µm in diameter. Their NC ratio varied from high to moderate. The cytoplasm was scarce to moderate, pale to moderately basophilic, and with moderately-defined cytoplasmic borders. Some of these cells also revealed the presence a fine pink intracytoplasmic granulation. Nuclei were oval, eccentrically placed, with smooth chromatin, and one to multiple prominent nucleoli. Overall cellular pleomorphism was mild to moderate with mild anisocytosis and mild to moderate anisokaryosis. Binucleation and presence of macronuclei were frequently seen, and rare mitotic figures were also noted (Figure 2).
The cytologic interpretation was round cell neoplasia and differential diagnosis based on the location of the mass and patient’s young age included an embryonal rhabdomyosarcoma, histiocytic disorder (metastatic histiocytoma, histiocytic sarcoma), plasmacytoma, amelanotic melanoma, and other embryonal neoplasms (primitive neuroectodermal tumors, PNETs).
The histopathological diagnosis was a sarcoma with features most likely consistent with an embryonal rhabdomyosarcoma. Immunohistochemistry was recommended for confirmation.
Rhabdomyosarcoma (RMS) is a rare and aggressive malignant neoplasm of mesenchymal origin, arising from skeletal muscle precursor cells. This tumor exhibits varying degrees of differentiation, ranging from immature myoblasts to more mature myocytes, leading to the classification of RMS into distinct subtypes, including embryonal, alveolar, and pleomorphic variants.
RMS predominantly affects young dogs, with a peak incidence in those ≤2 years of age. It is characterized by its aggressive clinical behavior, marked by a high incidence of local recurrence and distant metastasis. The age distribution, however, varies among the different RMS subtypes. For example, botryoid and alveolar embryonal RMS predominantly occur in dogs younger than 2 years, with over 60% of cases of botryoid embryonal and alveolar RMS, and approximately 50% of embryonal RMS cases, occurring in this age group. In contrast, non-otherwise specified (RMS-NOS) and pleomorphic RMS are more commonly diagnosed in adult and older dogs. Additionally, there is evidence of a sex predisposition, with females being more likely to develop the botryoid variant of embryonal RMS.
In terms of anatomical location, approximately 49% of RMS cases are found in the urinary bladder, with the botryoid variant being the most common subtype in this site. Other common locations include the head, neck, and face (37%), followed by the hip/pelvis/spinal column (3%), appendicular skeleton (8%), and skin (3%). Prognosis is largely dependent on the extent of local invasion and metastasis at the time of diagnosis, with an overall metastatic rate of 50%. The botryoid variant of RMS, however, has a lower metastatic rate of approximately 27%, suggesting a more favorable prognosis compared to other subtypes.
Embryonal RMS is the most frequently reported subtype in veterinary oncology and is characterized by three primary cellular variants: rhabdomyoblastic, myotubular, and spindloid. The rhabdomyoblastic variant consists predominantly of large, round cells with minimal evidence of cross-striation. The myotubular variant is composed of multinucleated, strap-like cells with frequent cross-striations, while the spindloid variant is characterized by fusiform to plump spindle cells arranged in streams. The botryoid variant of embryonal RMS is a well-recognized form, primarily affecting the trigone area of the urinary bladder in large-breed dogs. This variant is characterized by spindle to stellate cells embedded in a myxoid stroma, with the presence of elongated multinucleated myotube-like cells and undifferentiated myoblasts.
Alveolar RMS, the second most common subtype, is typically composed of small, undifferentiated round cells that may resemble lymphoid cells. Multinucleated cells are less commonly observed but may be present. Alveolar RMS is further subdivided into classic and solid patterns. Due to their morphological similarity to other small round cell neoplasms seen in human childhood cancers, alveolar, embryonal rhabdomyoblastic, and undifferentiated RMS are often grouped within the small round cell neoplasms of childhood.
RMS tumors that do not exhibit characteristics of the embryonal or alveolar subtypes are classified as pleomorphic RMS. This variant predominantly affects adult and older dogs, often involving major muscle groups such as the thigh. Pleomorphic RMS is characterized by the presence of haphazardly arranged, plump spindle cells, multinucleated “strap” cells, and large rhabdomyoblasts. It is often misclassified as anaplastic sarcoma due to its variable morphology and the absence of distinctive histological features.
Definitive diagnosis of RMS can be challenging based on cytological findings alone, given the tumor’s pleomorphic nature and variable differentiation into skeletal muscle. While the presence of cross-striations or strap-like cells may raise suspicion for RMS, cytology alone is insufficient for a definitive diagnosis. Histopathology, combined with immunohistochemistry (IHC) and transmission electron microscopy (TEM), is typically required for confirmation. Special stains, such as phosphotungstic acid hematoxylin (PTAH), can be employed to highlight striations and aid in diagnosis. Commonly used immunohistochemical markers for RMS include desmin, vimentin, muscle actin, sarcomeric actin, myoblast determination protein 1 (MyoD1), myoglobin, and myogenin. Although variability in marker expression is observed, the detection of at least one muscle-specific marker, in the absence of smooth muscle markers, is generally sufficient to establish a diagnosis of RMS. TEM remains the gold standard for confirming the diagnosis, though its availability may be limited.
The cytological features of RMS vary depending on the subtype and variant. In embryonal RMS, the morphology ranges from myoblasts to myotubular cells within a mucinous stroma. The myotubular variant is characterized by multinucleated, elongated, strap-like cells, while the rhabdomyoblastic variant consists of large round cells with abundant eosinophilic cytoplasm. The spindle cell variant features fusiform, elongated cells arranged in streams. Alveolar RMS is characterized by small, atypical round cells resembling lymphoid cells, often with frequent mitotic activity. Pleomorphic RMS is typified by plump, haphazardly arranged spindle cells and marked nuclear atypia, with frequent atypical mitotic figures.
Approximately 2-3 months following surgical resection, the dog in this case became lethargic, and thoracic radiographs revealed the presence of two large lung masses. Given the rapid clinical deterioration and the associated poor prognosis, the owners elected for humane euthanasia.
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