A 12-year old spayed female Cocker spaniel presented with an approximately 4x4x1cm, alopecia, nodular mass to the left of the vulva. The mass was present for an unknown period of time as the dog was recently obtained by a rescue group. The mass was sampled using a non-aspiration/pincushion technique and three slides were submitted.
The moderately to highly cellular slides had a clear background, few too many red blood cells, and a nucleated cell population largely predominated by spindle to stellate shaped cells found individually and in small to large aggregates. The cells had a moderate amount of lightly to moderate basophilic cytoplasm with indistinct, wispy cytoplasmic borders and few punctate vacuoles. The nuclei were round to oval with a coarsely stippled chromatin pattern and 1-3 prominent nucleoli. Anisokaryosis were mild to moderate; bi- and multinucleation was occasionally noted. See photos below.
The cytologic diagnosis was a perivascular wall tumor or nerve sheath tumor, with a comment stating that the cytomorphology was most consistent with hemangiopericytoma or other soft tissue sarcoma (other perivascular wall tumor, nerve sheath tumor). Some perivascular wall tumors have a benign biologic behavior (Avallone et al, 2007. The spectrum of canine cutaneous perivascular wall tumors: morphologic, phenotypic and clinical characterization, Vet Pathol 44(5):607-20). Histopathology was recommended to refine and solidify the diagnosis.
While fibrosarcoma (FSA) cannot be definitively ruled out based on this cytologic evaluation, FSA tumor cells typically do not exhibit the very pronounced ‘veiling’ cytoplasmic appearance found in these images. Further, FSAs are typically associated with a small to moderate amount of extracellular pink matrix, which is not typical of most perivascular wall tumors.
A biopsy was performed, and histopathology confirmed a hemangiopericytoma.
Hemangiopericytomas are a fairly common skin/subcutaneous tumor in dogs and are derived from pericytes, a cell type within blood vessel walls, adjacent to endothelium. Although not common, these tumors can be infiltrated by many lymphocytes which can be found upon cytologic evaluation. While most other mesenchymal tumors do not exfoliate readily, aspiration/non-aspiration techniques typically result in moderately cellular slides from this tumor. It is common for these tumors to exhibit mild to occasionally moderate anisokaryosis; binucleation and occasionally multinucleation in which the multiple nuclei can form a circular pattern can also be found.
While metastasis is rare, hemangiopericytomas are commonly very locally invasive and extremely challenging to excise with clean margins. If fine needle aspiration of a canine cutaneous or subcutaneous tumor results and numerous spindle to stellate cells a similar to those found here, consultation with a boarded surgeon and oncologist is recommended.
Image 1. Numerous spindle to stellate shaped cells with large oval nuclei and veiling cytoplasm
Image 2. More spindle to stellate shaped cells with veiling cytoplasm, few with small punctate vacuoles
Image 3. Spindle to stellate shaped cells exhibiting anisokaryosis and occasional binucleation