Author: Dante Meza Ruiz, DVM, M.Ed., DVSc, Dipl. ACVP/ASVCP

Case presentation

A 13-year-old spayed female domestic shorthair cat was presented to a veterinary clinic with a history of a large mass, approximately 8 – 10 cm in size, with an irregular texture. The mass was located on the left caudal border of the scapula and exhibited firm attachment, extending approximately 2 cm across the spine. A fine-needle aspiration (FNA) was performed, and the sample was submitted to NationWide Laboratories for evaluation.

Cytologic findings

The cytologic evaluation revealed a poorly cellular and mildly hemorrhagic sample, characterised by low numbers of poorly preserved, often lysed neutrophils, a few vacuolated macrophages, and rare small lymphocytes and plasma cells within a dense eosinophilic proteinaceous background (Figure 1).

Figure 1: Photomicrograph of direct smears from a fine-needle aspirate of a mass in a cat, stained with Leishman stain. Low numbers of poorly preserved neutrophils and a few vacuolated macrophages within a dense eosinophilic proteinaceous background, x40 objective.

Findings were consistent with the aspiration of proteinaceous fluid and mild mixed inflammation, predominantly neutrophilic, potentially associated with a cyst or cystic neoplasm. However, the exact identity of the lesion could not be determined from the sample.

A few days later, a second FNA was performed, and cytologic evaluation revealed a densely eosinophilic, proteinaceous background containing numerous neutrophils and highly vacuolated macrophages. Additionally, several spindle-shaped cells were observed, arranged individually, in loose aggregates, and occasionally associated with pink extracellular material. These spindle-shaped cells exhibited moderate to large amounts of wispy grey-blue cytoplasm with variably distinct borders, oval nuclei with finely stippled chromatin, and one to multiple prominent round nucleoli. Marked anisocytosis and anisokaryosis were noted, along with frequent giant multinucleated cells, some containing more than 20 nuclei. Occasional karyomegaly, anisonucleosis, and bizarre mitotic figures were also identified (Figures 2-5: Photomicrograph of direct smears from a fine-needle aspirate of a mass in a cat, stained with Leishman stain).

Figure 2: Several atypical spindle-shaped cells arranged individually and in loose aggregates, with frequent binucleation and multinucleation observed, x20 objective.

Figure 3: A cluster of atypical spindle-shaped cells associated with pink extracellular material, x40 objective.

Figure 4: Atypical spindle-shaped cells, with binucleated and multinucleated cells present. These cells have oval nuclei with finely stippled chromatin and one to multiple prominent round nucleoli, x40 objective.

Figure 5: Presence of a giant multinucleated cell, x60 objective.

Interpretation

A diagnosis of sarcoma with mixed inflammation, predominantly neutrophilic, was established. Considering the location, pronounced cellular pleomorphism, marked dysplastic features, and the presence of multinucleated giant cells, the primary differentials include injection-site sarcoma and undifferentiated pleomorphic sarcoma. However, the possibility of a reactive process or other mesenchymal tumours was not excluded.

Discussion

Soft tissue sarcomas (STS) are a frequently diagnosed group of malignant tumours in domestic cats, encompassing various histological subtypes and exhibiting diverse biological behaviours. The terminology surrounding these tumours can be challenging, due in part to the extensive range of terms used in the literature. Additionally, the existence of feline injection-site sarcomas (FISS) further complicates the classification and understanding of these neoplasms in cats1.

FISS are among the most serious vaccine-associated adverse events. These tumours typically develop at sites commonly used for vaccinations and injections, including the interscapular region, the lateral thoracic or abdominal wall, the lumbar region, and the area around the semimembranosus and semitendinosus muscles. The exact pathogenesis of these sarcomas remains unclear, and there is no definitive causal relationship or direct link with vaccination. However, chronic inflammatory reactions at the injection site are believed to serve as a trigger for subsequent malignant transformation2.

Most feline injection-site sarcomas (FISS) are fibrosarcomas, although other malignancies, including osteosarcomas, chondrosarcomas, rhabdomyosarcomas, malignant fibrous histiocytomas, and myofibroblastic sarcomas, have been rarely reported. FISS are generally more aggressive than sarcomas at other sites. The metastasis rate is relatively low, with the lungs being the most common site of metastasis, followed by regional lymph nodes and abdominal organs3.

The prognosis of FISS is primarily influenced by tumour size, with radical excision being crucial for a favourable outcome. Prognosis is also affected by the tumour’s location, its accessibility to surgery, and the ability to achieve tumour-free margins. Therefore, pre-operative diagnostic imaging is essential to assess the full extent of the tumour. Prognosis improves when radical surgery is combined with additional therapeutic options, such as radiotherapy or immunotherapy3.

References

1. Dobromylskyj M. Feline Soft Tissue Sarcomas: A Review of the Classification and Histological Grading, with Comparison to Human and Canine. Animals (Basel). 2022 Oct 12;12(20):2736. doi: 10.3390/ani12202736. PMID: 36290122; PMCID: PMC9597747.
2. Hartmann K, Day MJ, Thiry E, Lloret A, Frymus T, Addie D, Boucraut-Baralon C, Egberink H, Gruffydd-Jones T, Horzinek MC, Hosie MJ, Lutz H, Marsilio F, Pennisi MG, Radford AD, Truyen U, Möstl K; European Advisory Board on Cat Diseases. Feline injection-site sarcoma: ABCD guidelines on prevention and management. J Feline Med Surg. 2015 Jul;17(7):606-13. doi: 10.1177/1098612X15588451. PMID: 26101312; PMCID: PMC11148925.
3. Hartmann K, Egberink H, Möstl K, Addie DD, Belák S, Boucraut-Baralon C, Frymus T, Lloret A, Hofmann-Lehmann R, Marsilio F, Pennisi MG, Tasker S, Thiry E, Truyen U, Hosie MJ. Feline Injection-Site Sarcoma and Other Adverse Reactions to Vaccination in Cats. Viruses. 2023 Aug 8;15(8):1708. doi: 10.3390/v15081708. PMID: 37632050; PMCID: PMC10459272.