JPC SYSTEMIC PATHOLOGY

INTEGUMENTARY SYSTEM

September 2025

I-N16B

Slide A

Signalment (JPC 2799364): Age and breed unspecified cat

HISTORY: None

HISTOPATHOLOGIC DESCRIPTION: Subcutis (per contributor): Infiltrating the subcutis and compressing the panniculus carnosus is a 1.0 cm x 0.8 cm, unencapsulated, multilobulated, densely cellular neoplasm composed of spindle cells arranged in long interlacing streams and bundles separated by moderate fibromyxomatous matrix. Neoplastic cells have indistinct borders, a small amount of eosinophilic fibrillar cytoplasm, and an oval to elongate nucleus with finely stippled chromatin and1-3 prominent magenta nucleoli. Anisocytosis and anisokaryosis are moderate and there are 10 mitotic figures per 2.37mm squared. Scattered throughout the section, individual neoplastic cells have shrunken borders with hypereosinophilic cytoplasm and a pyknotic nucleus (individual cell necrosis). There are multifocal peripheral peri-tumoral and perivascular aggregates of macrophages that often contain intracytoplasmic, blue-gray, clumped, granular to globular material (vaccine material). These macrophages are admixed with often nodular aggregates of lymphocytes and fewer plasma cells and neutrophils. Subjacent myocytes are rarely shrunken with angular borders and hypereosinophilic sarcoplasm (atrophy) or swollen with vacuolated sarcoplasm (degeneration).

MORPHOLOGIC DIAGNOSIS: Subcutis (per contributor): Injection site-associated fibrosarcoma, breed unspecified, feline.

SYNONYMS: Feline injection site sarcoma; formerly known as feline vaccine-associated sarcoma, feline postvaccinal sarcoma

Slide B

Signalment (JPC 2800486): A domestic longhair cat

HISTORY: None

HISTOPATHOLOGIC DESCRIPTION: Subcutis (per contributor): Expanding the subcutis is an 8 mm x 10 mm nodule characterized by a central pseudocyst surrounded by variably mature granulation tissue, fibrosis, and inflammation. The central pseudocyst is characterized by a 7 mm x 3 mm irregular area of cellular dropout (necrosis) with replacement by anastomosing trabeculae of eosinophilic, lamellated, amorphous, polymerized fibrin admixed with a small amount of necrotic debris. The pseudocyst is rimmed by loosely arranged reactive fibroblasts often arranged perpendicularly to small caliber blood vessels lined by reactive endothelium (granulation tissue). The pseudocyst is further surrounded by abundant fibrosis and high numbers of large epithelioid macrophages that often contain intracytoplasmic, amphophilic, granular to globular material (vaccine material) admixed with moderate numbers of eosinophils and nodular perivascular aggregates of lymphocytes.

 

MORPHOLOGIC DIAGNOSIS: Subcutis: Panniculitis, granulomatous, focally extensive, marked, with central pseudocyst, granulation tissue, fibrosis, and intrahistiocytic vaccine material, domestic longhair, feline.

CONDITION: Post-injection panniculitis; Vaccine-associated granuloma

GENERAL DISCUSSION:

• Localized injection site reactions are common in domestic species; often associated with vaccines and therapeutics administered subcutaneously
• Malignant tumors composed of fibroblasts

Feline fibrosarcoma:

• Most common feline malignant mesenchymal neoplasm (4^th most common overall); 3 forms:
• Solitary (non-vaccine-associated): Common in older cats
• Arise in dermis or subcutis; dermal typically on digits and pinnae; subcutis typically on trunk and extremities
• Virus-induced from feline sarcoma virus (FeSV): Cause of multicentric fibrosarcoma in cats usually less than 5 years of age; rare
  • FeSV is a replication defective retrovirus; sarcoma formation requires feline leukemia virus (FeLV) as a helper virus > genetic recombination between viruses > induces multiple simultaneous rapidly growing fibrosarcomas
  • Typically locally invasive and metastasize to lung and other sites
• Injection site-associated sarcoma (formerly vaccine-associated sarcoma)
• Post-injection to tumor interval is 3 months to 3.5 years
  • Causes include vaccination (both adjuvanted and non-adjuvanted) but also associated with other injectables, foreign material, suture, trauma, and microchip implantation
  • Locally aggressive with a high rate of local recurrence weeks to months following excision
  • Occasionally metastasize (up to 24%), especially to the regional lymph nodes and lungs
  • Poor prognosis
  • The majority are fibrosarcomas, but other reported injection site-associated (vaccine-induced) sarcomas include (not limited to): osteosarcoma, malignant fibrous histiocytoma, chondrosarcoma, rhabdomyosarcoma, histiocytic sarcoma, and myxosarcoma

PATHOGENESIS:

• Hypothesis: Overzealous reparative response at injection site of inflammation or wound healing or both > malignant transformation of mesenchymal cells
  • Risk potentially influenced by antigen load and degree of inflammation at injection site
• Aluminum adjuvant historically identified in tumor associated macrophages; however, nonadjuvanted vaccines have also been associated with vaccine-associated sarcomas

TYPICAL CLINICAL FINDINGS:

• Median age of cats is 8 years, which is younger than cats with nonvaccine-associated fibrosarcoma (10.5 years); no known breed or sex predilection
• Common vaccination sites: Interscapular, dorsal neck, shoulder, flank, and femoral areas
• Local recurrence following surgical excision is frequent

TYPICAL GROSS FINDINGS:

• Typically well-circumscribed, irregular, firm, multilobular, white mass in the subcutis or skeletal muscle; typically larger (greater than 4 cm in diameter) than non-vaccine associated sarcomas
• Neoplasm often contains a cystic center containing a thin watery or mucinous fluid

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Interlacing streams and intersecting bundles of fibroblastic cells on a fibrous to fibromyxomatous matrix
• Neoplastic myofibroblasts, neoplastic multinucleated giant cells, often prominent cellular pleomorphism, often high mitotic rate
• Features associated with injection site-associated sarcomas (vs. non injection site-associated sarcomas:
  • Macrophages often contain an intracytoplasmic granular to crystalline gray-brown to bluish vaccine/foreign material
  • Prominent peripheral lymphoid aggregates with high proportion of T cells
  • Necrosis is often prominent; necrotic core with micro- or macro-cavitations surrounded by neoplastic cells
  • Tumors are often contiguous with granulation tissue

Vaccine reaction:

• Classically nodular aggregates of predominately lymphocytes arranged around a central core of caseous necrosis
• Macrophages with gray-brown (amphophilic) to bluish vaccine material granular vaccine substance material or often embedded in eosinophilic debris or found within phagocytes
• Strong antigenic stimulus provided by the exogenous antigen sometimes results in the formation of germinal centers; heterogeneity of the cell population and the lack of anaplastic characteristics in the lymphoid cells may differentiate these lesions, from genuine lymphoma

ULTRASTRUCTURAL FINDINGS:

• Spindle cells, giant cells, histiocytoid cells and cells with myofibroblastic features
• Similar to fibroblasts; subplasmalemmal dense attachment of plaques and thin cytoplasmic actin myofilaments

ADDITIONAL DIAGNOSTIC TESTS:

• Cytology: Abundant large, plump spindle cells individually arranged and in aggregates associated with pink, collagenous material with occasional multinucleated giant cells; +/- marked nuclear pleomorphism
• Histochemistry: Masson’s trichrome highlights collagen of fibrosarcomas
  • Both vaccine and non-vaccine associated sarcomas: Vimentin

DIFFERENTIAL DIAGNOSIS:

Injection Site-associated sarcoma:

• Gross differentials:
  • Primary or metastatic neoplasms
  • Inflammatory: Granuloma, abscess, foreign body reaction
  • Immune mediated: Post vaccine reaction
• Histologic differentials:
  • Non vaccine-associated fibrosarcoma (I-N15B): No peripheral lymphocytes / macrophages
• Feline cutaneous lymphomas at injection sites (CLIS): These are predominately large B-cell neoplasms that arise from the subset of recruited lymphocytes associated with injection/response. Histologically they also contain central cavities and mixed inflammation (macrophages, plasma cells, lymphocytes) and may contain foreign material. These may emerge from transformation and clonal expansion of lymphoid cells following persistent immune stimulation with feline leukemia virus (FeLV) reactivation and transformation (Roccabianca et al, 2025)
  • Reactive granulation tissue (especially in limited size biopsies)
  • Giant cell tumor of tendon sheath: Nuclear pleomorphism, abnormal mitoses
  • Hemangiopericytoma: Concentric whorls of spindle cells around capillaries; occurs almost exclusively in dogs
  • Keloidal vaccine-associated fibrosarcoma: Thick bands of hyalinized collagen
  • Leiomyosarcoma: Often retain cigar shaped nucleus
  • Liposarcoma (I-N18C): Lipocytes, pleomorphic, +/- scant collagen
  • Myxosarcoma: Spindloid to stellate cells separated by mucinous material
  • Myofibroblastic sarcoma
  • Nerve sheath tumors (I-N13): Antoni A and B patterns, Verocay bodies; S100, SOX10, NSE, GFAP immunoreactivity

COMPARATIVE PATHOLOGY:

Vaccine- or injection-associated disease:

• Dogs:
  • Vaccine associated sarcomas generally accepted to occur in canines; microchip associated fibrosarcoma also occur
  • Post-rabies vaccine panniculitis (poodle patch, I-M35): Immune-mediated dermatoses with cell-poor, mononuclear vasculitis with ischemic follicular atrophy and lymphocytic panniculitis; can present like lupus panniculitis
• Ferrets: Vaccine associated sarcomas reported
• Sheep: Aluminum-based adjuvant injection induces persistent, sterile, subcutaneous granulomas
• Horse: Injection-site eosinophilic granulomas are progressive (1-3 days post vaccination) nodules with necrotic cores; considered to be a response to the use of silicone-coated hypodermic needles (form of delayed hypersensitivity)
• Birds: Case report of three captive village weaver birds that developed sarcomas with intralesional foreign crystalline material resembling typical injection-site sarcomas in cats after use of autologous Yersinia pseudotuberculosis vaccine (Day et al, 2022).

References:

1. Day CE, Stidworthy MF, Cian F, Barrows M. Injection-Site Sarcoma in Three Village Weaver Birds (Ploceus cucullatus) Associated with Autogenous Yersinia pseudotuberculosis Vaccination. J Comp Pathol. 2022;199:43-50
2. Fisher DJ. Cutaneous and subcutaneous lesions. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and hematology of the dog and cat. 5th ed. St. Louis, MO: Elsevier; 2020:95-96.
3. Löhr CV, Stieger-Vanegas SM, Terry JL, Milovancev M, Medlock J. Targeting Peritumoral Lesions Identified by Computed Tomography and Magnetic Resonance Imaging in Feline Injection-Site Sarcomas for Microscopic Examination. Vet Pathol. 2021;58(5):923-934.
4. Maudlin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016:560, 725.
5. Raskin RE, Conrado FO. Integumentary system. In: Raskin RE, Meyer DJ, ed. Canine and Feline Cytology. 4th ed. St. Louis, MO: Elsevier; 2016:85-86.
6. Roccabianca P, Brunetti B, Dell'Aere S, Turba ME, Godizzi F, De Marino M, Avallone G. Clonality assessment and feline leukemia virus protein expression in injection-site sarcoma-associated lymphocytic infiltrates. Vet Pathol. 2025 7:3009858251367396. doi: 10.1177/03009858251367396.
7. Welle MM, Linder KE. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7^th ed. St. Louis, MO: Mosby Elsevier; 2022:1141,1143,1158.

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