JPC SYSTEMIC PATHOLOGY
Signalment (JPC # 1957907): Age and breed unspecified dog
HISTOPATHOLOGIC DESCRIPTION (Slide A): Haired skin and subcutis: Infiltrating the dermis and subcutis, elevating the epidermis, separating and surrounding adnexa, skeletal muscle fibers and collagen bundles, and extending to the margins of the submitted tissue is an unencapsulated, poorly-circumscribed neoplasm composed of sheets of neoplastic mast cells. Neoplastic cells are round with distinct cell borders, moderate amounts of amphophilic cytoplasm that occasionally contain fine basophilic granules, and generally centrally located, round nuclei with coarsely stippled chromatin and 1 to 2 variably distinct nucleoli. The mitotic count averages 1 per 10 400X hpf. Scattered throughout the neoplasm are moderate to high numbers of eosinophils and fewer lymphocytes, plasma cells, and hemosiderin-laden macrophages. Multifocally, there are ectatic lymphatic vessels (edema) and blood vessels are congested.
(Slide B) Luna mast cell stain: Neoplastic cells contain numerous metachromatic granules.
MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Mast cell tumor, low grade (grade II), breed unspecified, canine
Signalment (JPC # 2924576): 13-year-old standardbred gelding
HISTORY: A subcutaneous mass on the right flank
HISTOPATHOLOGIC DESCRIPTION (Slide C): Subcutis (per contributor): Infiltrating, separating, and surrounding collagen bundles is an unencapsulated, poorly- circumscribed, relatively paucicellular neoplasm composed of sheets of neoplastic mast cells. Neoplastic cells are round with distinct cell borders, moderate amounts of amphophilic cytoplasm that often contain fine basophilic granules, and centrally located, oval nuclei with coarsely stippled chromatin and indistinct nucleoli. There is moderate anisocytosis and anisokaryosis. The mitotic rate is 1 per 10 hpf. There are multiple large, irregular, up to 5 mm diameter areas of necrosis composed of brightly eosinophilic cellular and karyorrhectic debris, neoplastic mast cells, and multifocal brightly eosinophilic, hyalinized bands of collagen (collagenolysis), that are bordered by deeply basophilic fragmented mineral, and dense fibrous connective tissue, admixed with fine, beaded amphophilic mucin, epithelioid macrophages, and multinucleated giant cells. Scattered throughout the neoplasm, there is hemorrhage, moderate numbers of hemosiderin-laden macrophages, and occasional eosinophils.
MORPHOLOGIC DIAGNOSIS: Subcutis (per contributor): Mast cell tumor, standardbred, equine
SYNONYMS: Mastocytoma, cutaneous mastocytosis
- Mast cell tumors are common cutaneous round cell neoplasms of dogs and cats, which occasionally occur in horses, cattle, pigs, and ferrets; there is species variation in location and biological behavior
- Mast cell tumors occasionally develop in the intestine, liver, spleen, or elsewhere either de novo or as metastasis from a cutaneous site
- Solitary and multiple mast cell tumors, and cutaneous and systemic mastocytosis recognized in dogs and cats
Normal Mast Cell Production and Function:
- Mast cells originate from hematopoietic precursors in bone marrow that undergo differentiation and maturation in tissues under the influence of primarily IL-3, IL-4, IL-6 and the c-kit ligand (specific KIT receptors for the stem cell factor)
- They are the mediators of type I hypersensitivity and may also play an important role in initiation of innate immune responses
- Mast cells have high-affinity membrane receptors for the Fc portion of IgE, and cross-linking of IgE molecules on these receptors by specific antigens causes signal transduction, resulting in degranulation of primary (pre-formed) mediators and de novo synthesis of secondary mediators
- The binding of C5a and C3a (anaphylatoxins) may also trigger mast cell release of mediators
- Primary mediators include biogenic amines (histamine, adenosine and serotonin), chemotactic factors (eosinophil chemotactic factor and neutrophil chemotactic factor), enzymes (lead to generation of kinins and activation of complement), and proteoglycans (heparin, chondroitin sulfate)
- Secondary mediators include leukotrienes (LTC4, LTD4, B4), prostaglandin D2, platelet activating factor, and cytokines (TNF-alpha, IL-1, 3, 4, 5 and 6)
- The combined effects of these mediators are edema, mucus secretion, smooth muscle spasm, and the recruitment of additional leukocytes
- Pathogenesis is unknown
- There may be an underlying genetic defect in certain predisposed breeds
- Mutations in the c-kit protooncogene that codes for KIT protein (stem cell factor receptor) may be responsible for development or progression of mast cell tumors
- In canine mast cell tumors, the c-kit mutations frequently involve the negative regulatory juxtamembrane domain
- KIT expression is inversely related to the degree of differentiation of canine mast cell tumors (increased KIT = less differentiated tumor)
- p53 abnormalities, loss of p27 expression, and aberrant p21 expression have also been associated with canine mast cell tumor development and progression
- MMP-2, MMP-9 and VEGF-A increased and TIMP-2 decreased with increasing histologic grade suggesting a role in tumor invasion
- COX-2 contributes to the development of malignancies through inhibition of cellular apoptosis, promotion of tumor angiogenesis, increasing tumor cell motility and invasiveness, immunomodulatory activity, and the conversion of procarcinogens to carcinogens
TYPICAL CLINICAL FINDINGS:
- Typically adult dogs (average 8 years old); no sex predilection; all are considered potentially malignant
- Boxers, Boston terriers, English bulldogs, fox terriers, bull terriers, Labrador retrievers, dachshunds, beagles, pugs, golden retrievers, weimaraners and shar peis are at increased risk
- Shar peis develop lesions at a younger age and are more likely to develop multiple lesions
- Usually adult males; no breed predilection
- Foals may have multiple tumors resembling urticaria pigmentosa of humans
Subgroup of poorly differentiated equine cutaneous MCT in which aberrant KIT expression is associated with high proliferative rate and potential aggressive behavior Cats:
- Solitary and multiple mast cell tumors, and cutaneous and systemic mastocytosis recognized
- Typically older cats (between 9-11 years)
- Solitary or multiple nodules; usually benign; usually on head or neck
- Morphologic features usually associated with malignancy such as pleomorphism and infiltrative growth do not correlate with malignant behavior
- Siamese cats are predisposed to atypical poorly granulated mast cell tumors (histiocytic variant)
- An association between feline immunodeficiency virus (FIV) infection and the development of multiple cutaneous mast cell tumors has been reported
TYPICAL GROSS FINDINGS:
- Solitary to multicentric, dermal to subcutaneous, nodular to pedunculated masses
- Ulceration is common in larger tumors
- May be erythematous and edematous (wheal formation - Darier sign)
- Canine lesions are most commonly found on the trunk and limbs and less frequently on the head
- Equine lesions typically occur on the head (lips, nostrils, jaw, and periorbital), neck, trunk, and limbs
- Gastroduodenal ulceration due to excess histamine release may occur in dogs
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- An unencapsulated, variably circumscribed neoplasm composed of sheets of well- to poorly-differentiated mast cells
A Grenz zone is typically present; may be epitheliotropic
- Well-differentiated neoplastic cells; often involves the subcutis; numerous eosinophils (often more numerous than neoplastic mast cells) and focal to multifocal areas of necrosis and mineralization are frequently present
- Edema, fibrosis, tissue eosinophilia, mucinous stroma and collagenolysis are common; necrosis may be present
- Patnaik grading scale:
- Grade 1/Well-differentiated: Small, well-circumscribed, and typically within the superficial dermis; resembles normal mast cells; few mitotic figures; and moderate to high numbers of eosinophils
- Grade 2/Intermediate: Larger, deeper, often involve the subcutis, and less well- circumscribed; larger, slightly pleomorphic neoplastic cells; low to moderate mitotic activity; fewer eosinophils; and may be ulcerated
- Grade 3/Poorly-differentiated: Large, ulcerated, poorly-circumscribed and often extend into the subcutis; granules are often indiscernible without special stains; cells are highly pleomorphic; mitotic rate is moderate to high and many atypical mitoses are present; and eosinophil numbers are usually scant to low
Well-differentiated; medium-sized; distinct granules
Moderate; fine, distinct granules
indistinct or no granules
round, condensed chromatin; no nucleoli
round to indented
binucleated and giant cells
Large; pleomorphic; fine chromatin; 1 or more nucleoli;
binucleated and multinucleated giant cells are common
Mitotic index average/hpf
Superficial dermis→ interfollicular
Deep dermis to SQ
SQ to deeper tissues
Occasional edema; necrosis
Edema; necrosis common
- Patnaik grading scale biologic behavior (strong correlation with neoplastic grade; dogs):
- Grade 1: Rarely recur and have a 3.5 year survival rate of approximately 90%
- Grade 2: Have a low to moderate metastatic rate and 3.5 year survival rate of approximately 55%
- Grade 3: Have a high metastatic rate and a 3.5 year survival rate of approximately 15%
- Changing the grading system to 2-tiers by re-classifying Grade 2 into either Grade 1 or 3 reportedly predict survival probability well for canines.
- Mitotic index is an indirect measure of cellular proliferation and is a strong predictor of outcome/metastasis, especially in grade II category (more than 5 mitotic figures per 10 hpf equal to decreased prognosis/survival time)
- Two to three centimeter wide lateral and one fascial plane deep surgical margins are recommended for grades II and III
- Modified proportional margins surgical technique: Surgical margins of the widest diameter of the MCT are measured (e.g., 1.5 cm in diameter; the lateral marginused for excision is 1.5 cm); 85% of MCTs were deemed to have been excised with clear margins using this technique
- Increased expressions of argyrophilic nucleolar staining organizing regions (AgNOR), proliferating cell nuclear antigen (PCNA), and Ki-67 have been associated with a poorer prognosis
- 2-Tier histologic grading system for cutaneous MCTs (high grade vs. low grade)
- High-grade MCT: At least 7 mitotic figures in 10 hpf; at least 3 multinucleated (3 or more nuclei) cells in 10 hpf; at least 3 bizarre nuclei (indented, segmented) in 10 hpf; karyomegaly (nuclear diameters of at least 10% of neoplastic cells vary by at least two-fold)
- High-grade MCTs were significantly associated with shorter time to metastasis or new tumor development, and with shorter survival time (median survival time was less than 4 months for high-grade MCTs but more than 2 years for low-grade MCTs)
- No association between tumor grade and occurrence of multiple tumors
- Subcutaneous MCTs
- Often categorized as grade II or higher (Patnaik scale)
- Decreased survival time linked to mitotic index (more than 4 mitotic figures per 10 hpf); infiltrative growth pattern; presence of multinucleated cells
- The majority of subcutaneous MCTs have a favorable prognosis, with extended survival times and low rates of reoccurrence and metastasis
Lymph node cytology:
- Metastasis to a lymph node should be diagnosed by cytology if mast cells represent more than 3% of the cell population in the aspirate (25% of dogs would be diagnosed with a metastatic MCT)
- Evaluation of the degree of cytological differentiation and clustering or aggregation of cells should be considered in the final cytological diagnosis of metastatic disease; surgical excision and histological assessment of the lymph node should be considered in suspicious cases
- Resemble normal mast cells
- Cytoplasm contains many membrane bound granules, which appear as single or fused vesicles that lack microvilli; fine fibrillar material forms a loose network within these vesicles
ADDITIONAL DIAGNOSTIC TESTS:
- Cytology: Granules may not stain with Diff-Quik
- Histochemistry: Giemsa, toluidine blue, Luna mast and the periodic acid-Schiff (PAS) reaction highlight metachromatic granules
- Immunohistochemistry: CD117 (c-kit), mast cell tryptase; CD25 (increased in grade III canine cutaneous MCTs)
Other round cell neoplasms (for grade III usually, since they are poorly differentiated):
- Transmissible venereal tumor
Horse (for eosinophilic inflammation):
- Equine nodular collagenolytic granuloma
- Cutaneous habronemiasis: Habronema muscae, Habronema microstoma, Draschia megastoma (spiruid nematodes)
- Cutaneous pythiosis: Pythium insidiosum (fungal-like Oomycete)
- Cutaneous mast cell tumors occur most commonly in older animals, with Siamese cats and possibly males predisposed
- They are usually benign, superficial, well-circumscribed, and multicentric, with a predilection for the head and neck
- Normal and neoplastic feline mast cells have a "fried egg" appearance and usually do not have visible granules on H&E
- Collagenolysis and high numbers of eosinophils are not typically present in feline mast cell tumors
- Poor prognosis is associated with mitotic index (strongest predictor; more than 5 mitotic figures per 10 hpf), increased ki67 index, dissemination of the neoplasm (multiple lesions), pleomorphic phenotype (partial/total loss of cytoplasmic granules), and increased KIT labeling
- Pleomorphic subtype: Neoplastic cells with marked anisocytosis, anisokaryosis, nuclear pleomorphism, uninucleate and multinucleate giant cells, and occasional erythrophagocytosis and endocytosis of inflammatory cells; considered benign if the mitotic rate is low; has higher mitotic index, ki67 index, and KIT labeling compared to other categories of feline MCTs
- Histiocytic subtype: Occurs primarily in young Siamese cats (6 weeks to 4 years old); spontaneously regresses; and is composed of large polygonal to round cells with abundant cytoplasm (resembling epithelioid macrophages or granulomatous inflammation), moderate numbers of eosinophils, and lymphoid aggregates
- Ferrets: Similar in morphology and behavior to well-differentiated feline mast cell tumors
- Cattle: Most occur in the skin; usually multiple; malignant; and have a high metastatic potential
- Swine: Rare, single or multiple, cutaneous or visceral neoplasms that typically occur in young (6-18 month old) pigs; mast cell nodules have been descbribed as tumors and as inflammatory aggregates, perhaps in response to Eperythrozoon
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