October 2019




Signalment (JPC # 1957907): Age and breed unspecified dog




HISTOPATHOLOGIC DESCRIPTION: 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 on a preexisting collagenous stroma. Neoplastic cells are round with distinct cell borders, moderate amounts of amphophilic cytoplasm that occasionally contains 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 2.37 mm2. Scattered throughout the neoplasm are moderate to high numbers of eosinophils and fewer lymphocytes, plasma cells, and hemosiderin-laden macrophages. Multifocally, lymphatic vessels are ectatic (edema) and blood vessels are congested.



Luna mast cell stain: Neoplastic cells contain numerous intracytoplasmic metachromatic granules.


MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Mast cell tumor, low grade, breed unspecified, canine



Signalment (JPC # 2924576): 13-year-old Standardbred gelding


HISTORY: A subcutaneous mass on the right flank


HISTOPATHOLOGIC DESCRIPTION: Subcutis (per contributor): Infiltrating, separating, and surrounding collagen bundles is an unencapsulated, poorly- circumscribed neoplasm of low cellularity composed of sheets of neoplastic mast cells separated by abundant collagen and large foci of lytic necrosis. 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 count is 1 per 2.37 mm2. There are multiple large, irregular, up to 5 mm diameter areas of lytic necrosis composed of brightly eosinophilic cellular and karyorrhectic debris, often degenerate neoplastic mast cells, and multifocal brightly eosinophilic, hyalinized bands of collagen that are bordered by deeply basophilic fragmented mineral (collagen flame figures). Necrotic foci are bordered by dense fibrous connective tissue admixed with fine, beaded, amphophilic mucin, epithelioid macrophages, and multinucleated giant cells (foreign body and Langhans type). Scattered throughout the neoplasm are occasional eosinophils and foci of hemorrhage, fibrin, and edema admixed with moderate numbers of hemosiderin-laden macrophages.


MORPHOLOGIC DIAGNOSIS: Subcutis (per contributor): Mast cell tumor, Standardbred, equine



Signalment (JPC #4014162-02): 13 year old, spayed female domestic shorthair cat


HISTORY: Several cutaneous masses; this mass on the left shoulder


HISTOPATHOLOGIC DESCRIPTION (Slide D): Expanding the dermis, elevating the overlying epidermis, and separating adnexa is an unencapsulated, well-demarcated, densely cellular neoplasm composed of round cells arranged in sheets on a scant to moderate pre-existing collagenous matrix. Neoplastic cells have distinct cell borders, a scant to abundant amount of eosinophilic cytoplasm, and a round to oval nucleus with coarsely clumped chromatin and 1-2 prominent nucleoli. Anisocytosis and anisokaryosis are marked and there are frequent karyomegalic and pleomorphic cells. Mitotic count is1 per 2.37 mm2. Entrapped collagen fibers are brightly eosinophilic and hyalinized. There are multifocal dense perivascular lymphocytic infiltrates.


MORPHOLOGIC DIAGNOSIS: Pleomorphic mast cell tumor


SYNONYMS: Mastocytoma, cutaneous mastocytosis



·      Mast cell tumors:

·      Common cutaneous round cell neoplasms of dogs and cats and occasionally occur in horses, cattle, pigs, and ferrets; there is species variation in location and biological behavior

·      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 syndromes are 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, also known as CD117)

·      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

·      Mutations in the c-kit proto-oncogene that codes for KIT protein (stem cell factor receptor) may be responsible for development or progression of mast cell tumors

·      Mast cell tumors with mutations in exon 8 (extracellular ligand-binding domain) or 11 (negative regulatory juxtamembrane domain) have increased therapeutic success with tyrosine kinase inhibitors

·      Mast cell tumors with activating duplication in exon 11 have shorter disease-free interval



·      Dogs:

·      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

·      Horses:

·      Usually adult males; no breed predilection

·      Foals may have multiple tumors resembling urticaria pigmentosa of humans

·      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)



·      Solitary to multicentric, dermal to subcutaneous, nodular to pedunculated masses

·      Ulceration is common in larger tumors

·      May be erythematous and edematous

·      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



·      An unencapsulated, variably circumscribed neoplasm composed of sheets of well- to poorly-differentiated mast cells

·      A Grenz zone is typically present

·      Mature eosinophils are often scattered diffusely among neoplastic cells

·      +/- Collagen flame figures (old term: collagen degeneration, collagenolysis)

·      Horses:

·      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

·      Cats: Mastocytic and histiocytic (atypical) types

·      Mastocytic type:

· Uniform, with poor granularity that does not consistently stain well with toluidine blue or Giemsa

· Giant nuclei, binucleated, and trinucleated cells can be seen and are not a prognostic indicator

· Mitotic rate is the best mitotic indicator

·      Histiocytic (atypical) type:

· Tumor cells resemble histiocytes

· Stain poorly with toluidine blue and Giemsa

· May be mistaken for granulomatous inflammation

· Similar to behavior to mastocytic type


Grading Systems:

·      2 grading systems commonly used; changing the grading system from 3 (Patnaik) to 2-tiers (Kiupel) by re-classifying Grade 2 into either Grade 1 or 3 predicts survival probability well for canines:

·      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

· Survival rate of approximately 90%

· 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

· Have a low to moderate metastatic rate and 3.5 year survival rate of approximately 55%

· 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

· Have a high metastatic rate and a 3.5 year survival rate of approximately 15%


Grade 1

Grade 2

Grade 3


Well-differentiated; medium-sized; distinct granules

Moderate; fine, distinct granules

Poorly- differentiated;

indistinct or no granules



round, condensed chromatin; no nucleoli

Mild pleomorphism;

round to indented

scattered chromatin;

occasional nucleoli;

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

Aggressive features


Occasional edema; necrosis

Edema; necrosis common

· 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)

· Surgical recommendations:

· 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

·      Kiupel (2-Tier) grading scale for cutaneous MCTs (high grade vs. low grade):

· Mitotic index is distinct indicator of clinical behavior

· High-grade MCT: Presence of any one of the following:

· 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



·      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



·      Cytology:

·      Granules may not stain with Diff-Quik

·      A recent study (Camus Vet Pathol. 2016) found good correlation between a cytologic 2-tier grading scheme and histologic 2-tier grade diagnosis; high-grade MCT was diagnosed if mast cells were poorly granulated or had at least 2 of:

·      Mitotic figures, binucleated or multinucleated cells, nuclear pleomorphism, or >50% anisokaryosis

·      Histochemistry: Giemsa, toluidine blue, Luna mast, and the periodic acid-Schiff (PAS) reaction highlight metachromatic granules

·      Immunohistochemistry:

·      Mast cell markers: CD117 (c-kit), mast cell tryptase

· Prognostic markers: Increased expressions of argyrophilic nucleolar staining organizing regions (AgNOR), proliferating cell nuclear antigen (PCNA), and Ki-67 have been associated with a poorer prognosis



·      Other round cell neoplasms (for grade III, poorly differentiated mast cell tumors):

·      Lymphoma

·      Plasmacytoma

·      Histiocytoma

·      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)



·      Ferrets (Vilalta; J Comp Pathol; 2016):

·      Similar in morphology and behavior to well-differentiated feline mast cell tumors

·      Frequently associated with crusting, commonly located on the extremities and trunk

·      MC granules failed to stain with toluidine blue on histological sections

·      KIT expression, eosinophilic infiltration, and “collagenolysis” noted

·      Cattle: Most occur in the skin; usually multiple; malignant; and have a high metastatic potential

·      Case series reporting mastocytosis in the livers of 5 Japanese black cattle discovered at slaughter (Ohfuji; Vet Pathol; 2018)

·      Horses:

·      There is a subgroup of equine cutaneous mast cell tumors that are poorly differentiated and have a correlation between KIT xpression and potentially aggressive behavior (Ressel; J Comp Pathol; 2015)

·      Case series reporting mast cell tumors with large, pleomorphic cells with abundant granular cytoplasm; IHC and special stains confirmed mast cell origin (Elbahi; J Comp Pathol; 2018)

·      Cats:

·      A newly recognized entity of epitheliotropic mastocytic conjunctivitis has been reported; apparently benign (Beckwith-Cohen; Vet Pathol; 2017)

·      A proposed grading scheme classifies cutaneous tumors based on mitotic count in 10 400x fields and two of the following criteria: >1.5cm diameter, irregular nuclear shape, nucleolar prominent/chromatin clusters (Sabattini; Vet Pathol; 2019)

·      Stem cell factor (the ligand for Kit) expression may be related to tumorigenesis (Sakurai; J Comp Pathol; 2018)

·      Swine: Rare, single or multiple, cutaneous or visceral neoplasms that typically occur in young (6-18 month old) pigs; mast cell nodules have been described as tumors and as inflammatory aggregates, perhaps in response to Eperythrozoon

·      Possums: Case series found disseminated mast cell tumors as spontaneous neoplasms in Virginia opossums (Pope; J Vet Diagn Invest; 2017)



1.     Abbas AK, Aster JC, Kumar V, et. al. Diseases of the immune system. In: Kumar V, Abbas AK, Aster JC, eds. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Philadelphia, PA: Elsevier Saunders; 2015:202-204.

2.     Beckwith-Cohen B, Dubielzig RR, Maggs DJ, Teixeira LB. Feline epitheliotropic mastocytic conjunctivitis in 15 cats. Vet Pathol. 2017;54(1):141-146.

3.     Camus MS, Koehler JW, Priest HL, et. al. Cytologic criteria for mast cell tumor grading in dogs with evaluation of clinical outcome. Vet Pathol. 2016; 53(6):1117-1123.

4.     Elbahi A, Kipar A, Ressel L. Histocytic-like atypical mast cell tumours in horses. J Comp Pathol. 2018;162:14-17.

5.     Hargis AM, Meyers S. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2016:755, 1119, 1297, 1248.

6.     Mauldin 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: Saunders Elsevier; 2016: 519, 730-732.

7.     Ohfuji S. Hepatic mastocytosis in Japanese black cattle. Vet Pathol. 2018;55(1):182-186.

8.     Pope JP, Donnell RL. Spontaneous neoplasms in captive Virginia opossums (Didelphis virginiana): a retrospective case series (1989-2014) and review of the literature. J Vet Diagn Invest 2017;29(3):331-337.

9.     Ressel L, Ward S, Kipar A. Equine cutaneous mast cell tumors exhibit variable differentiation, proliferation activity, and KIT expression. J Comp Pathol. 2015;153(4):236-243.

10.  Sabattin S, Bettini G. Grading cutaneous mast cell tumors in cats. Vet Pathol. 2019;56(1):43-49.

11.  Sabattini S, Scarpa F, Berlato D, Bettini G. Histologic grading of canine mast cell tumor: is 2 better than 3? Vet Pathol. 2015;52(1):70-73.

12.  Sakurai M, Iwasa R, Sakai Y, Chambers JK, Uchida K, Morimoto M. Expression of stem cell factor in feline mast cell tumor. J Comp Pathol. 2018;163:6-9.

13.  Morrison JA, Pearl DL, Thompson JJ, et. al. Receptor tyrosine kinase expression profiles in canine cutaneous and subcutaneous mast cell tumors. Vet Pathol. 2016;53(3):548-558.

14.  Vilalta L, Melendez-Lazo A, Doria G, Ramis A, Solano-Gallego L, Pastor J, Martorell J. Clinical, cytological, histological and immunohistochemical features of cutaneous mast cell tumours in ferrets (Mustela putorius furo). J Comp Pathol. 2016;155(4):346-355.

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