Read-Only Case Details Reviewed: Mar 2010

JPC SYSTEMIC PATHOLOGY
ENDOCRINE SYSTEM
January 2022
R-N10

Slide A: Signalment (JPC #2026123): A cat

HISTORY: This cat had diffusely swollen and enlarged mammary glands along the entire mammary chain.

HISTOPATHOLOGIC DESCRIPTION: Mammary gland: Diffusely, the mammary gland is expanded by variably sized lobules composed of hyperplastic ducts surrounded by multiple layers of myofibroblasts embedded within loose stroma that blends peripherally into dense bands of collagenous stroma. Ducts are lined by 1-3 layers of cuboidal to columnar epithelial cells. The surrounding spindle cells (myoepithelial cells) have indistinct cell borders, a scant amount of eosinophilic fibrillar cytoplasm, and an oval nucleus with finely stippled chromatin and 1-2 nucleoli. Multifocally ducts are filled with small amounts of necrotic debris, sloughed epithelial cells, and few neutrophils. Within the periductal stroma there are moderate numbers of foamy macrophages and fewer lymphocytes and plasma cells. Diffusely the superficial dermis is mildly expanded by increased clear space and ectatic lymphatics (edema). The epidermis is mildly hyperkeratotic.

MORPHOLOGIC DIAGNOSIS: Mammary gland: Hyperplasia, fibroepithelial, diffuse, moderate, breed unspecified cat, feline.

CAUSE: Prolonged elevated levels of progesterone

CONDITION: Mammary fibroadenomatous hyperplasia

SYNONYMS: Fibroepithelial hyperplasia, feline mammary hypertrophy

Slide B: Signalment (JPC #4154528): A nine-year-old spayed-female Siamese cat.

HISTORY: This cat had multiple left mammary chain masses for an unknown duration.

HISTOPATHOLOGIC DESCRIPTION: Haired skin, subcutis, and mammary gland: Expanding the subcutis and dermis, compressing overlying adnexa, and elevating the overlying epidermis is an encapsulated, well demarcated, densely cellular neoplasm composed of neoplastic epithelial cells predominantly arranged in mildly ectactic and tortuous tubules and fewer solidly cellular areas supported by a variably dense fibrovascular to occasionally desmoplastic/scirrhous stroma. Neoplastic epithelial cells have distinct cell borders, a moderate amount of eosinophilic cytoplasm, and a round to oval nucleus with coarse to vesiculated chromatin and one prominent magenta nucleolus. Anisocytosis and anisokaryosis are marked and there are 72 mitotic figures per 10 high power fields (2.37mm²). Tubules are typically lined by a single layer of columnar to cuboidal neoplastic epithelial cells that frequently have palisading nuclei. In other regions tubules are up to four cells thick (pluristratification) with loss of basilar orientation (cellular atypia) and marked nuclear pleomorphism, or are lined by attenuated epithelium. Tubule lumens are variably expanded by lightly basophilic to amphophilic homogenous fluid (secretory product), macrophages with eosinophilic foamy cytoplasm, and sloughed necrotic epithelial cells characterized by hypereosinophilic cytoplasm and pyknotic nuclei. The fibrovascular stroma is multifocally expanded by hemorrhage, fibrin, and edema. There is a focally extensive region of necrosis characterized by both loss of differential staining and retention of cellular architecture (coagulative necrosis) and loss of cellular architecture (lytic necrosis) within the central aspect of the mass; similar smaller necrotic foci are found throughout the remaining regions of viable neoplastic cells. Neoplastic cells multifocally infiltrate into the adjacent subcutis and are separated and surrounded by dense collagenous stroma that contains numerous spindle cells (myofibroblasts). The subcutis adjacent to the neoplasm is multifocally infiltrated by few lymphocytes and plasma cells, and lymphatics are ectatic.

MORPHOLOGIC DIAGNOSIS: Haired skin, subcutis, and mammary gland: Carcinoma, tubular, grade III, infiltrative, Siamese, feline.

CONDITION: Tubular mammary carcinoma

GENERAL DISCUSSION:

Mammary fibroadenomatous hyperplasia:

Benign, nonneoplastic proliferation of mammary ducts and connective tissue of young intact female cats (<2yrs old), pregnant cats, or older neutered male and female cats on prolonged progesterone therapy (megestrol acetate)
Most commonly occurs in the spring and with first several estrus cycles
Regression following ovariohysterectomy or termination of pregnancy or progesterone therapy

Feline mammary neoplasia

Much less common in cats than dogs
Predominantly occurs in queens but also occurs in males
75-90% are carcinomas and majority eventually metastasize
Siamese and other short-haired pure breeds are predisposed
Intact animals at slightly greater risk; cats spayed prior to 1 year of age have much less risk (although association between ovariohysterectomy and incidence is not as strong as in the dog); 7-9 year risk plateau

Criteria of malignancy associated with mammary neoplasms:

Metastasis to regional lymph node(s) (always indicative of malignancy)
Lymphatic invasion (always indicative of malignancy)
Infiltration of surrounding tissue with desmoplasia/scirrhous reaction
Tumor histotype (e.g. solid carcinoma, comedocarcinoma, adenosquamous carcinoma, etc.)
Marked nuclear and cellular pleomorphism (i.e. prominent anisocytosis/anisokaryosis, hyperchromatic cells, and loss of cytoplasm (not in ductal tumors)
Mitotic count with ≥ 6 per 2.37 mm² used to distinguish between some benign and malignant tumors
Randomly distributed areas of necrosis (not to be confused with central regions of ischemic necrosis)
Pluristratification with loss of cellular orientation with the basement membrane and nuclear pleomorphism

Feline mammary carcinoma grading systems

Two grading systems exist (Nottingham Human Grading System (NHG) and Three-Tiered “Mills System” introduced in 2015); no current consensus on which should be used for feline mammary carcinomas; combined use suggested (Avallone et al, Vet Pathol. 2021)
Nottingham Human Grading System

Histologic feature
Tubule formation	Score
Comprises majority of tumor (>75%)	1
Present to a moderate degree (10-75%)	2
Little or none present (<10%)	3
Nuclear pleomorphism	Score
Small regular uniform nuclei	1
Moderate increase in size, vesiculation, and variability	2
Vesicular nuclei with marked variation in size and shape	3
Mitotic count (2.37mm²⁾	Score
0-8 mitoses (0-50*)	1
9-16 mitoses (51-70*)	2
≥17 mitoses (≥71*)	3
Histological grade	Total Score
Grade I (low grade)	3-5
Grade 2 (medium grade) Grade 3 (high grade)	6-7 8-9
*Assessed at periphery or in most mitotically active parts of the tumor. Recently proposed modifications to the mitotic cutoffs are reported in parentheses (Mills et al.)

3-tier Grading System (Mills system)

Histologic feature
Lymphovascular invasion	Score
Absent	0
Present	1
Nuclear shape*	Score
≤5% abnormal	0
>5% abnormal	1
Mitotic count (2.37mm²)**	Score
≤62	0
>62	1
Histological grade	Total Score
Grade 1 (low grade)	0
Grade 2 (medium grade)	1
Grade 3 (high grade)	2-3
* Abnormal nuclear from includes any deviation from smooth nuclear contour or round/oval nuclear shape, such as clefting, angularity, corrugation, or ameboid morphology assessed at high power in the least differentiated and/or most invasive portion of the tumor. The number of nuclei exhibiting the abnormal nuclear form is estimated and expressed as a percentage of the total number of nuclei within any given field. **Assessed at periphery or in most mitotically active parts of the tumor.

PATHOGENESIS:

Mammary fibroadenomatous hyperplasia:

Coincides with the luteal phase of estrus, early in pregnancy, or after progestin therapy

The mechanism for progesterone induced hyperplasia involves IGF-1 and growth hormone

Prolonged exposure to endogenous or exogenous progesterone
Exact mechanism unknown

Feline mammary carcinoma:

Both estrogen and progesterone increase risk of malignant transformation of mammary epithelium; exact mechanism is unknown
Deregulation of the Wnt/β-catenin and Hippo pathways compared to healthy tissues (similar in dogs and humans) (Sammarco et al, Vet Pathol. 2020)
Aggressiveness correlated with expression of permeability glycoprotein (P-gp) and metallothionein (MT) (Manoel et al, J Comp Pathol. 2021)

TYPICAL CLINICAL FINDINGS:

Mammary fibroadenomatous hyperplasia:

Sudden, rapid, uniform enlargement of one or more mammary glands
Firm and painless, unless ulcerated

Feline mammary carcinoma:

Firm, nodular masses in one or more mammary glands with the time of onset unrelated to estrus
Involved nipples may be erythematous and swollen; may exude a tan or yellow fluid
Ulceration of the overlying skin is common
May be adhered to overlying skin or underyling abdominal wall
+/- lymphadenopathy

TYPICAL GROSS FINDINGS:

Mammary fibroadenomatous hyperplasia:

One or all mammary glands markedly enlarged
Overlying skin: Pink to blue, +/- necrosis
Cut surface: White to pink, bulges, multinodular with blue fibrous septa forming a reticulated pattern, edema, hemorrhage, necrosis
Intraductal pattern: Project from wall of fluid‑filled space as a polypoid or dome shaped mass or more annular growth around a central cavity

Feline mammary carcinomas:

Solitary masses are as common as multiple tumors; in cases of multiple tumors, masses are commonly found throughout the mammary chains
Primary tumor is usually adjacent to the nipple
May be >3cm diameter

TYPICAL LIGHT MICROSCOPIC FINDINGS:

Mammary fibroadenomatous hyperplasia:

Unencapsulated mass composed of lobules of branching ductal structures lined by epithelial cells surrounded by edematous myofibroblastic stroma
Duct-like structures lined by several layers of cuboidal to columnar epithelial cells with vesiculate nuclei
Mitoses may be common in both ductal and myofibroblast populations

Tubular mammary carcinoma:

Tubules are predominant feature but may coexist with other patterns (e.g. solid or papillary)
Tubular lumina may vary in width and may be lined by more than one layer of cells, which are often pleomorphic; nuclei range from hypo- to normo- to hyperchromatic
Mitotic count is ≥6 per 2.37mm²(often >30)
Multifocal regions of necrosis are common
Invasion into adjacent mammary tissue elicits a desmoplastic/scirrhous response with extensive proliferation of (myo)fibroblasts

DIFFERENTIAL DIAGNOSIS:

Mammary fibroadenomatous hyperplasia:

Neoplasia: Usually does not involve all glands; usually a discrete nodule; epithelial component disorganized and/or invasive
Other non-neoplastic mammary gland proliferations
- Cystic dilation of mammary ducts: due to obstruction of duct leading to dilation; common in dogs and cats
- Regular lobular hyperplasia: proliferation of intralobular ducts and alveoli; epithelial cells exhibit no atypia
- Lobular hyperplasia with secretory activity (lactational): alveoli variably distended and lined by actively secreting cuboidal cells (lactational change); alveolar cells often have intracytoplasmic vacuoles (lipid) and apical surface of epithelial cells may protrude (blebbing)
- Lobular hyperplasia with fibrosis; prominent interlobular and intralobular fibrous connective tissue
- Lobular hyperplasia with atypia: epithelial cells exhibit changes including nuclear hyperchromasia, anisokaryosis, anisocytosis, and variable numbers of mitoses; lobular architecture may be slightly disorganized
- Epitheliosis (epithelial proliferation in ducts): regular, with marked atypia (may progress to carcinoma); carcinoma diagnosis made based on nuclear pleomorphism, increased mitoses, and/or necrosis
- Papillomatosis: florid epithelial proliferation in ducts forming papillary projections
- Galactostasis: engorged, hot, painful mammae secondary to milk retention (failure of let-down d/t inhibition of oxytocin release)

Primary mammary gland neoplasia in felines (not all encompassing):

Ductular
Ductal adenoma: most common benign feline mammary tumor; bilayered epithelium of CK8+/18+ luminal epithelial cells and round to oval suprabasal or or round to oval interstitial myointerstitial cells which express myoepithelial markers (CK5, CK6, CK14, smooth muscle actin, calponin); minimal ansiokaryosis and anisocytosis; few mitoses (<6 per 2.37mm²)
Ductal carcinoma: neoplastic cells arranged in cords, tubules, and solid areas, but solid areas are larger than ductal adenomas; variably present slit-like lumina often lined by double layer of luminal epithelial and myoepithelial cells with little to no matrix; differentiated from ductal adenoma by more extensive anisokaryosis and anisocytosis, an increased mitotic count (≥6 per 2.37mm²), and loss of well-differentiated ductal architecture which are replaced by sheets of neoplastic luminal epithelial cells with occasional foci of squamous differentiation
Simple: neoplastic component composed of epithelium
Simple adenoma: very rare benign mammary tumor in felines; nodular, well-demarcated, non-infiltrative lesion composed of luminal epithelial cells arranged in tubules; anisocytosis and anisokaryosis are minimal with few mitotic figures (<6 per 2.37mm²)
Tubular carcinoma: common in cats; see description under “light microscopic findings”
Tubulopapillary carcinoma: very common in cats; similar to tubular carcinoma but >20% of tubules have papillae supported by fibrovascular connective tissue stroma protruding into lumina
Solid carcinoma: common in cats; epithelial cells arranged in nests and islands without lumina and are separated by delicate stroma; anisokaryosis and anisocytosis are moderate to severe and number of mitoses is variable; note: solid carcinomas can never score a 1 for tubule formation
Comedocarcinoma: common in cats; characterized by a multinodular pattern within a single tumor; within tumor nodules there are variably sized and very well defined, central, circular areas of necrosis surrounded by neoplastic cells; peripheral epithelial cells may have multiple growth patterns, including solid, tubulopapillary, tubular, and micropapillary; mitotic count is variable but is typically medium to high (>20 per 2.37mm²); lymphatic vessel infiltration and lymph node metastasis common at time of diagnosis
Note: Although rare, tumors composed of malignant epithelial cells and myoepithelial cells have been reported in cats (Sammarco et al, Vet Pathol. 2020)

COMPARATIVE PATHOLOGY:

Mammary hyperplasia in other species:

Mice: virgin female FVB/N develop lobuloalveolar hyperplasia with secretory product in alveoli and ducts; increases with age and associated with prolactin secreting cells in pars distalis of pituitary; high incidence of pituitary adenomas in FVB/N mice; persistent hyperplasia may be present in absence of pituitary signs
Rabbits: mammary hyperplasia can be caused by the administration of cyclosporine
Water Buffalo: mammary gland fibroadenomatoid hyperplasia; probable hormone influence

Mammary neoplasia in other species

Dogs: More than 35 phenotypes described; 7 are benign with remaining considered histologically malignant; >50% of cases are benign with either an epithelial or mixed epithelial and myoepithelial components; simple carcinomas, solid carcinomas, and complex carcinomas are the most common malignant mammary neoplasms; grading scheme for mammary carcinomas is similar (but not identical) to Nottingham Human Grading System used for feline mammary carcinomas; mammary neoplasms associated with short survival times following diagnosis include mammary osteosarcoma, anaplastic carcinoma, lipid-rich carcinoma, micropapillary invasive carcinoma, inflammatory carcinoma, comedocarcinoma, and squamous cell carcinoma
Mice: Related to insertional mutagenesis of endogenous and exogenous mammary tumor viruses; tend to grow by blunt expansion rather than invasion; 60% metastasize to lung despite commonly having morphologically benign features; up to 100% C3H females develop mammary tumors by 9mo of age; common in multiparous FVB/N mice; BALB/c strain resistant
Rats: Mammary gland benign fibroadenomas; elevated prolactin; common in older female Sprague-Dawley rats; markedly reduced in ovariectomized rats; malignant mammary tumors are rare
Guinea pigs: Mammary adenocarcinomas occur in both males and females; majority are ductal in origin; may metastasize to regional lymph nodes while others are low grade local malignancy; mammary gland adenoma and malignant mixed mammary tumors also reported
Rabbits: Multiple breeds affected, including laboratory rabbits; typically arise around 3-4yrs of age; majority are carcinomas, including tubular, papillary, tubulopapillary, solid, adenosquamous, comedo, complex, ductal, cribriform, anaplastic, and spindle cell; cystic mastitis thought to be a prelude to neoplastic transformation from benign adenomas to adenocarcinomas; lymph node, lung, and other organ metastasis reported
Pinnipeds: Mammary carcinoma among the most common neoplasias of captive sea lions
Panthers: Mammary carcinoma was the most commonly reported neoplasm in a recent study (Kloft et al, J Comp Pathol. 2019)
Reports of mammary carcinoma in llamas, alpacas, dromedary camels, suids, wild felids, beluga whales, bats, and a male orangutan

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