Round Cell or Discrete Tumors
Cells are round to oval and discrete, although they may cluster if in large numbers.
 

Histiocytoma

Clinical

  • Occurs only in the dog and is the most common skin tumor in this species.
  • Occurs in young animals (50% are less than 2 years old)
  • Rapidly growing, dome shaped, often ulcerated lesions approximately 2 cm in diameter.
  • Skin of the head is the most common site.
  • Do not metastasize and may spontaneously regress
Cytology
  • Round to oval cells with moderately distinct cell membranes
  • Round to oval nuclei with finely stippled chromatin and sometimes a fine cleft. Nucleoli are indistinct or not observed.
  • Abundant pale blue cytoplasm, which may occasionally have vacuoles but does not exhibit evidence of phagocytosis.
 
Cutaneous Malignant Lymphoma

Clinical

  • Epitheliotrophic (mycosis fungoides)
  • A focal band of neoplastic lymphocytes occurs in the superficial dermis; these cells form nests within the epidermis. Most are multiple and of T- cell origin

  • Nonepitheliotrophic
  • Neoplastic lymphocytes occur as dermal nodules or diffuse dermal infiltrates and do not involve the epidermis. They usually are multiple and involve other tissues.

Cytology
  • The cytology is similar to that of malignant lymphoma in other tissues.
  • Cells are large with abundant blue cytoplasm; peripheral clearing occasionally occurs around the nucleus.
  • Nuclei are round to oval to irregular and sometimes cleaved. Chromatin is dispersed, and nucleoli are often present.
 
Extramedullary Plasmacytoma

Clinical

  • Solitary, dome-shaped, alopecic nodules, usually less than 2 cm. in diameter.
  • Most common sites are the head and feet; may occur on gingiva.
  • Average age of affected dogs is 10 years.
  • Considered benign tumors, usually do not recur following surgical removal.
  • Rarely are functional, i.e., produce monoclonal gammopathy.
Cytology
  • Round, eccentric nuclei with dense chromatin; bi- and trinucleate cells are common and characteristic.
  • Abundant blue cytoplasm, often with prominent Golgi or perinuclear clear zone.
 
Mast Cell Tumor

Clinical

  • Usually well-circumscribed cutaneous nodules, 1-10 cm. In diameter, but may appear as poorly defined edematous swellings.
  • Hind quarters are the most common location; they are usually solitary, but 25% of the cases may have multiple tumors.
  • Average age of occurrence is 8 years
  • Considered potentially malignant.
  • They may disseminate to lymph nodes, spleen, liver, bone marrow, and occasionally other organs.
  • Potential to disseminate is difficult to predict, but degree of differentiation is the best indicator.
Cytology
Well-differentiated:
Many granules
Poorly stained nuclei
Poorly-differentiated:
Poorly granulated
Anisokaryosis
  • Round nuclei and moderate amounts of cytoplasm containing purple granules (metachromatic).
  • Nucleus may be poorly stained in heavily granulated cells.
  • The more undifferentiated tumors have fewer granules and more variation in nuclear and cell size (anisokaryosis and anisocytosis).
  • Eosinophils are frequently present
 
Transmissible Venereal Tumor

Clinical

  • Cauliflower, pedunculated, papillary, or nodular growths involve the external genitalia (i.e., prepuce, vulva, vagina).
  • Extragenital tumors can occur: skin, nasal cavity, conjunctiva, mouth.
  • Single or multiple lesions
  • Metastasis may occur to regional lymph nodes but is uncommon.
  • Some tumors may regress.
Cytology
  • Large, round to oval cells with round nuclei.
  • Nuclei have fine ropy chromatin and single blue nucleolus; mitoses may be observed.
  • Abundant cytoplasm contains distinct clear vacuoles.
 
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