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Veterinary Focus

Oncology

Canine round cell tumor cytology

Published 04/10/2024

Written by Candice P. Chu

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Understanding the cytological features of canine round cell tumors is critical for a veterinarian to provide an accurate diagnosis, prognosis and effective therapeutic strategies to manage these conditions. 

Cell characteristics

Key points

Round cell tumors in dogs encompass a range of conditions, each with their own distinct cytologic characteristics and prognosis.


Canine lymphoma manifests in various forms, with survival times and treatment efficacy varying significantly across types.


Mast cell tumors are the most common skin tumor in dogs, with prognosis and treatment dependent on the tumor grade and location.


Other tumor types may have similar cytologic features as round cell tumors; if in doubt, submit the cytology slides to a clinical pathologist. 


Introduction

Round cell tumors are diverse neoplasms commonly encountered in veterinary medicine, particularly in dogs, with the category including transmissible venereal tumors (TVT), lymphomas, mast cell tumors, plasma cell tumors, and histiocytomas. When viewed under a microscope, these tumors are characterized by a round appearance of the cells, but each has distinct cytologic appearance and clinical features. This article offers a brief guide for the first opinion clinician in what to look for when doing initial investigations, and also what features can be misleading and possibly lead to an erroneous diagnosis.

Transmissible venereal tumor (TVT)

TVT is a unique type of contagious cancer that affects dogs, transmitted through direct contact, usually during mating. It predominantly affects the external genitalia but can also appear in extragenital locations. TVT cells are distinctive with their cytoplasmic vacuoles; these are discrete and punctate, and are frequently described as a “string of pearls” (Figure 1). 

Cell characteristics of Transmissible Venereal Tumor (TVT).

Figure 1. Transmissible venereal tumor (TVT); the cells frequently contain small to moderate numbers of punctate vacuoles, resembling a string of pearls. (40x magnification) 
© Dr. Shih-Ming Liu, Taiwan.

Lymphoma

Lymphoma in dogs is a common presentation, and a preliminary diagnosis can be made on clinical examination (Figure 2), but it is recommended to make identification definitive, as this will help in advising the owner on treatment and prognosis. In cytology reports, lymphoma is often diagnosed as “intermediate cell lymphoma,” “intermediate to large cell lymphoma,” or “large cell lymphoma”. These terms may lead to a misconception that “large cell lymphoma” is a distinct disease type, but in fact there are more than 30 types of lymphoma based on WHO classification 1. Many of them could be diagnosed as “large cell lymphoma”, but the presentation and exact immunophenotype can vary widely, from indolent to aggressive forms, and hence the prognosis will also vary correspondingly. Discussion here will center on a few lymphomas that have distinct cytologic features which may be recognized on cytology without ancillary tests, as an understanding of these features may provide a rough idea of possible diagnosis and prognosis; however, the complete classification and a definitive diagnosis of canine lymphoma can only be achieved by combining cytology, histopathology, immunohistochemistry, and advanced molecular testing (e.g., flow cytometry).

Lymphadenopathy is often detected on palpation during physical examination.

Figure 2. A clinical diagnosis of lymphoma is usually made on physical exam, as in many cases the peripheral lymph nodes will be markedly enlarged on palpation. However, cytology is required to ascertain what type of lymphoma is present. 
© Shutterstock

  • The large or intermediate to large cell lymphoma: This type of lymphoma primarily comprises lymphocytes that are intermediate to large (more than 2-3 times the size of red blood cells). This is the most common diagnosis of lymphoma seen on a cytology report. Typically, large neoplastic lymphocytes have a high nuclear-to-cytoplasmic ratio (N:C ratio) and small to scant amounts of basophilic cytoplasm (Figure 3). Based on the clinical presentation and the immunophenotype, the lymphoma could be a “Diffuse Large B Cell Lymphoma (DLBCL)” or “Peripheral T Cell Lymphoma Not Otherwise Specified (PTCL-NOS)”.
  • Diffuse Large B Cell Lymphoma (DLBCL): DLBCL is the most common canine lymphoma, and it generally affects middle-aged to older dogs, with a median age of onset around 7-9 years. Certain breeds, including Golden Retrievers, Labrador Retrievers, Bernese Mountain Dogs, and German Shepherds, are at higher risk. Dogs with lymphoma typically present with generalized lymphadenopathy. The median survival time is approximately 300-500 days 2.
  • Peripheral T-Cell Lymphoma (PTCL-NOS): PTCL-NOS accounts for roughly 15% of all canine lymphoma cases. This type tends to present in middle-aged Boxers and is slightly more common in male dogs. The disease often involves the mediastinal lymph nodes and can be associated with hypercalcemia. Unfortunately, PTCL-NOS is associated with a poor prognosis, with a median survival time of only about 158 days 2

Cell characteristics of large cell lymphoma.

Figure 3. Large cell lymphoma from a gastrointestinal mass. The neoplastic lymphocytes are larger than neutrophils, with a high N:C ratio and scant basophilic cytoplasm. (40x magnification) 
© Dr. Chih-Chien Tsai, Taiwan.

Given the cytologic similarity and prognostic difference between DLBCL and PTCL-NOS, flow cytometry or histopathology with immunohistochemistry is strongly recommended to confirm the diagnosis.

  • T-Zone Lymphoma (TZL): TZL is a small to intermediate cell lymphoma that has a better prognosis than other types. It often presents with a “hand mirror” or “tadpole” appearance cytologically (Figure 4). However, flow cytometry is necessary for definitive diagnosis, as TZL cells will typically be positive for CD3, CD5, CD21, and MHC II cell markers 3. This type of lymphoma frequently involves the blood, leading to lymphocytosis. Golden Retrievers, Maltese Terriers and Shih Tzus are breeds commonly affected. The indolent nature of TZL means a better median survival time, which can be as long as 637-760 days 3.
  • Large Granular Lymphocyte (LGL) Lymphoma: LGL is characterized by cytotoxic T cells or natural killer (NK) cells that often have pink to magenta cytoplasmic granules containing enzymes such as perforin and granzymes. In dogs this type of lymphoma is primarily found in the liver and spleen, but it can also be present in the gastrointestinal tract or as a disseminated disease. Diagnosis is typically confirmed through cytology, because histopathology, which does not show cytoplasmic granules well, will often indicate a low-grade lymphoma. The aggressive nature of the disease is indicated by a median survival time of only 28 days, even though around 75% of dogs may initially respond to chemotherapy. A positive prognostic factor includes finding an intermediate lymphocyte size (versus large size) on cytology, which correlates with a median survival time of more than 100 days in some patients 4.
  • Marginal Zone Lymphoma (MZL): MZL is best diagnosed through histopathology, although the cytologic feature of a single prominent nucleolus can be indicative. This type of lymphoma can have a variable prognosis depending on the affected organ. In the spleen, it can have a good long-term prognosis with splenectomy alone, whereas lymph node involvement can be as aggressive as DLBCL 2.
Cell characteristics of T-zone lymphoma

Figure 4. T-zone lymphoma. The neoplastic lymphocytes are small to intermediate in size. They frequently have cytoplasmic tails that resemble hand mirrors or tadpoles. (40x magnification) 
© Dr. Chih-Chien Tsai, Taiwan.

Mast cell tumor (MCT)

These are the most common skin tumors in dogs, representing 16.8% of all cutaneous neoplasms 5. The mean age for MCT presentation is around nine years, and they can occur in any breed, but some are predisposed; these include Boxers, Pugs, Labrador Retrievers, Golden Retrievers, and Weimaraners. The tumors are often solitary but they can be multiple, and are typically found on the trunk (50%), limbs (40%), and head and neck (10%). The clinical appearance can range from erythematous alopecic masses or plaques to raised nodules (Figure 5) 5.

Mast cell tumors have various presentations

Figure 5. Mast cell tumors can have a markedly variable appearance, ranging from relatively innocuous raised nodules to ulcerated dysplastic masses.
© Shutterstock

Cytological features

MCT are characterized cytologically by centrally located round nuclei with purple intracellular granules that vary in size and number. Note that the granules may not be visible if using an in-clinic aqueous Romanowsky stain (e.g., Diff-QuikTM stain) (Figure 6). Other components commonly associated with MCTs include collagen fibers, eosinophils, and fibroblasts.

Different stain techniques show distinctive features of mast cell tumor under microscope: Diff-QuikTM stain

a

Different stain techniques show distinctive features of mast cell tumor under microscope: Giemsa stain

b

Figure 6. The varying visibility of mast cell tumor (MCT) granules using Diff-QuikTM stain (a) and Giemsa stain (b). The lack of visible granules may prevent clinicians or even inexperienced cytologists from diagnosing mast cell tumors. (50x magnification) 
© Dr. Candice Chu. 

Cytologic grading

Tumor grading assesses how normal or abnormal the cancer cells look under a microscope, and cytologic grading is a crucial aspect of diagnosing MCT. Generally, the more abnormal the cells appear, the more aggressive the cancer is, and the faster it is likely to grow and spread. The cytologic grading for canine cutaneous MCT involves examining specific criteria based on morphologic characteristics of neoplastic cells, including karyomegaly, multinucleation, nuclear pleomorphism, and mitotic figures. A recent paper has described using cytological identification of granulation alongside a set of criteria to determine the grading of a MCT 6. Essentially, poor granulation on cytologic evaluation indicates high grade, but if the cells are well-granulated, fulfilling at least two of the following criteria would also indicate a high-grade MCT (Box 1):

  • Presence of mitotic figures
  • Nuclear pleomorphism (variation in nuclear size and shape)
  • Binucleation or multinucleated cells
  • Anisokaryosis, which refers to a significant (> 50%) variation in nuclear size
Cell characteristics of plasma cell tumor

Figure 7. Flame cells in a plasma cell tumor. Note the cells have a clumped chromatin, perinuclear Golgi zone, and pink cytoplasm. (40x magnification) 
© Dr. Szu-Wei Lin, Taiwan.

A recent study suggested a new development for cytologic grading 7. It reported that examination of slides with a Romanowsky stain could allow inclusion of additional factors for grading, such as fibroblasts and collagen fibrils. A moderate to intense concentration of these elements suggests a more benign character associated with increased survival and a lower histopathologic grade.

Candice P. Chu

Tumor grading assesses how normal or abnormal the cancer cells look under a microscope, and cytologic grading is a crucial aspect of diagnosing MCT. Generally, the more abnormal the cells appear, the more aggressive the cancer is, and the faster it is likely to grow and spread.

Candice P. Chu

Plasma cell tumor

Also known as plasmacytoma, plasma cell tumors in dogs are regarded as benign neoplasms that can appear “ugly” on cytology. These tumors often occur in middle-aged to older dogs, with common sites including the limbs and areas of the head such as the lip and ear margins. Cytologically, they have a characteristic perinuclear clearing known as the Golgi zone and may present bi- and multi-nucleation. Their nuclei often have clumped chromatin, often described as “clock-face” or “soccer ball” in appearance. Occasionally, plasma cells may have a pink fringe known as “flame cells” (Figure 7).

(a)	The gross image and cytologic morphology of histiocytoma (dome-shaped feature).

a

(b)	The gross image and cytologic morphology of histiocytoma (fried egg appearance).

b

Figure 8. A histiocytoma on the nose of a 4-year-old Shiba Inu. The gross image (a) shows the distinct dome-shaped feature of a histiocytoma. The cytologic morphology (b) resembles a fried egg appearance. Note that there is a mild lymphocytic infiltration, indicating a regression process. (40x magnification) 
© Dr. Yuan-Ru Lai, Taiwan.

Histiocytoma

Histiocytomas are benign skin tumors most commonly found in younger dogs under two years old. They appear as solitary, firm, dome-shaped masses primarily located on the head and limbs, and are often ulcerated. Under the microscope these tumors show a “fried egg” appearance, with pale blue to colorless cytoplasm and few mitotic figures (Figure 8). Histiocytomas often regress spontaneously without treatment.

Cytological identification of granularity helps determine the grading of a MCT

Box 1. Grading of MCT via granularity of cells and other criteria (from 6).

Conclusion

Round cell tumors in dogs comprise a variety of neoplasms that are relatively common in veterinary practice. Each type has unique characteristics that require specific diagnostic and treatment approaches. Understanding the cytological features of these tumors is critical for veterinarians to provide accurate diagnoses, prognoses, and therapeutic strategies to manage these conditions effectively in canine patients.

References

  1. Valli VE, San Myint M, Barthel A, et al. Classification of canine malignant lymphomas according to the World Health Organization criteria. Vet. Pathol. 2011;48:198-211.

  2. Avery AC. The genetic and molecular basis for canine models of human leukemia and lymphoma. Front. Oncol. 2020;10:23.

  3. Seelig DM, Avery P, Webb T, et al. Canine T-zone lymphoma: unique immunophenotypic features, outcome, and population characteristics. J. Vet. Intern. Med. 2014;28:878-886.

  4. Yale AD, Crawford AL, Gramer I, et al. Large granular lymphocyte lymphoma in 65 dogs (2005-2023). Vet. Comp. Oncol. 2023 (pub on line https://doi.org/10.1111/vco.12959)

  5. London CA, Thamm DH. Mast cell tumor. In: Vali DM, Thamm DH, Liptak JM (eds.) Withrow and MacEwen’s Small Animal Clinical Oncology E-Book; Elsevier Health Sciences; 2019;335-355.

  6. Camus M, Priest H, Koehler J, et al. Cytologic criteria for mast cell tumor grading in dogs with evaluation of clinical outcome. Vet. Pathol. 2016;53:1117-1123.

  7. Paes PR, Horta RS, Luza LC, et al. Inclusion of fibroblasts and collagen fibrils in the cytologic grading of canine cutaneous mast cell tumors. Vet. Clin. Pathol. 2022;51:339-348.

Candice P. Chu

Candice P. Chu

Dr. Chu is a board-certified veterinary clinical pathologist and a tenure-track assistant professor at Texas A&M University Read more