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How to stage cancer in veterinary medicine

Overview of the staging process

Staging is used to determine the extent of a cancer spread in patients. The type of cancer should first be ascertained before undergoing staging so that a more informed approach can be undertaken. The cancer type is determined through biopsy and cytologic or histopathologic investigation of a sample. It is important to establish the stage of cancer as it is used to create a treatment plan and indicate prognosis. Staging should be completed before starting corticosteroid treatment as it can prevent an accurate reading.

 

Staging evaluation points

Certain data points are necessary to attain before determining the overall cancer stage. These include:

  • Site, size, cell type, and grade of the primary tumour 
  • Lymph node involvement
  • Metastasis presence or absence 
  • Number of tumours

Staging systems

TNM System

TNM system classifies the stage of cancer by analysing 3 components: the primary tumour (T), lymph nodes (N), and metastases (M). These components are categorised into subsections.

Tumours:

  • T0: no main tumour found
  • T1: small tumour, less than 3cm in diameter
  • T2: tumour 3-5cm in diameter
  • T3: large tumour, greater than 5cm in diameter or with an invasion of subcutis 
  • T4: tumour invades beyond the subcutis

Lymph Nodes:

  • N0: no lymph node involvement found
  • N1: mobile lymph nodes on one side of the body
  • N2: bilateral mobile lymph nodes
  • N3: non-mobile lymph nodes
  • (a) - nonmetastatic node
  • (b) - metastatic node

Metastases:

  • M0: no distant metastasis found
  • M1: distant metastasis present

Once the severity of each of these components has been determined, the overal stage can be interpreted. 

Lymphoma system

Lymphoma has a unique staging system as it is inherently spread throughout the body due to being a cancer of the lymphatic system. Therefore, the stages are not based on metastases but rather the organs involved and clinical signs. Stage 1 dictates the involvement of one lymph node and stage 2 dictates the involvement of multiple nodes in one region. Stage 3 is categorised by general peripheral lymphadenopathy, stage 4 dictates involvement with the liver or spleen, and stage 5 is categorised by involvement with bone marrow or other organ systems. Substage a is used for patients without systemic signs and substage b is for those showcasing systemic signs.

Types of staging tests

Physical examinations

Staging usually begins with physical examinations. These examinations can determine the size, location and mobility of the tumour. They may also determine if lymph nodes are involved, however, physical examinations alone cannot confirm the extent of lymph node interaction. If the tumour is internal, imaging may be required to assess the full extent of the cancer. 

Laboratory tests

Complete blood count, serum biochemistry panel, and urinalysis tests should be carried out early to gain general data about the patient.

Imaging

Many imaging types can be used in staging. Radiography can be used to quickly gain an overview of the tumours present. Ultrasonography can visualise the outlines of organs and lymph nodes. This makes its most helpful as a guide when performing aspiration which may be necessary to determine if cells present are cancerous. CT scans show a high definition of structures and are most efficient for visualising metastases. While MRI scans are also high definition, MRI scans are only necessary for brain tumours in animals.

Cytology and histopathology

Cytology and histopathology are used to confirm what physical examination and imaging cannot. Assessment of multiple lymph nodes should be undertaken as cancer spread through the lymphatic system can be highly variable. Cytology provides information by examining the microscopic cell appearance. These cells can be easily provided from fine-needle sampling and do not always require biopsy. Cytology can be used to ascertain the cell type, however, it cannot determine the tumour grade. Histopathology analyses tissue structure and is used to establish tumour grade from a biopsy sample. 

Staging procedures for common tumour types

Lymphoma staging protocol

Lymphoma can spread anywhere throughout the body, therefore multiple diagnostic tests are needed. 3-view thoracic radiographs should be completed. Cytology can be used to diagnose lymphoma, however, histopathology can be used when cytology is questionable, or when nodes are solitary or slow growing. Advanced imaging (CT or MRI scans) should be used to more accurately confirm location after the radiograph.

Mammary gland cancer

Mammary gland tumours can be detected from a 3-view thoracic radiograph as they are located within the breast tissue. Aspiration should be used to distinguish malignant tumours from benign. Regional lymph node aspiration should also be applied to determine lymph node involvement. 

Mast cell tumours staging protocol

Histopathology should be used to determine tumour grade. Aspiration of regional lymph nodes should be completed. Aspiration of the spleen and liver should also be completed if they are enlarged or have nodal metastases as the mast cell metastases pass through the lymphatic system. 3-view thoracic radiographs should be performed. 

Melanoma staging protocol

Oral malignant melanoma is the most common form of melanoma in animals. They are common in lungs and lymph nodes, therefore 3-view thoracic radiographs should be performed. Aspiration and possible surgical removal of mandibular lymph node may be necessary for histopathology. CT or MRI scans may also be used to aid in surgical planning.

Hemangiosarcoma staging protocol

Splenic hemangiosarcoma is one of the most common forms of hemangiosarcoma. Anaemia is a factor to be looked for in lab tests. Abdominal ultrasonography and 3-view thoracic radiographs should be performed. Metastases are likely to be found in liver and lungs. 

When to refer

A referral to an oncology specialist may be considered for multiple reasons. Referrals are appropriate when pet owners wish to have a greater range of treatment options or want to proceed with treatment that the primary care veterinarian cannot provide. Additionally, referrals are also appropriate for clients who want to participate in clinical trials that primary care veterinarians do not have access to. Before engaging with a specialist, the client’s emotional attachment and financial resources should first be considered. 

Prognosis of each stage

Animals diagnosed with a higher stage have a lower survival rate. For example, for each stage of oral malignant melanoma, the median amount of days survived after treatment with curative-intent surgery has been studied. Stage 1 has a median survival time of 874 days, stage 2 has 818 days, and stage 3 has 207 days. Stage 4 was not treated in this study. 

Communicating stages to pet owners

Proper communication with pet owners regarding staging is a crucial component of treating animals with cancer. An explanation is necessary, as the client’s full understanding of the situation will aid in determining how they wish to proceed with treatment. However, revealing the stage of a pet can be an emotional and confronting experience for pet owners, especially those who may not have realised the severity of their pet’s condition. For this reason, clinicians should be patient and empathetic in their explanations. Some clinicians may find that relating the stages of animal cancer to that of humans can improve the client's comprehension of the topic. 

References

Biller B. 2016. AAHA Oncology Guidelines for Dogs and Cats. J Am Anim Hosp Assoc. 52(4):181-204.

DeCormier D. 2021. Staging Cancer: Digging Deeper Than the Diagnosis. Today’s Veterinary Nurse.

Elliot J. 2014. Clinical evaluation of small animal cancer patients. In Practice. 36(5):218-228.

Murphy S. Small animal oncology: how to give a client a more accurate prognosis. In Practice. 41(3):98-105.