​Tumor Growth Monitoring and Endpoint Criteria in Research Animals

Policy

Monitoring: The frequency of monitoring and care, if applicable, of study animals must be described in the protocol. Different tumor types necessitate different monitoring procedures based on tumor biology and pathology, for example tumor site, growth rate, tendency to metastasize and/or ulcerate. Investigative groups are expected to monitor animals at an interval that is appropriate for the tumor model that is used. Visual monitoring should be accompanied by tumor size measurement with calipers, if appropriate for the experiment (e.g., subcutaneous tumor models). Imaging may be appropriate for internal tumor models. Typically, the frequency of monitoring increases as tumor growth progresses. Animals are generally monitored a minimum of three times per week at the start of experiments. More frequent monitoring (as often as daily) is warranted when animals approach their endpoints based on tumor size and/or clinical disease. Records of monitoring must be maintained and available for review at any time, including records of tumor size measurement, if applicable.

Endpoint criteria: Experiments involving tumorigenesis must describe specific experimental and humane endpoint criteria. Animals may require euthanasia at earlier timepoints based on veterinary recommendations. Most experiments should be terminated when animals begin to exhibit one or more clinical signs of disease. Compelling scientific justification must be given for using endpoint criteria more severe than those outlined below.

​Specific signs of clinical disease that require euthanasia:

1. Poor body condition (e.g., body condition score less than 2 out of 5 in mice)¹
2. Impairment of ability to eat, drink, or ambulate normally
3. Moist ulceration, necrosis, or discharge from tumor
4. Labored respiration (dyspnea)
5. Hypothermia (cold to the touch)
6. Anemia (pale paws)
7. Abdominal distension greater than 20% increase in pre-treatment/pre-tumor body girth (approximating the normal appearance of a pregnant animal)²
8. Inability or reluctance to move when stimulated, or moribund condition
9. For intracranial tumors, neurological signs including lethargy, ataxia, and seizures
10. Total estimated tumor mass equal to or greater than 10% of animal mass (see below)

Tumor size limitation and calculation: Compelling scientific justification is required to allow growth of superficial tumors to proceed beyond a total estimated tumor mass per animal (sum of all tumors) equal to or greater than 10% of an animal’s mass. Ideally this is based on pre-tumor/treatment body weight for adult animals, or age-matched control animals. The tumor volume is measured and converted to an approximate mass based on the density of water, 1 g/cm³.

Calculation of ellipsoid volume:

 

 

​​​​​​​​where l = length, w = width, h = height of tumor in millimeters

Example:
A 25 gram adult mouse enters is enrolled in an experiment in which subcutaneous tumor growth is induced. A total tumor volume of 2500 mm3 is approximately equivalent to 2.5 g, or 10% of the body mass of a 25 g mouse. Therefore, this mouse must be euthanized when the total (sum) volume of all its tumors meets or exceeds 2500 mm3. Allowable tumor dimensions could be 18 mm long, 17 mm wide, and 15 mm high for a single tumor, or 15 mm long, 12 mm wide, and 10 mm high (each tumor) for two same-sized tumors.

A modified ellipsoid formula using only length and width measurements such as that described by Feldman et al.³ may also be used to calculate tumor volume:

 

 

where l = length, w = width in millimeters