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Tumor implantation in research animals is a critically important experimental activity which requires consideration of the effect of the tumor or tumors on the animal. Effective monitoring systems and endpoints should include limits on the tumor burden and severity of tumor-associated disease. The use of altered physiological, biochemical, and other biomarkers are suggested as potentially more objective and reproducible endpoints than clinical signs.

These guidelines limit the tumor burden an animal experiences to that which does not cause excessive pain or distress, and are for cumulative tumor burden per animal. If multiple tumors occur, the total tumor burden cannot exceed the parameters noted below.


  1. Tumor Implantation or Production
    1. Subcutaneous or intradermal: The tumor(s) should be placed into a site or sites that will not interfere with normal body functions such as ambulation, eating, drinking, defecation and/or urination. Subcutaneous or intradermal sites on the back or in the flank are considered to cause the least distress. Sites involving the special senses should be avoided, and intramuscular implantation should be avoided as distention of muscle is considered to be painful to the animal. Extra attention must be paid if multiple sites are used, and no more than four tumors should be implanted in one animal.
    2. De novo and metastatic tumor models: For each tumor model, a PI must evaluate the possible adverse effects, likely incidence of adverse effects, proposed methods of controlling severity (e.g. analgesia, anesthetic, sedation), and the definition and implementation of humane endpoints.
  1. Tumor size
    1. Optimally, one observer should perform all tumor measurements in a given study.
    2. Calipers must be used to measure tumor size in order to avoid discrepancies.
    3. Tumor size must not exceed 20mm (2.0cm) at the largest diameter in an adult mouse and 40 mm (4.0cm) in adult rats. Larger size tumors must be specifically justified in the IACUC protocol. If multiple subcutaneous tumors are implanted, smaller maximum tumor sizes must be described in the IACUC protocol.
    4. Health limitations may be evident before the tumor reaches the maximum standards above. Some limitations may include mobility restriction, the inability to access food and water, pressure on internal organs or sensitive regions of the body, or body condition score (BCS) of < 2. Animals displaying such signs must be euthanized even if the maximum tumor size has not been reached.
  1. Monitoring: Clinical observations and/or palpation will be necessary to monitor for deterioration of clinical condition. Special examination techniques may be required for specific sites (e.g. respiratory rate for lung involvement, neurological disturbance for brain neoplasms, and blood cell counts for leukemias) [3].
    1. Schedule
      1. Mice and rats with developing tumors are to be observed no less than  three times weekly until a palpable tumor nodule is present (5-7.5mm in diameter), followed by daily monitoring (including weekends and holidays). Deviations from this monitoring schedule must be discussed and justified in the protocol.
      2. If tumors are located in a location that is not palpable, a monitoring schedule should be established based on pilot studies. Pilot studies can be used to familiarize the animal researcher to possible adverse effects and to define the critical time scale of adverse effects. Features to consider include tumor site, growth rate, invasion, distension, ulceration, metastasis, and production of cachectic factors.
      3. If tumor growth is rapid in the days before termination, twice daily monitoring may be necessary.
    1. Variables: Clinical signs which may be associated with tumor progression include
      1. General Appearance; including dull or closing eyes
      2. Decreased food/water intake
      3. Dehydration
      4. Weight loss (assess by weighing) and/or Body Condition Score (BCS)
      5. Depressed or restless activity or abnormal aggression
      6. Vocalizations/ Respiratory difficulty
      7. Cranial deformity/neurological signs
      8. Rough hair coat  and/or hunched posture
      9. Skin pathology
      10. Restricted mobility
      11. Changes in feces/urine and/or perianal soiling
      12. Aggression
      13. Eye/nose discharge
  1. Endpoints:  The overall well-being of the animal takes priority over precise tumor measurements in decisions regarding euthanasia or other interventions. Tumors induced in body cavities (cranium, orbit, abdomen, or thorax) may have additional limitations as to the maximum acceptable size.  These animals must be monitored very closely for any severe impairment in physiological or neurological function and be euthanized as soon as such signs become apparent.
    1. The following clinical signs are indications of morbidity. Tumor-bearing animals exhibiting these signs should be euthanized based on severity of clinical signs:
      1. BCS < 2; Muscle atrophy or emaciation
      2. Hypothermia
      3. Bloodstained or mucopurulent discharge from any orifice
      4. Labored respiration – particularly if accompanied by nasal discharge or cyanosis
      5. Ulcerated tumors
        • Note: Ulceration may be acceptable for certain skin tumors. This condition and the treatment must be discussed in the protocol.
      6. Significant abdominal distension
      7. Incontinence, inappetence or prolonged diarrhea
  1. IACUC Protocol: In Section J of the IACUC protocol, you must answer the following question: If producing tumors, will you follow the UCSF Guideline for Tumor Production in Rats and Mice. If you answer yes, you are expected to follow this guideline as written. If no, departures must be described. Possible departures that must be described may include:
    1. Tumor size larger than 20 mm (2.0cm) for mice or 40 mm (4.0cm) for rats
    2. Maximum tumor size of multiple implanted subcutaneous tumors
    3. Justification of tumor ulceration
    4. Endpoints particular to your model not described above.


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