Definition
Melanoma is a cancer that originates from melanocytes (pigment producing cells).
Clinical Signs (Skin Melanoma)
- Light mass (amelanotic melanoma)
- Dark mass (may resemble a mole or hematoma)
- Nodular mass
- Flat discolored area (superficial spreading melanoma)
- Bleeding
- Ulceration
- Itching (pruritus)
Clinical Signs (Intraoccular Melanoma)
- Bulging of the eye from the orbit (exophthalmia)
- Opacity
- Swelling
*Note: for information on recognizing various signs of pain or discomfort refer to:
Signs of Pain In Rats.
Etiology
Melanocytes are the cells that produce melanin (the pigment found in the hair, skin, and eyes) which are responsible for color. Melanoma occurs when these cells become neoplastic. In rats melanomas have been found on the pinna (ear), scrotum, tail, eyelids, anal area, and in the eye (intraocular).
Melanomas can be either melanotic (containing melanin pigment) or amelanotic (without melanin pigment). Melanomas can differ in shape. Some are nodular masses that progress downward (nodular melanoma) while others tend to spread out (superficial spreading melanoma).
Nodular melanomas can be misdiagnosed initially as small hematomas (blood blister). However, a dark nodule that does not resolve and increases in size most likely is not a hematoma and should be looked at by a veterinarian.
Natural melanoma is not a common occurrence in the rat and most of the information found comes from spontaneous melanomas that occur during carcinogenicity studies.
During one extensive study the issue of metastisis was addressed:
Only the tumors arising in the pinna metastasized to the lung and/or mandibular lymph nodes. The metastatic rate was 19% (16/84) of the tumors and was clearly increased with an increase in tumor size. (Yoshitomi et al 1995)
Causes of melanoma are:
- Exposure to UV light
- Genetic predisposition
- Exposure to carcinogens
Prognosis for rats having melanoma is dependent upon the location of the growth and severity.
Figures
- Fig. 1: Melanoma of the ear: Apollo
Diagnostics
FNA (Fine needle aspiration): A small sample of cells can be removed from the tumor and examined microscopically for melanocytes and melanin.
Histopathology: Tissue (either from the removed mass or from a biopsy) can be put in formalin and sent to a diagnostic laboratory. A cytologic examination of prepared tissue can sometimes be difficult for a pathologist to interpret, particularly if the melanoma is amelanotic. In such cases a special immunhistochemical test can be performed using the S-100 protein marker to support the diagnosis of a melanoma.
Treatment
Surgical removal of the tumor and surrounding marginal tissue is the best option for a melanoma. Laser surgery has been used successfully to remove a hematoma and marginal tissue from an ear.
Detection and removal of affected lymph nodes is most likely not feasible in rats.
Intraoccular melanoma in a rat would most likely be asymptomatic or misdiagnosed. Those with severe clinical signs (unresponsive to treatment) involving the eye may benefit from an enucleation (eye removal).
Rats do experience pain with surgical procedures. The type of pain medication used post-op should be determined based on extent of procedure and the anticipated severity of pain.
If surgery is required the following is recommended for pain control post-op:
For severe pain or first 24 hours post-op: Buprenex (buprenorphine), or Torbugesic (butorphanol).
For mild to moderate pain: Banamine (flunixin meglumine), Metacam (meloxicam), or carprofen. Do not use if a corticosteroid has been prescribed.
For mild pain: Tylenol (acetaminophen).
For information regarding medications refer to the Rat Medication Guide.
Nursing Care
- Provide hospital cage during recovery, or if there are concerns that their cage mates may groom surgical wound site.
- Provide clean bedding daily such as cloths or ink-free paper towels. Avoid litter-type bedding post-op until healed, to prevent the chance of wound contamination or infection.
- Provide additional warmth using a hot water bottle or heating pad, if needed, in order to maintain body temperature within normal limits. Use low heat settings and place under one half of cage to ensure rat does not become overheated or dehydrated.
- Medicate for post-op pain as needed.
- Provide proper nutrition
- Keep the rat well hydrated
- Contact the vet if any of the following are observed: swelling, redness, or pain at the incision site, or if there are signs of increased weight loss, lethargy, or changes in habits.
Outcome
- Post-op pain relieved.
- Incision site free from infection.
- Any recurrences of melanoma detected early and treated.
Prevention
There is no known prevention for melanoma in rats. Routine health checks will aid in early detection of reoccurring melanomas. Detection and treatment when the growth is small decreases surgical time, enhances recovery period, lessens the chance of metastasis, and improves outcome.
References
- Pathology and Incidence of Amelanotic Melanomas of the Skin in F-344/N Rats
Katsuhiko Yoshitomi , Michael R. Elwell , Gary A. Boorman
Posted on January 13, 2010, 13:58,
Last updated on April 10, 2010, 18:03
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