Sarcoma

Definition

Sarcoma: a malignant tumor (cancer) of the bone, cartilage, fat, muscle, tendons, fibrous tissue, nerves, blood vessels, or other connective and supportive tissue.

Clinical signs

May observe any of the following:

A swelling / growth not accompanied by any infection.
A firm, fixed growth that seems to enlarge.
A painless mass.
Signs of pain may be gradual, may be dependent on size of growth and location, (e.g., growth involving a limb may cause the rat to favor the limb or to limp).
A systemic illness related to metastasis.
Rat easily tires.
A loss of weight.
A loss of appetite.

*Note: for additional information on recognizing various signs of pain or discomfort refer to: Signs of Pain In Rats.

Etiology

Sarcoma, a Greek word meaning “fleshy growth”, is a type of tumor that is often highly malignant. These tumors generally develop rapidly and can metastasize via the lymph channels. Composed of cells derived from connective and supportive tissues, sarcomas are often separated into two categories:

Soft Tissue Sarcomas: tumors that develop in the soft tissues (also known as the mesenchyma) of the body such as the: muscles, tendons (bands of fiber that connect the muscles to bones), fibrous tissues, fat, blood vessels, nerves, and synovial tissues (tissues around joints) that connect, support, or that surround the structures and organs in the body.
Osteosarcoma: tumors that develop in the bone, which have a different development than those of soft tissue sarcomas. Those tumors that arise in bone and contain both cartilage and bone are called compound osteosarcomas. Primary tumors of bone are not a common finding in pet rats, but when present, treatment is primarily palliative.

Sarcomatous growths can arise most anywhere on the body such as: back, chest, the hip and leg area, and in the abdomen.

Soft tissue sarcomas are further classified into subtypes based on the specific tissue type they affect, not typically from where they arise.

Fibrosarcomas

These types of sarcoma develop in cells called fibrocytes, and histiocytes. Normal developing cells make up the fibrous tissues that join structures of the body such as muscles to bones. This type of tumor may be noticed as a firm lump that is not painful to the rat.

Malignant Fibrous Histiocytomas or “MFH” , currently classified as “undifferentiated pleomorphic sarcoma”

A soft tissue sarcoma that has several histologic variations such as the following: storiform-pleomorphic, myxoid, inflammatory, giant cell, and angiomatoid. Once such variant, Myxosarcoma or Myxofibrosarcoma is of fibroblast origin containing mucopolysaccharides in a myxoid matrix (mucin or mucus like), and may affect any part of the body but is most frequently found to occur in the chest or limbs.
In general, all of these tumors tend to be aggressive.
Desmoid tumors

Is a fibrous sarcoma that tends to grow slowly. While these tumors do not tend to spread to distant sites in the body, they may extend into nearby tissues.

Liposarcomas

This type of sarcoma develops in the fat cells, usually in the layer nearest the skin, and can grow anywhere in the body. Their benign counterpart is called a lipoma.

Synovial sarcomas

Sarcomas appear as hard lumps that can not only develop near joints but in other parts of the body as well. Some studies indicate that this type of tumor has not been identified in rats.

Rhabdomyosarcomas

Sarcomas that develop in the active muscles known as skeletal muscle or striated muscle (muscle under our control). This type of tumor can occur in the head, neck, pelvis, and limbs. In some laboratory studies where rats have been exposed to 2, 6-xylidine, rhabdomyosarcomas have been found to occur in the nasal cavity (Boorman and Everitt, 2005).

Leiomyosarcomas

These tumors develop from smooth muscle cells, and can occur anywhere in the body. Smooth muscle is involuntary muscle (muscle not under voluntary control) that forms the walls of the rat’s bicornuate uterus, the stomach, intestine and the blood vessels.

Malignant peripheral nerve sheath tumours (MPNST)

This type of sarcoma develops in the cells that cover nerves and can occur anywhere in the body. Cells which form the sheath around the nerve cells are called Schwann cells. Tumors that develop from these cells can also be called malignant schwannomas or neurofibrosarcomas. Their benign counterparts are schwannomas (neurilemoma) or neurofibromas.

Angiosarcoma

Sarcomas that develop from cells that make up the walls of blood or lymph vessels.

Hemangiosarcomas: developed from blood vessels.
Lymphangiosarcomas: developed from lymph vessels.

Gastrointestinal stromal tumors

Sarcomas that develop from the cells of the connective tissues that support the organs of the digestive system. Primary GI stromal tumors are not common in pet rats.

Two additional categories

Kaposi’s sarcoma and Ewings sarcoma, are not mentioned herein as there are no documented cases for pet rats at this time.

In addition to sarcoma being classified into subtypes based on the specific tissue the tumor affects, the classification of soft-tissue and bone tumors are based on biological behavior, structure, and by results of immunohistochemical and cytogenetic studies.

Histologically sarcomas can be graded as low, intermediate, or high grade for rate of metastasis. The grade is also based on tumor morphology: predominant cell type (e.g., pleomorphic, round cell, spindle cell, myxoid), atypia, mitosis, and necrosis.¹ The classification, type, patterns and characteristics help to determine how slow or fast growing the tumor is, and how likely it is to spread.

More than one grading or staging system may be recognized. The following sites can be viewed for more information on grading and staging.

Soft tissue sarcomas, in the early stage of tumor growth, do not often cause discomfort since soft tissue is relatively elastic (stretches to accommodate growth). This means tumors can often grow fairly large before they start to cause discomfort or pain by pushing on nearby structures such as organs, muscle, or nerves. Where soft tissue sarcomas are present, removal of the tumor with clean margins is often the treatment of choice while the tumor is small.

In cases where these tumors cannot be excised due to the size of tumor, organ involvement, or metastasis, the prognosis is grave.

While the cause of sarcoma is not generally known, research has suggested that some types of sarcoma tumors may be due to a damaged gene that controls the activity of supporting tissues.

Besides genetics, other factors may influence or increase the chance of sarcoma development in pet rats. These factors may Include:

Dietary influence (increased fat, high protein, and high calorie intake)
Chemical exposure (pine and cedar bedding)
Infection
Hormonal
Aging
Poor immune system

Figures

Case Histories of Sarcoma

Soft tissue sarcoma

Fig. 1: Sarcoma in adult male rat (Neo)
Fig. 2: Sarcoma in adult male rat
Fig. 3: Hemangiosarcoma in adult male rat. *Warning! These photos are very graphic in nature.*
Fig. 4: Gastric sarcoma in adult male rat. *Necropsy photos and case history graphic in nature.*
Fig. 5: Myxosarcoma in 20-month-old male rat(D’Lochlan)
Fig. 6: Vaginal sarcoma in a 26-month-old female rat (Jango)
Fig. 7: Round cell sarcoma in a 22-month-old female rat (Agnes)
Fig. 8: Fibrosarcoma of the leg in a 9-month-old female rat (Victoria) *Includes photos and video with case history.*
Fig. 9: Sarcoma of cecum causing intussusception: resecting tumor, reduce and repair of intussusception in 20-month-old male rat (Tugger). *Warning; includes graphic necropsy photos along with case history.*
Fig. 10: Soft Tissue Sarcoma of the forelimb and subsequent amputation in 22-month-old male rat (Ace)

Osteosarcoma

Fig. 1: Osteosarcoma in adult female rat (Zinnia)

Diagnostics

Fine needle biopsy is not always recommended due to the risk of introducing malignant cells back through the biopsy tract.

While an x-ray may confirm bone tumor or an ultrasonogram a soft tissue sarcoma, these are not always feasible in a rat, and may not be affordable for a pet owner.

Confirmation may in some cases only come if able to do a core biopsy, or if a necropsy is permitted following the rat’s death.

Microscopic examination of tissue from a core biopsy can show the extent of cellular abnormality. This will help to determine if the tumor is low-grade (unlikely to metastasize) or high-grade (more likely to metastasize).

Treatment

Excision of soft tissue sarcomas, if feasible and health permits, is recommended.
If possible, the veterinarian may remove the cancer and a safe margin of the healthy tissue around it in order to prevent further spread of the cancer.

While in some cases amputation may be considered with soft tissue sarcoma of the leg, it is not always practical. However, should amputation be the choice of treatment, it is recommended that a broad-spectrum antibiotic along with medication for pain control be given.

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.

Palliative Care

Provide pain medication if: rat exhibits signs of pain, lesion is in advanced stage, or if the rat is not physically able to tolerate surgery. A nonsteroidal anti-inflammatory agent such as Banamine or Metacam, or a corticosteroid such as dexamethasone or prednisone may be included to help reduce inflammation and swelling of surrounding tissue.

Medication to be given by mouth or injection will be based on the severity, or if the rat is able or willing to take meds by mouth.

For information regarding medications refer to the Rat Medication Guide.

*Note: Always check with a veterinarian when giving any medications to determine appropriate use or contraindication.

Nursing Care

If surgery is not an option:

Give medications as prescribed by veterinarian.
Provide a safe environment.
If mobility is impaired, provide a one level cage with soft bedding/litter.
Keep food and water within easy reach.
Provide additional food and fluid supplements if appetite is poor (see suggestions listed under: If surgery is an option).

If surgery is an option:

Provide hospital cage during recovery, or if there are concerns that cage mates may groom sutures or wound site.
Provide clean bedding daily such as felt, soft t-shirt type material or ink-free paper towels. Avoid using material such as terry cloth type towels that can ravel. Also avoid litter-type bedding, post-op, until healed to prevent the chance of wound contamination or infection.
Provide additional warmth to maintain body temperature within normal limits. It is essential that the rat does not become overheated or dehydrated. The rat should also be able to move away from the heat source if it becomes uncomfortable. If the rat is unconscious or immobile extreme care must be taken to keep the heat low and stable.
- You can use an isothermic product that is heated in the microwave such as SnuggleSafe®. Make sure to follow the product directions carefully and wrap in a towel before placing in the cage. SnuggleSafe® will provide heat for 12 hours before needing to be reheated. Other similar types of products may vary in re-heat time. Check directions for individual product.
- If using a heating pad (good for long term use) use only the low heat setting, put a thick towel in between the pad and the cage bottom, and place beneath a corner of the cage.
- If none of these options are available you can use a plastic bottle filled with hot water, and wrapped in a towel, in the corner of the cage.
Medicate for post-op pain as needed.
Body wrap may be required if thread sutures are used.
In the event of dried or excess drainage, the incision site may be cleaned with a moistened Q-tip (swab), using warm water or normal saline.
In cases where limb amputation has been performed: prevent wound area contamination by removing soiled litter-free bedding daily, and replace with fresh bedding.
Contact veterinarian if any of the following signs are observed: swelling, redness, or pain at the incision site, or if there are signs of increased weight loss, lethargy, or changes in habits.
For 24 to 48 hours post-op, feed iron-rich foods to prevent anemia (cooked liver, scrambled or hard-boiled eggs).
During recuperation provide additional high calorie foods or food supplements such as Nutri-Cal Paste, canned Ensure, soy or soy formula. Include multi-vitamin supplement (can be found in pet store) if food intake is poor.
The health supplement co-enzyme Q10 may also be of benefit (refer to the Rat Medication Guide for dosing).
Encourage fluid intake while recuperating, such as water, Jell-O water or electrolyte replacement drinks such as Pedialyte or Gatorade (which can be found in local grocery stores).
- Please note that Pedialyte is only good refrigerated for 24 hours after opened, but can be frozen as ice cubes and thawed as needed.
- *Note: a juicy type of fruit also provides an additional fluid source in the diet.

Outcome

Discomfort is controlled or relieved.
In the event surgery is performed, incision site remains free from infection, and post-op pain is relieved.
Maintains adequate elimination of bladder and bowel.
Maintained in safe one level environment for easy access to food and water, and to reduce discomfort from having to climb ramps or ladders.
Emotional support for those having to consider euthanasia for their rat.

Prevention

While there are no guidelines for the prevention of soft tissue sarcoma since the exact cause is unknown, the following tips may be of benefit:

Performing daily health checks on your rat may help to catch abnormal growths in the early stages. For a reference on how to perform a health check, refer to Basic Health Check or Advanced Health Check in the Rat Guide.
Treating illnesses in the early stages to help maintain a healthy immune system.
Providing a healthy approved diet for rats.
Avoid litter/bedding that are made of chemically treated wood products.

References

Shmookler B, Bickels J, Jelinck J, Sugarbaker P, Malawer M. Bone and soft-tissue sarcomas: epidemiology, radiology, pathology and fundamentals of surgical treatment. In: MalawerMM, SugarbakerTH, editors: Musculoskeletal cancer surgery. Treatment of sarcomas and allied diseases. Dordrecht: Kluwer Academic Publishers, 2001: 3–36.
Boorman, G., Everitt, J. (2005). Neoplastic Disease. In M. Suckow, Weisbroth, S., & Franklin, C. (Eds.), The Laboratory Rat, Second Edition (American College of Laboratory Animal Medicine) (pp. 479-512). Toronto: Academic Press.
Kirkpatrick, C., Alves, A., Köhler, H., Kriegsmann, J., Bittinger, F., Otto, M., Williams, D., & Eloy, R. (2000). Biomaterial-induced sarcoma: A novel model to study preneoplastic change. Am J Pathol, 156(4), 1455-67. Retrieved December 21, 2008, from http://ajp.amjpathol.org/cgi/content/full/156/4/1455.