Skin Mast Cell Tumors in Dogs
What are skin mast cell tumors?
Mast cells are an important component of the immune system, playing important roles in inflammation and allergic
reactions. A mast cell (or mastocyte) is a resident cell of several types of tissues and contains many granules rich in
histamine and heparin. Mast cells are present in most tissues and are especially prominent in the skin, lining of the lungs
and digestive tract, mouth and nose. Mast cell tumors (MCTs) are formed by the abnormal proliferation of mast cells and
when the entire body is affected, then the disease is referred to as mastocytosis.
The behavior or skin mast cell tumors ranges from being benign (non-cancerous) to highly malignant (cancerous). There
is a wide variation in the histological pattern of MCTs and the tumor grade (level of aggressiveness) is a strong predictor
of how well the dog will do. The metastatic potential (ability to spread to other organs) is not known for MCTs. It is
estimated that well-differentiated tumor cells (those that still resemble mast cells) have a low-to-moderate risk of
metastasis but undifferentiated tumor cells (those that no longer resemble mast cells) have a high metastatic potential.
Most of these tumors spread first to regional lymph nodes and then mainly to the spleen and liver.
How common are these tumors?
Mast cell tumors are the most common cutaneous (skin) tumors in the dog, accounting for 16%-21% of all skin tumors.
The average age at diagnosis is 8 years but puppies as young as 4 months have been reported. There is some
evidence that certain breeds may be at higher risk of developing skin mast cell tumors, including boxers, bulldog breeds,
bullmastiffs, Boston terriers, Staffordshire, Rhodesian ridgbacks, pugs, Labradors retrievers, golden retrievers,
weimaraners, and beagles. Although mast cells are also present in the lung and gastrotintestinal tract, the development
of tumors in these sites is not as common as for the skin. Skin MCTs are most commonly found on the trunk (50-60%)
and the limbs (25%). MCTs that are located underneath the skin appear as soft nodules and can be misdiagnosed as
lipoma.
What are the symptoms?
Skin MCTs are usually detected by owners as raised lumps on the skin which can have a wide range of appearance,
from a wart-like nodule to a soft subcutaneous lump to an ulcerated skin mass. The appearance can mimic other types
of skin tumors, therefore biopsy should be done to determine the tumor's type. This is important prior to initiating
treatment since different tumor types may require different treatment strategies.
Most tumors are solitary (single mass) but 11-14% of dogs can have multiple tumors. The appearance of these tumors
depends on how differentiated the tumor cells are (how much their appearance resembles mast cells). Tumors that are
composed of cells that still resemble mast cells under the microscope are usually slow growing and not ulcerated,
although the hair may be lost in the area. Tumors that are composed of cells that no longer resemble mast cells are
typically rapidly growing and ulcerated. Surrounding tissues may become inflamed and edematous (swollen due to fluid
accumulation) and small nodules (masses) can develop in the surrounding tissues. The tumor's size may fluctuate in size
and degree of redness due to the release of chemicals by the cells within the tumor environment.
Although boxers, bulldogs and pugs are at increased risk of developing skin mast cell tumors, they tend to have less
aggressive tumors. Labrador retrievers tend to have more aggressive tumors and golden retrievers tend to have multiple
tumors.
What are the stages of the disease?
The World Health Organization has developed a clinical staging system for canine mast cell tumors, reflecting the natural
progression of the disease. This system is by no means absolute and serves only as a guide.
How is the diagnosis made?
Mast cell tumors are initially diagnosed based on the basis of fine-needle aspiration (FNA) cytology but this technique
does not provide any information about the tumor's grade (level of aggressiveness). Taking out a piece of the tumor
through a procedure called biopsy in order to obtain histologic assessment of the tumor cells is, therefore,
recommended. Fine needle aspiration cytology is usually also used to evaluate the nearby lymph nodes since malignant
tumor cells travel through the lymphatic system. If the cytology test confirms the presence of tumor cells in the lymph
nodes, the dogs will typically face a poorer prognosis. It should be noted that the interpretation of lymph node cytology
results should be done with caution since mast cells can be found in the lymph nodes of healthy animals as well. The
veterinarian will usually also do blood tests to check the overall health of the pet, abdominal ultrasound to check for
metastasis to the spleen or liver, chest X-rays to check for metastasis to the lungs and fluid accumulation in the chest,
and bone marrow aspiration to check for infiltration.
What are the treatment options?
Treatment strategies will largely depend on the tumor's grade (level of aggressiveness) and stage of the disease. The
most widely used treatment options are summarized below.
Low or intermediate grade tumors easily accessible for complete surgical removal
For easily accessible MCT tumors, surgical removal is the treatment choice, followed up by histologic evaluation to
ensure completeness of surgery. The surgery will typically remove not only the tumor itself but also normal tissue
surrounding the tumor to make sure that all cancer cells that may have locally spread are removed. If the histologic
evaluation confirms that no cancer cells were present in the healthy tissues, then the dogs should either come for
routine check ups every 3 months to monitor possible tumor recurrence/metastasis or the pet owners can agree to
chemotherapy. In this setting, the goal of chemotherapy would be to prevent or minimize the risk of metastasis and/or
local tumor recurrence. Because of the lack of adequate studies, veterinary oncologists seem to disagree on the
potential benefit of chemotherapy in this particular scenario. If the histologic evaluation confirms the presence of cancer
cells in the healthy tissues, then additional treatments are necessary to prevent the tumor from growing back. These will
typically include either additional surgery that will remover larger area around the tumor or radiation therapy or
chemotherapy.
Low or intermediate grade tumor not easily accessible for complete surgical removal
For tumors that are not easily accessible for complete surgical removal such as distal extremities (eg paws), it is usually
recommended that an incisional biopsy is done prior to deciding on a definitive treatment plan. In this case, three main
treatment options exist: amputation, radiation therapy and combination of surgery with radiation therapy. Amputation is
an aggressive treatment option with the best chance of controlling the tumor but it will also affect the dog's function.
Radiation therapy alone has had varying degrees of success. Combining radiation with surgery offers an attractive
alternative to limb amputation, in which the surgery removes as much of the mass as possible and the radiation beam
kills any remaining cancer cells left behind. Alternative treatments may include surgical removal followed by
chemotherapy or chemotherapy alone but the benefit of these treatments is not established. Regardless of the
procedure chosen, the dogs should be evaluated on regular basis to monitor for local tumor recurrence and possible
metastasis.
High grade MCT easily accessible for complete surgical removal
For easily accessible MCT tumors, complete surgical removal is the treatment of choice, followed up by histologic
evaluation to ensure completeness of surgery. The surgery will typically remove not only the tumor itself but also
surrounding normal tissue to make sure that all cancer cells that may have locally spread are removed. If the histologic
evaluation confirms that no cancer cells were present in the healthy tissues, then chemotherapy is used to try to kill any
circulating tumor cells that are likely to be present due to the aggressive behavior of high grade tumors. The dogs are
then followed up every 3 months for any signs of local recurrence and/or metastasis development. If the histologic
evaluation confirmed the presence of cancer cells in the healthy tissues, then either another surgery is attempted that
will excise a larger region or the area can be subjected to radiation therapy, followed by chemotherapy.
High grade MCT not easily accessible for complete surgical removal
For tumors that are not easily accessible for complete surgical removal such as distal extremities (eg paws), several
options are available:
Recently, several chemotherapy drugs have been evaluated for their efficacy. There is accumulating evidence in both
human and veterinary oncology that certain combination of different classes of chemotherapy drugs may offer the best
chance of controlling the spread of the tumor throughout the body compared to single drugs. The following table
summarizes the response to commonly used chemotherapy drugs in the setting of mast cell tumors:
What is the prognosis?
The prognosis for dogs with MCTs is dependent on a variety of different factors such as the tumor's size, location, grade
(level of aggressiveness), age, or symptoms. Histologic tumor grade appears to be the most consistent prognostic factor
for dogs with MCTs and the survival times associated with specific tumor grades are summarized the following table. High
grade, undifferentiated tumor corresponds to a highly aggressive tumor and most dogs die within 1 year after surgery as
a result of local recurrence or metastasis.
Dogs diagnosed with visceral MCTs (in their internal organs) face poorer prognosis compared to those in the skin.
These dogs may show symptoms such as anorexia, vomiting, dark feces, wide-spread inflammation, and edema due to
the release of substances from the tumor mast cells. One study showed that only 40% of dogs with gastrointestinal
MCTs were alive 30 days after diagnosis and less than 10% were alive at 6 months. Therefore, an aggressive treatment
is warranted upon the initial diagnosis to improve the pet's long-term prognosis.
Are there any clinical trials?
Below is a summary of available clinical trials. For more details, please click here.
Study: Canine recurrent or nonresectable grade 2 or 3 mast cell tumor(s) without nodal or
visceral metastasis.
Location: Gulf Coast Veterinary Specialists, South Houston, TX
Phone: 713-692-1111
www.gcvs.com
Study: Canine recurrent or nonresectable grade II or III mast cell tumor without metastasis or lymph node involvement.
Location: Veterinary Oncology&Hematology Clinic, Norwalk, CT
Phone: 203-838-6626
www.oncovet.com/vohc-clinic.htm
Additional online resources:
Sources:
Mast cells are an important component of the immune system, playing important roles in inflammation and allergic
reactions. A mast cell (or mastocyte) is a resident cell of several types of tissues and contains many granules rich in
histamine and heparin. Mast cells are present in most tissues and are especially prominent in the skin, lining of the lungs
and digestive tract, mouth and nose. Mast cell tumors (MCTs) are formed by the abnormal proliferation of mast cells and
when the entire body is affected, then the disease is referred to as mastocytosis.
The behavior or skin mast cell tumors ranges from being benign (non-cancerous) to highly malignant (cancerous). There
is a wide variation in the histological pattern of MCTs and the tumor grade (level of aggressiveness) is a strong predictor
of how well the dog will do. The metastatic potential (ability to spread to other organs) is not known for MCTs. It is
estimated that well-differentiated tumor cells (those that still resemble mast cells) have a low-to-moderate risk of
metastasis but undifferentiated tumor cells (those that no longer resemble mast cells) have a high metastatic potential.
Most of these tumors spread first to regional lymph nodes and then mainly to the spleen and liver.
How common are these tumors?
Mast cell tumors are the most common cutaneous (skin) tumors in the dog, accounting for 16%-21% of all skin tumors.
The average age at diagnosis is 8 years but puppies as young as 4 months have been reported. There is some
evidence that certain breeds may be at higher risk of developing skin mast cell tumors, including boxers, bulldog breeds,
bullmastiffs, Boston terriers, Staffordshire, Rhodesian ridgbacks, pugs, Labradors retrievers, golden retrievers,
weimaraners, and beagles. Although mast cells are also present in the lung and gastrotintestinal tract, the development
of tumors in these sites is not as common as for the skin. Skin MCTs are most commonly found on the trunk (50-60%)
and the limbs (25%). MCTs that are located underneath the skin appear as soft nodules and can be misdiagnosed as
lipoma.
What are the symptoms?
Skin MCTs are usually detected by owners as raised lumps on the skin which can have a wide range of appearance,
from a wart-like nodule to a soft subcutaneous lump to an ulcerated skin mass. The appearance can mimic other types
of skin tumors, therefore biopsy should be done to determine the tumor's type. This is important prior to initiating
treatment since different tumor types may require different treatment strategies.
Most tumors are solitary (single mass) but 11-14% of dogs can have multiple tumors. The appearance of these tumors
depends on how differentiated the tumor cells are (how much their appearance resembles mast cells). Tumors that are
composed of cells that still resemble mast cells under the microscope are usually slow growing and not ulcerated,
although the hair may be lost in the area. Tumors that are composed of cells that no longer resemble mast cells are
typically rapidly growing and ulcerated. Surrounding tissues may become inflamed and edematous (swollen due to fluid
accumulation) and small nodules (masses) can develop in the surrounding tissues. The tumor's size may fluctuate in size
and degree of redness due to the release of chemicals by the cells within the tumor environment.
Although boxers, bulldogs and pugs are at increased risk of developing skin mast cell tumors, they tend to have less
aggressive tumors. Labrador retrievers tend to have more aggressive tumors and golden retrievers tend to have multiple
tumors.
What are the stages of the disease?
The World Health Organization has developed a clinical staging system for canine mast cell tumors, reflecting the natural
progression of the disease. This system is by no means absolute and serves only as a guide.
How is the diagnosis made?
Mast cell tumors are initially diagnosed based on the basis of fine-needle aspiration (FNA) cytology but this technique
does not provide any information about the tumor's grade (level of aggressiveness). Taking out a piece of the tumor
through a procedure called biopsy in order to obtain histologic assessment of the tumor cells is, therefore,
recommended. Fine needle aspiration cytology is usually also used to evaluate the nearby lymph nodes since malignant
tumor cells travel through the lymphatic system. If the cytology test confirms the presence of tumor cells in the lymph
nodes, the dogs will typically face a poorer prognosis. It should be noted that the interpretation of lymph node cytology
results should be done with caution since mast cells can be found in the lymph nodes of healthy animals as well. The
veterinarian will usually also do blood tests to check the overall health of the pet, abdominal ultrasound to check for
metastasis to the spleen or liver, chest X-rays to check for metastasis to the lungs and fluid accumulation in the chest,
and bone marrow aspiration to check for infiltration.
What are the treatment options?
Treatment strategies will largely depend on the tumor's grade (level of aggressiveness) and stage of the disease. The
most widely used treatment options are summarized below.
Low or intermediate grade tumors easily accessible for complete surgical removal
For easily accessible MCT tumors, surgical removal is the treatment choice, followed up by histologic evaluation to
ensure completeness of surgery. The surgery will typically remove not only the tumor itself but also normal tissue
surrounding the tumor to make sure that all cancer cells that may have locally spread are removed. If the histologic
evaluation confirms that no cancer cells were present in the healthy tissues, then the dogs should either come for
routine check ups every 3 months to monitor possible tumor recurrence/metastasis or the pet owners can agree to
chemotherapy. In this setting, the goal of chemotherapy would be to prevent or minimize the risk of metastasis and/or
local tumor recurrence. Because of the lack of adequate studies, veterinary oncologists seem to disagree on the
potential benefit of chemotherapy in this particular scenario. If the histologic evaluation confirms the presence of cancer
cells in the healthy tissues, then additional treatments are necessary to prevent the tumor from growing back. These will
typically include either additional surgery that will remover larger area around the tumor or radiation therapy or
chemotherapy.
Low or intermediate grade tumor not easily accessible for complete surgical removal
For tumors that are not easily accessible for complete surgical removal such as distal extremities (eg paws), it is usually
recommended that an incisional biopsy is done prior to deciding on a definitive treatment plan. In this case, three main
treatment options exist: amputation, radiation therapy and combination of surgery with radiation therapy. Amputation is
an aggressive treatment option with the best chance of controlling the tumor but it will also affect the dog's function.
Radiation therapy alone has had varying degrees of success. Combining radiation with surgery offers an attractive
alternative to limb amputation, in which the surgery removes as much of the mass as possible and the radiation beam
kills any remaining cancer cells left behind. Alternative treatments may include surgical removal followed by
chemotherapy or chemotherapy alone but the benefit of these treatments is not established. Regardless of the
procedure chosen, the dogs should be evaluated on regular basis to monitor for local tumor recurrence and possible
metastasis.
High grade MCT easily accessible for complete surgical removal
For easily accessible MCT tumors, complete surgical removal is the treatment of choice, followed up by histologic
evaluation to ensure completeness of surgery. The surgery will typically remove not only the tumor itself but also
surrounding normal tissue to make sure that all cancer cells that may have locally spread are removed. If the histologic
evaluation confirms that no cancer cells were present in the healthy tissues, then chemotherapy is used to try to kill any
circulating tumor cells that are likely to be present due to the aggressive behavior of high grade tumors. The dogs are
then followed up every 3 months for any signs of local recurrence and/or metastasis development. If the histologic
evaluation confirmed the presence of cancer cells in the healthy tissues, then either another surgery is attempted that
will excise a larger region or the area can be subjected to radiation therapy, followed by chemotherapy.
High grade MCT not easily accessible for complete surgical removal
For tumors that are not easily accessible for complete surgical removal such as distal extremities (eg paws), several
options are available:
- surgical removal combined with radiation therapy (including regional lymph nodes) and chemotherapy
- amputation combined with chemotherapy
- surgical removal combined with chemotherapy
- chemotherapy alone
- radiation therapy combined with chemotherapy
- radiation therapy alone
Recently, several chemotherapy drugs have been evaluated for their efficacy. There is accumulating evidence in both
human and veterinary oncology that certain combination of different classes of chemotherapy drugs may offer the best
chance of controlling the spread of the tumor throughout the body compared to single drugs. The following table
summarizes the response to commonly used chemotherapy drugs in the setting of mast cell tumors:
What is the prognosis?
The prognosis for dogs with MCTs is dependent on a variety of different factors such as the tumor's size, location, grade
(level of aggressiveness), age, or symptoms. Histologic tumor grade appears to be the most consistent prognostic factor
for dogs with MCTs and the survival times associated with specific tumor grades are summarized the following table. High
grade, undifferentiated tumor corresponds to a highly aggressive tumor and most dogs die within 1 year after surgery as
a result of local recurrence or metastasis.
Dogs diagnosed with visceral MCTs (in their internal organs) face poorer prognosis compared to those in the skin.
These dogs may show symptoms such as anorexia, vomiting, dark feces, wide-spread inflammation, and edema due to
the release of substances from the tumor mast cells. One study showed that only 40% of dogs with gastrointestinal
MCTs were alive 30 days after diagnosis and less than 10% were alive at 6 months. Therefore, an aggressive treatment
is warranted upon the initial diagnosis to improve the pet's long-term prognosis.
Are there any clinical trials?
Below is a summary of available clinical trials. For more details, please click here.
Study: Canine recurrent or nonresectable grade 2 or 3 mast cell tumor(s) without nodal or
visceral metastasis.
Location: Gulf Coast Veterinary Specialists, South Houston, TX
Phone: 713-692-1111
www.gcvs.com
Study: Canine recurrent or nonresectable grade II or III mast cell tumor without metastasis or lymph node involvement.
Location: Veterinary Oncology&Hematology Clinic, Norwalk, CT
Phone: 203-838-6626
www.oncovet.com/vohc-clinic.htm
Additional online resources:
- www.vet.uga.edu/vpp/clerk/Dahm/Index.php
- www.kateconnick.com/library/mastcelltumor.html
- www.caninecancerawareness.org/html/CanineCancerMastCell.html
- www.vetsurgerycentral.com/mct.htm
- http://veterinarycancer.com/mastcelltumors.html
- www.peteducation.com/article.cfm?cls=2&cat=1638&articleid=461
- www.answers.com/topic/mast-cell-tumor
Sources:
- Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
- Morrison Wallace B. Cancer in Dogs and Cats: Medical and Surgical Management. Baltimore: Williams&Wilkins,
1998.
| PET CANCER CENTER Comprehensive guide to cancer diagnosis and treatment in cats and dogs |
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
Last updated 1/8/08
|
Please make a donation to support our goals of funding veterinary cancer research and
continuing to provide free online resources for pet owners
continuing to provide free online resources for pet owners