Mast Cell Tumors in Dogs
What are mast cell tumors?
Mast cells are present in most tissues, and are especially prominent in the skin, lining of the lungs and digestive tract,
mouth and nose. They are an important component of the immune system, with functions in inflammation and allergy
reactions. Mast cell tumors (MCTs) are formed by the abnormal proliferation of mast cells, and when the entire body is
affected, the disease is referred to as mastocytosis. Mast cells can release several biologically active chemicals such as
histamine and heparin, which can be very damaging to the body when released in excess by the tumor cells.
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?
Skin MCTs are the most common type of mast cell tumors, accounting for 16%-21% of all skin tumors in the dog.
Although MCTs can also be found in the liver, spleen, gastrotintestinal tract and bone marrow, they are not as common
as in the skin. The average age at diagnosis is 9 years and there is some evidence that boxers, Boston terriers,
Labrador retrievers, beagles, and schnauzers may be at increased risk for developing MCTs.
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 (mass) to a soft subcutaneous lump to an ulcerated skin mass. MCTs that are located
underneath the skin appear as soft nodules and can be misdiagnosed as lipoma.
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 still resembles mast cells). Tumors that
are composed of cells that 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), with small nodules forming nearby. The clinical symptoms of dogs with MCTs may be complicated due to
the release of chemicals by the tumor cells, which can cause gastrointestinal ulceration (open sores). In this case, the
dogs may experience vomiting, anorexia, dark feces and abdominal pain.
How is the diagnosis made?
Mast cell tumors are initially diagnosed by fine-needle aspriation (FNA) cytology. While FNA is useful, it does not provide
any information about the tumor's grade (level of aggressiveness), and biopsy is therefore recommended. Additional
diagnostic steps will vary depending on the tumor's location, size and initial findings. If the tumor is located at a site easily
accessible for surgical removal, and no obvious negative prognostic factors are present, the tumor is usually surgically
removed and the sample sent to pathology lab for evaluation. If the tumor develops at a site where complete surgical
removal is not possible and/or if additional negative prognostic factors are present, further diagnostic tests are done
prior to initiating treatment. The veterinarian will typically examine the regional lymph nodes to check for metastasis
(spread of cancer cells), do blood test to check for overall health of the pet, buffy coat smear to check for circulating
mast cell tumor cells throughout the body, 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
MCT dissemination (spread). Advanced imaging such as CT (computed tomography) can be a very useful diagnostic
tool that can evaluate the extent of the disease in more detail and can aid the oncologist in planning an appropriate
treatment strategy.
Do these tumors cause pain?
It is imperative that pets with cancer are provided with pain medication to relieve discomfort caused by the disease as
well as by the treatments. To learn more, please visit the Cancer Pain Management section.
What food should be fed to pets with cancer?
Pets with cancer need to be fed diets specifically designed to support a cancer patient. To learn more, please visit the
Cancer Nutrition section.
What are the treatment options?
Treatment strategies will largely depend on the tumor's grade (level of aggressiveness) and how advanced the disease
is. In addition to treating the tumor itself, some cases may also require treatment with anti-histamine medication and/or
drugs to relieve gastrointestinal ulceration associated with MCTs.
Low or intermediate grade (mildly aggressive) MCT easily accessible for complete surgical removal
For easily accessible MCTs, 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 ensure that no cancer cells are left behind. 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 on regular basis 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 (cancer spread) 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 and spreading to distant sites.
These will typically include either additional surgery that will remover larger area around the tumor and/or radiation
therapy and/or chemotherapy.
Low or intermediate grade (mildy aggressive) 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), 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 (highly aggressive) MCT easily accessible for complete surgical removal
For easily accessible MCTs, 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 ensure that no cancer cells are left behind. 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 on regular basis 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 around the tumor or
the area can be subjected to radiation therapy, followed by chemotherapy.
High grade (highly aggressive) 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:
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Are there any clinical trials investigating new treatments?
There are only a few available clinical trials investigating new treatments for mast cell tumors in pets. To learn more
about clinical trials open to enrollment in the United States, please visit Clinical Trials for Mast Cell Tumors section. To
learn more about veterinary clinical trials in general, please visit the Pet Clinical Trials section.
To help Pet Cancer Center conduct more mast cell tumor clinical trials across the United States in order to make novel
and more efficacious therapies available to pets, please
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. 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.
.
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Dogs diagnosed with visceral MCTs (tumors 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.
Additional online resources about mast cell tumors:
Sources:
Mast cells are present in most tissues, and are especially prominent in the skin, lining of the lungs and digestive tract,
mouth and nose. They are an important component of the immune system, with functions in inflammation and allergy
reactions. Mast cell tumors (MCTs) are formed by the abnormal proliferation of mast cells, and when the entire body is
affected, the disease is referred to as mastocytosis. Mast cells can release several biologically active chemicals such as
histamine and heparin, which can be very damaging to the body when released in excess by the tumor cells.
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?
Skin MCTs are the most common type of mast cell tumors, accounting for 16%-21% of all skin tumors in the dog.
Although MCTs can also be found in the liver, spleen, gastrotintestinal tract and bone marrow, they are not as common
as in the skin. The average age at diagnosis is 9 years and there is some evidence that boxers, Boston terriers,
Labrador retrievers, beagles, and schnauzers may be at increased risk for developing MCTs.
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 (mass) to a soft subcutaneous lump to an ulcerated skin mass. MCTs that are located
underneath the skin appear as soft nodules and can be misdiagnosed as lipoma.
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 still resembles mast cells). Tumors that
are composed of cells that 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), with small nodules forming nearby. The clinical symptoms of dogs with MCTs may be complicated due to
the release of chemicals by the tumor cells, which can cause gastrointestinal ulceration (open sores). In this case, the
dogs may experience vomiting, anorexia, dark feces and abdominal pain.
How is the diagnosis made?
Mast cell tumors are initially diagnosed by fine-needle aspriation (FNA) cytology. While FNA is useful, it does not provide
any information about the tumor's grade (level of aggressiveness), and biopsy is therefore recommended. Additional
diagnostic steps will vary depending on the tumor's location, size and initial findings. If the tumor is located at a site easily
accessible for surgical removal, and no obvious negative prognostic factors are present, the tumor is usually surgically
removed and the sample sent to pathology lab for evaluation. If the tumor develops at a site where complete surgical
removal is not possible and/or if additional negative prognostic factors are present, further diagnostic tests are done
prior to initiating treatment. The veterinarian will typically examine the regional lymph nodes to check for metastasis
(spread of cancer cells), do blood test to check for overall health of the pet, buffy coat smear to check for circulating
mast cell tumor cells throughout the body, 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
MCT dissemination (spread). Advanced imaging such as CT (computed tomography) can be a very useful diagnostic
tool that can evaluate the extent of the disease in more detail and can aid the oncologist in planning an appropriate
treatment strategy.
Do these tumors cause pain?
It is imperative that pets with cancer are provided with pain medication to relieve discomfort caused by the disease as
well as by the treatments. To learn more, please visit the Cancer Pain Management section.
What food should be fed to pets with cancer?
Pets with cancer need to be fed diets specifically designed to support a cancer patient. To learn more, please visit the
Cancer Nutrition section.
What are the treatment options?
Treatment strategies will largely depend on the tumor's grade (level of aggressiveness) and how advanced the disease
is. In addition to treating the tumor itself, some cases may also require treatment with anti-histamine medication and/or
drugs to relieve gastrointestinal ulceration associated with MCTs.
Low or intermediate grade (mildly aggressive) MCT easily accessible for complete surgical removal
For easily accessible MCTs, 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 ensure that no cancer cells are left behind. 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 on regular basis 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 (cancer spread) 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 and spreading to distant sites.
These will typically include either additional surgery that will remover larger area around the tumor and/or radiation
therapy and/or chemotherapy.
Low or intermediate grade (mildy aggressive) 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), 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 (highly aggressive) MCT easily accessible for complete surgical removal
For easily accessible MCTs, 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 ensure that no cancer cells are left behind. 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 on regular basis 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 around the tumor or
the area can be subjected to radiation therapy, followed by chemotherapy.
High grade (highly aggressive) 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:
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Are there any clinical trials investigating new treatments?
There are only a few available clinical trials investigating new treatments for mast cell tumors in pets. To learn more
about clinical trials open to enrollment in the United States, please visit Clinical Trials for Mast Cell Tumors section. To
learn more about veterinary clinical trials in general, please visit the Pet Clinical Trials section.
To help Pet Cancer Center conduct more mast cell tumor clinical trials across the United States in order to make novel
and more efficacious therapies available to pets, please
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. 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.
.
Source: Withrow Stephen J, and David M. Vail. Small Animal Clinical Oncology. St Louis: Saunders Elsevier, 2007.
Dogs diagnosed with visceral MCTs (tumors 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.
Additional online resources about mast cell tumors:
- 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.
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Last updated 8/2708
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