Race Foster, DVM
Holly Nash, DVM, MS
Foster & Smith, Inc. Mammary
tumors are the most common
tumors in female dogs who have
spayed. Mammary tumors can be small,
simple nodules or large, aggressive,
metastatic growths. With early
detection and prompt treatment, even some of
the more serious tumors can be successfully
treated. Cats also suffer from mammary
tumors and they have their own unique set of
problems that are discussed in a separate
Which dogs are at
risk for developing mammary tumors?
Mammary tumors are more common in
unspayed, middle-aged female dogs (those
between 5 and 10 years of age), although
they can, on rare occasions, be found in
dogs as young as 2 years. These tumors
are rare in dogs that were spayed under
2 years of age. Occasionally, mammary
tumors will develop in male dogs and
these are usually very aggressive and
have a poor prognosis.
|The risk of breast
cancer is almost eliminated in dogs
that are spayed before their
greatly reduces the chances
of a female dog developing this
condition. In those females spayed prior
to their first heat cycle, breast cancer
is very, very rare. The risk of
malignant mammary tumors in dogs spayed
prior to their first heat is 0.05%. It
is 8% for dog spayed after one heat, and
26% in dogs spayed after their second
heat. It is believed that the
elimination or reduction of certain
hormonal factors causes the lowering of
incidence of the disease in dogs that
have been spayed. These factors would
, a similar
hormone or possibly a combination of two
or more of these.
What are the types
of mammary tumors in dogs?
There are multiple types of mammary
tumors in dogs. Approximately one-half of
all mammary tumors in dogs are
benign, and half are
malignant. All mammary tumors
should be identified through a biopsy and
histopathology (microscopic examination of
the tissue) to help in the treatment of that
particular type of tumor.
The most common benign form of canine
mammary tumors is actually a mixture of
several different types of cells. For a
single tumor to possess more than one kind
of cancerous cell is actually rare in many
species. This combination cancer in the dog
is called a 'benign mixed mammary tumor' and
contains glandular and connective tissue.
Other benign tumors include complex
adenomas, fibroadenomas, duct papillomas,
and simple adenomas.
The malignant mammary tumors include:
tubular adenocarcinomas, papillary
adenocarcinomas, papillary cystic
adenocarcinomas, solid carcinomas,
anaplastic carcinomas, osteosarcomas,
fibrosarcomas, and malignant mixed tumors.
What are the
symptoms of mammary tumors?
Mammary tumors present as a solid mass or
as multiple swellings. When tumors do arise
in the mammary tissue, they are usually easy
to detect by gently
palpating the mammary glands.
When tumors first appear they will feel like
small pieces of pea gravel just under the
skin. They are very hard and are difficult
to move around under the skin. They can grow
rapidly in a short period of time, doubling
their size every month or so.
The dog normally has five mammary glands,
each with its own nipple, on both the right
and left side of its lower abdomen. Although
breast cancer can and does occur in all of
the glands, it usually occurs most
frequently in the 4th and 5th.
In half of the cases, more than one growth
is observed. Benign growths are often
smooth, small and slow growing. Signs of
malignant tumors include rapid growth,
irregular shape, firm attachment to the skin
or underlying tissue, bleeding, and
ulceration. Occasionally tumors
that have been small for a long period of
time may suddenly grow quickly and
aggressively, but this is the exception not
It is very difficult to determine the
type of tumor based on physical inspection.
biopsy or tumor removal and
analysis are almost always needed to
determine if the tumor is benign or
malignant, and to identify what type it is.
Tumors, which are more aggressive may
metastasize and spread to the surrounding
lymph nodes or to the lungs. A
chest x-ray and physical inspection of the
lymph nodes will often help in confirming
Mammary cancer spreads to the rest of the
body through the release of individual
cancer cells from the various tumors into
the lymphatics. The lymphatic system
includes special vessels and
lymph nodes. There are regional
lymph nodes on both the right and left sides
of the body under the front and rear legs.
They are called the
'axillary' and 'inguinal' lymph
nodes, respectively. Mammary glands 1, 2,
and 3 drain and spread their tumor cells
forward to axillary lymph nodes, while cells
from 3, 4, and 5 spread to the inguinal
ones. New tumors form at these sites and
then release more cells that go to other
organs such as the lungs, liver, or kidneys.
What is the
Upon finding any mass within the breast of a
dog, surgical removal is recommended unless
the patient is very old. If a surgery is
done early in the course of this disease,
the cancer can be totally eliminated in over
50% of the cases having a malignant form of
cancer. The area excised depends on the
judgment and preference of the practitioner.
Some will only remove the mass itself.
Others, taking into consideration how the
cancer spreads, will remove the mass and the
rest of the mammary tissue and lymph nodes
that drain with the gland. For example, if a
growth were detected in the number 2 gland
on the left side, we would therefore remove
glands, 1, 2, and 3 and the axillary lymph
node on that side. If it were found in the
number 4 gland on the right side, then
glands 3, 4, 5, and the inguinal lymph node
on that side would be completely removed.
With some tumor types, especially sarcomas,
complete removal is very difficult and many
of these cases will have tumor regrowth at
the site of the previously removed tumor.
Owners may confuse a surgical removal of
a mammary gland in the dog with a radical
mastectomy in humans, with all of the
associated problems. In humans, this type of
surgery would affect the underlying muscle
tissue which complicates the recovery. In
the dog, however, all of the breast tissue
and the related lymphatics are outside of
the muscle layer, so we only need to cut
through the skin and the mammary tissue.
This makes the surgery much easier and
recovery much faster. A radical mastectomy
in a dog means all the breasts, the skin
covering them, and the four lymph nodes are
all removed at the same time. Although this
is truly major surgery, suture removal
usually occurs in 10 to 14 days with normal
activity resuming at that point.
Many veterinarians will spay a dog having
a mastectomy (unless she is very old). The
value of this in decreasing the recurrence
of tumors is still controversial.
Radiation Therapy: Chemotherapy has
not been a very successful nor widely used
treatment for mammary tumors in dogs.
However, with the constantly changing and
improving drugs available, a veterinary
oncologist should be consulted to find out
if there is an effective drug available for
your dog's particular type of mammary
cancer. The effectiveness of radiation
therapy has not been thoroughly researched.
Some anti-hormonal drug regimens are being
tested in dogs. At this point in time,
surgical removal of the tumors is the
treatment of choice.
How can I prevent
mammary cancer in my dog?
There are few cancers that are as easily
prevented as mammary cancer in dogs. There
is a direct and well-documented link between
the early spaying of female dogs and the
reduction in the incidence in mammary
cancer. Dogs spayed before coming into their
first heat have an extremely small chance of
ever developing mammary cancer. Dogs spayed
after their first heat but before 2.5 years
are at more risk, but less risk than that of
dogs who were never spayed, or spayed later
in life. We all know the huge benefits of
spaying females at an early age, but every
day, veterinarians still deal with this
easily preventable disease. Early spaying is
still one of the best things pet owners can
do to improve the health and ensure a long
life for their dogs.
Mammary cancer is a very common cancer
and can often be successfully treated,
if caught early. If all non-breeding
dogs and cats were spayed before their
first heat this disease could be almost
completely eliminated. If you find a
growth or lump in the mammary tissue of
your dog, you should inform your
veterinarian immediately and not take a
"wait and see" attitude.
Bonagura, J. Current
Veterinary Therapy 12. W.B. Saunders Co.
Philadelphia, PA; 1995.Ettinger, S. Textbook
of Veterinary Internal Medicine. W.B.
Saunders Co. Philadelphia, PA; 1989. Rutteman,
GR; Withrow, SJ; MacEwen, EG. Tumors of the
mammary gland. In Withrow, SJ; MacEwen, EG (eds).
Small Animal Clinical Oncology. W.B.
Saunders Co. Philadelphia, PA; 2001455-477.
© 2007 Drs. Foster and Smith, Inc.
Reprinted as a courtesy and with
On-line store at
Free pet supply
Neutering-Why it's a
Other than population control, there are
lots of very, very good reasons to
castrate (remove the testicles from)
male dogs. They basically fall into one
of two categories – they are either
behavioral or medical. Regardless of
which category we are talking about,
most of the unwanted characteristics or
conditions are caused by the male
hormone testosterone, which is produced
within the testicle. That is the major
reason vasectomies have never been that
popular in veterinary medicine. A
vasectomy eliminates successful
breeding, but it does not reduce any of
the undesirable problems of the intact
male, since it does affect testosterone
production or its distribution
throughout the rest of the dog's body.
advantages of neutering
Aggression: One of the most
important behavioral advantages of
castration is that as adults, these dogs
will tend to be less aggressive both
toward other male dogs and also people.
The androgen (male) hormones, of which
testosterone is the most important, are
responsible for the development of many
behavioral patterns. When young puppies
are sexually mounting their 7 and 8-week
old litter mates this is because of
androgen surges in their bodies. The
same is true with aggressive behavior.
Some medications that have
androgenic hormonal activity
often cause increased aggression (an
example would be the birth control
medication, Cheque Drops, which contains
one of these androgen-type chemicals).
The degree castration has on suppressing
aggression varies between animals and
the age at which it is done. Its effect
is greatest if it is done before one
year of age.
Roaming: A second behavioral
advantage of neutering is that these
dogs will not 'roam' when they sense a
female in heat. Male dogs can sense
females in heat through pheromones.
These are airborne chemical attractants
that are liberated from the female when
she is cycling. They travel through the
air for great distances. We grew up on a
farm where the next closest house was
over a mile away, but when one of our
female dogs was in heat, the males would
come for miles from upwind, downwind,
and crosswind. Pheromones are, to say
the least, very effective stimuli. In
the seventies, it was briefly popular to
do vasectomies on dogs thinking that we
would not be taking the 'joy of sex'
away from our canine counterparts. The
problem with this reasoning was that
many of us keep our dogs restricted in
our homes, a kennel, or on a chain. Now
think of the psychological stress the
vasectomized male is under when he is
locked up, but yet smells that female in
heat four blocks away. There is no joy
of sex, as he is trapped on your
property unable to go and mate with her.
He is, in effect, teased continuously
for three to fourteen days while the
female is in
estrus and he is unable to
mate with her. If dogs are neutered at
an early age, they will not sense or
respond to pheromones, and would
certainly be less stressed and tend to
behavioral advantage occurs when you
are training or working your dog, or
using him for field work. If
neutered, he will be a much better
student with a much longer attention
span when there are females nearby
that are in heat. This is because he
will not be constantly distracted by
numerous behavioral and medical
benefits to neutering your dog.
The medical advantages are numerous
and even more significant. Again,
all are caused by the effects of
testosterone on the body or are
physical problems that arise within
the testicles themselves. Here
again, a vasectomy would not serve
any real or meaningful purpose.
Testicular Tumors: There are
several different tumor types, both
malignant, that arise
within the testicles. As with most
cancers, these usually are not noted
until the animal reaches 5 or more
years of age. Therefore, these would
not be a problem in those
individuals castrated at the
Genetics: We all agree that a
male carrying a harmful genetic
trait like hip
dysplasia or epilepsy
should be neutered. We must do all
that is possible to prevent the
spread or continuation of these
conditions and others like them.
Hernias: A hernia is a protrusion
of an organ or parts of an organ or
other structure through the wall of a
cavity that normally contains it.
Perianal hernias occur when the colon,
urinary bladder, prostate, or fat
protrude from the abdominal cavity,
through the muscular wall by the anus
and then lie just under the skin. This
type of hernia is far more common in
older, unneutered male dogs. The levels
of testosterone and other hormones
appear to relax or weaken the group of
muscles near the anus. When the animal
then strains to defecate or urinate, the
weakened muscles break down and the
abdominal organs and fat bulge out under
the skin. In shorthaired breeds, this
large bulge is noted by the owner almost
immediately, but in the longhaired dogs,
the problem may go on for months before
anyone realizes there is an abnormality.
Left untreated, these organs may become
damaged, unable to function or even die
from loss of blood supply. Additionally,
because of the displacement of organs
into this area, the animal may not be
able to defecate or urinate correctly or
completely and may become constipated or
have urinary incontinence (dribble
urine). The surgery to repair this
condition is not simple and today can
easily cost $700 to $1500 or more,
depending on the severity.
Perianal Tumors: There are tumors
whose growth is stimulated by
testosterone. These occur near the anus
and are called perianal adenomas
(benign) or perianal adenocarcinomas
(malignant). As with the hernias, these
usually do not occur until the dog is at
least 7-years old. They require surgical
treatment and should be caught early in
their development to prevent recurrence.
These tumors and the above hernia are
very, very rare in those individuals
castrated at 7 to 8-months of age.
Prostate Problems: The most
common medical problems eliminated in
dogs neutered at an early age are those
prostate. Over 80% of all unneutered
male dogs develop prostate disease.
Prostate conditions such as benign
cysts, and infection are all
related to the presence of testosterone.
In the United States, most dogs are
neutered between 5 and 8 months of age.
Many humane shelters and veterinarians
are starting to neuter male animals at a
younger age, even 6-14 weeks of age.
This early neutering does not affect the
growth rate, and there are no
appreciable differences in skeletal,
physical, or behavioral development
between those animals neutered early
than those neutered at a more
traditional age. It must be remembered
that younger animals may need different
anesthetics and are more prone to
hypothermia (lower than
normal body temperature) during surgery.
As long as procedures are modified to
account for these differences, early
neutering is very safe. In fact, animals
neutered at a younger age often have
faster recoveries than those neutered
when they are older.
None of the behavioral or medical
problems caused by testosterone are
rare. Veterinarians deal with them on a
daily basis. To say it in a way that may
not sound very nice but is certainly
true – veterinarians would make a lot
less money if everyone neutered their
male dogs before they were a year of
© 2007 Drs. Foster and Smith, Inc.
Reprinted as a courtesy and with
On-line store at
Free pet supply