VetInfo Digest September 2007
Table of Contents:
Most common skin lumps
Non-cancerous lumps
Cancerous lumps
This Month's Note:
I decided to write this issue (and next month’s issue) of the VetInfo Digest on skin tumors after a couple of you asked questions about them this month. I know that it would be better to have photos to accompany the text when discussing skin tumors. As time goes on I will try to put some photos online. However, even when this happens it is extremely important to remember that even though there are often descriptions of “typical” appearances for tumors, the tumors themselves don’t care. It is very difficult to identify skin tumors by appearance alone in many instances, even after seeing thousands of the tumors in person. Don’t get too hung up on trying to match skin tumors to pictures. Have the tumor identified through microscopic examination of aspirates from the tumor, through biopsy of a small piece of the tumor or after removal of the entire tumor, if there is any question about what type of tumor might be present.
Visible lumps in dogs and cats
Lumps in the skin and under the skin are easy for veterinary clients to see or feel, so they are the cause for a fairly large percentage of visits to the veterinary office. There are a lot of different types of lumps and bumps, some of which are very significant and other of which can be safely ignored. It is not possible to cover all types of lumps in the skin in one newsletter but I thought it might be useful to list the most common ones and some of the current treatment recommendations.
The data that has been collected on the types of lumps and how frequently they occur is difficult to interpret well enough to give accurate estimates of the percentage of dogs that are likely to be affected by each type of lump or tumor, due to naming differences, incomplete lists of tumors in some studies and differences in reporting technique. Some studies report the percentages of each type of tumor found and other studies report estimates of the percentages of dogs affected in the overall population. I think a fairly accurate list of the most common types of lumps can be put together but it is not possible to give reliable data about the incidence of types of lumps in dogs or cats in the overall population. For this reason, I am going to rely on common and uncommon to describe the incidence of lumps. I’m sorry that I can’t be more specific than that.
Most common lumps (includes the most common cancers) on Dogs:
Sebaceous hyperplasia and sebaceous adenoma (related, hard to tell apart)
Epidermal inclusion cysts (also called sebaceous cysts or epidermoid cysts)
Lipomas
Mast cell tumors
Perianal adenomas (hepatoid adenoma), usually dogs who have not been spayed or neutered
I could not find a really good list of incidence for skin lumps in dogs that were not cancerous. This makes it hard to rank the relative frequency of the lumps but I think that the first three on the list above are the most common lumps in dogs in our practice.
Most common skin cancers affecting dogs (in approximate order):
Mast cell tumors
Sebaceous adenoma
Perianal adenomas
Lipomas
Fibrosarcoma
Melanomas
Histiocytoma
Spindle cell sarcomas
Hemangiopericytoma
Basal cell tumor
Most Common Skin Lumps on Cats (all types):
Basal cell tumor
Mast cell tumors
Squamous cell carcinoma
Fibrosarcoma
Sebaceous hyperplasia/adenoma
Apocrine gland cysts (sweat gland cysts)
In cats basal cell tumors are the most common skin tumor, comprising about 25% of the skin lumps found in cats. The distribution of the next three types of lumps is pretty uniform with none of them clearly standing out as the next most common tumor overall. It is possible that the incidence of fibrosarcomas may lessen since it is now recognized that some cases of fibrosarcoma occur due to tissue irritation and many veterinarians are taking measures to try to avoid excessive tissue irritation, such as increasing the length of time between vaccinations and avoiding the use of injectable medications when alternatives will suffice.
Skin cancers behave differently in cats than in dogs so as I go along describing individual tumor types it will be important to note whether or not the patient is a cat.
Abscesses are by far the most common lumps found in cats. I have mistaken an abscess for a tumor on several occasions in cats when it wasn’t possible to find the pocket of pus with a needle and there was an obvious lump. Abscesses are often warm to the touch and handling an abscess is often painful, which isn’t a common finding with the skin tumors. It is important to keep in mind the strong possibility of an abscess whenever there is a subcutaneous lump in a cat.
Vaccine reactions cause formation of lumps in a very large percentage of cats after vaccination. Note where your vet puts vaccines and if a lump appears in that region after a few weeks remember that it may just be a vaccine reaction. If the lump persists for more than three weeks it is a good idea to let your vet know. Most vaccine lumps are small, generally about the size of a pea. They occur more commonly after rabies vaccination than after other vaccinations.
Descriptions of Non-cancerous Lumps Affecting Dogs and Cats
Epidermoid cysts:
The most common non-cancerous lump in dogs is an epidermoid cyst, also known as an epidermal inclusion cyst or a sebaceous cyst. These are usually rounded lumps that appear to be arising from subcutaneous tissues but that clearly involve the skin itself, as well. They are not nearly as common in cats as they are in dogs but they do occur in cats.
Epidermoid cysts may become quite irritated and can burst open and leak sebaceous material or bleed. There is some disagreement about the best approach to these cysts. When I was in veterinary school the recommendation was to remove these surgically in most, or all, cases. As research data continued to be collected it was evident that most dogs and cats who develop epidermoid cysts will not experience any significant problem due to the presence of the cyst. So it seems reasonable not to remove the cyst unless it causes problems. This is currently our approach to epidermoid cysts. I suspect that we remove 3 or 4 of these a year and we see them much more frequently than that. While we are comfortable with this approach even some very new dermatology references still state that surgical removal is better.
Epidermoid cysts are fairly easy to remove, there is not much risk of surgical complications but there is some anesthetic risk. If you decide that you would like the lump removed bearing these things in mind, the decision is reasonable. On the other hand, it is also very reasonable to leave them alone.
Apocrine (sweat gland) cysts
Apocrine cysts occur in both dogs and cats but are not especially common in either species. They can appear to grow quite rapidly and sometimes seem to disappear quickly, as well. I think this occurs because they are just water (sweat) filled cysts and they leak fairly easily, shrinking quickly when the sweat leaks out. In most cases they fill back up fairly quickly, so draining them is usually frustrating.
We usually leave apocrine cysts alone unless they are getting large enough to be annoying. Like epidermoid cysts they are easy to remove, if desired. Some veterinarians recommend removing these cysts routinely because it is possible to confuse them with apocrine tumors in some cases, although this is rare. The tumors are solid, usually solitary and often have an ulcerated surface. Every now and then one does seem to be associated with a sweat gland cyst, though. Despite this, I think it is reasonable to treat them as separate entities and to leave the cysts alone unless they are causing problems.
Papillomas (Warts)
Young dogs sometimes develop warts. These are thought to be viral in dogs and will usually disappear in a few months. Older dogs also sometimes develop warts but when they occur in older dogs they are less likely to be viral and more closely resemble sebaceous hyperplasia lesions.
The warts that occur in young dogs have a distinctly cauliflower look to them as they are usually white and rough surfaced. They are most commonly found around the mouth. There are often large numbers of warts present. In older dogs warts tend to be fewer in number, often solitary, and they tend to occur on the head and feet. There is some concern that warts that occur in older dogs may be precursors to squamous cell carcinoma and so removal or cryosurgery are considered to be the best therapies.
Warts occur in cats but are considered to be a rare problem in this species.
Histiocytoma (button tumor)
This is a lump that occurs in young dogs, usually when they are less than 18 months of age. Much less commonly they can occur in older dogs. They are smooth, well circumscribed lesions that often appear to be mildly inflamed. In almost all cases they will regress on their own, without treatment, within 2 to 3 months. It is unusual to see more than one of these lesions on the same dog. They are also called button tumors because they really do seem to resemble a button that suddenly appears on the dog’s skin surface. They do not seem to occur in cats.
When we see a lump that resembles a histiocytoma on young dog we advise the owners to wait two to three months before removal. If the tumor persists beyond that time we feel that it is reasonable to remove it. Once again, dermatologists often seem to want to remove these tumors even with the knowledge that they will likely regress. I am not sure that I understand the logic behind this inclination, in this case.
There is a similar condition referred to as cutaneous histiocytosis in which multiple plaques occur in the skin. This is most common in golden retrievers and will often respond to treatment with corticosteroids.
Sebaceous hyperplasia
Almost anyone who has seen older poodles has seen sebaceous hyperplasia. While poodles are not the only breed affected by this condition, they get it quite commonly. An older dog prone to sebaceous hyperplasia will often have multiple pink, warty looking bumps scattered around its body. Cats are affected by sebaceous hyperplasia, as well. It is less common in cats and most cats who develop this condition have one or two lesions rather than the multiple lesions seen in dogs.
When a dog or cat has a solitary lesion that appears to be sebaceous hyperplasia it is probably reasonable to leave it alone. In most cases these lesions cause no problems. A small percentage of dogs who have sebaceous hyperplasia lesions seem unable to resist licking or chewing the lesion and this will often result in infection. Cats are less likely to do this for some reason but we have seen one or two of these lesions become infected in cats, as well. The hyperplasia can be significant enough that the lesion gets to be large enough to get in the way of grooming procedures. In these cases it is generally quite easy to remove the lesion in order to resolve the problems associated with it. We can often do this in our practice using a sedative or narcotic combined with local anesthesia. For very compliant patients local anesthesia alone would probably be sufficient.
It is harder to figure out what to do when a dog seems to have a propensity for developing these lesions and develops a number of them over time. When dogs have large numbers of sebaceous hyperplasia lesions it seems inevitable that some of them will become infected. It is reasonable to remove any that are and to leave the rest. It also seems reasonable in these dogs to occasionally remove lesions before they become a problem when it seems likely that they will eventually cause problems. It is possible to remove these lesions through cryosurgery (usually application of liquid nitrogen), by laser surgery (this is a time when lasers seem to be better than conventional surgery) or by conventional surgical removal. We have tried using nitrous oxide applications using a surgical tool that controls gas flow from nitrous oxide cartridges but this method of removal does not seem as reliable as the previously mentioned techniques, at least in our hands.
In general we try hard to leave sebaceous hyperplasia lesions alone but many vets feel they should be removed, if for no other reason than to ensure that another skin tumor is not misidentified as sebaceous hyperplasia and allowed to grow. I think that this risk is minimal but there is no question that I have been fooled in the past on at least a couple of occasions by lesions that really appeared to be sebaceous hyperplasia and turned out to be mast cell tumors. If you wish to be very cautious it is reasonable to remove all sebaceous hyperplasia lesions but it does mean that a great deal of unnecessary surgery is done to ensure that a very small number of other skin tumors are not missed.
Cancerous Lumps Affecting Dogs and Cats
Just a note before moving on to the descriptions of cancerous lumps, in human medicine it seems to be the trend to call benign cancers “tumors” and to call malignant cancers “cancer”. In veterinary medicine the trend is to call all unregulated cell growths cancer and to use the term “benign cancer” for lesions that are not likely to spread and “malignant cancer” to describe lumps that are likely to spread. So when your veterinarian says your dog has cancer it may not be the devastating situation that most people expect when they hear the word cancer from their physician. Be sure to listen for the description and to differentiate between benign cancers and malignant cancers.
Lipomas
In the veterinary literature concerning cancers, which mostly represents the work of internal medicine specialists who treat oncology cases, lipomas are ranked lower than mast cell tumors in occurrence. Personally, I think that this just reflects the patient load of specialists versus the patient load of general practitioners. I almost never refer a patient to an internal medicine specialist for treatment of a lipoma. On rare occasions I do refer a patient I think has a lipoma for confirmation of this fact because mast cell tumors sometimes closely mimic lipomas in appearance, feel and even by examination of aspirated tissue. My best guess is that lipomas are the most common subcutaneous tumor in dogs and are nearly as common in cats as the other tumors that occur in this region.
Lipomas do not generally affect the skin surface, although they can. Most lipomas are soft fluctuant masses that occur in the subcutaneous tissue. They are often easy to move around under the skin. They can become quite large. Most lipomas have a smooth appearance and are the same consistency throughout the tumor when palpated. They tend to occur on the sternum, in the armpit region, along the tops of the hips and on the flanks most commonly. They can occur anywhere, including internally, though. When a lipoma forms in the skin rather than in the subcutaneous tissue it can strongly mimic other tumor types, especially basal cell tumors in dogs.
We have removed lipomas that were as large as a Nerf ™ football. Clients often worry about lipomas when they become large but these tumors only rarely act aggressively. The exception to this rule may be lipomas that are identified in dogs that are less than a year of age. We have seen liposarcomas ( malignant lipomas ) in two dogs over the years and both were less than a year of age when the lipomas were identified.
Some vets feel very strongly that every tumor that looks like a lipoma should be examined to determine that it is actually a lipoma. This is fairly easy to do, as lipomas are generally easily identified by aspirating a small amount of the tumor tissue through a needle and then expressing the tissue onto a microscope slide for identification. However, many dogs who have one lipoma will develop multiple lipomas. Once it seems clear that a patient is prone to lipoma formation and if the lipomas that develop appear typical in all ways, I have a hard time advocating aspiration of every single one that occurs. I’m perfectly happy to do that for my cautious clients but I understand when my less cautious or more money conscious clients ask me if it is really necessary to aspirate yet again, when a dog is working on its fifth or sixth or tenth lipoma.
Cats do get lipomas. We are very careful to make an effort to positively identify the tumor as a lipoma when it occurs in a cat, as there is a stronger tendency in cats to have malignant tumors of other types, in the subcutaneous tissues. We are also more careful to identify lipomas through microscopic examination when they occur on the lower limbs in dogs, as this is a common site for soft tissue sarcomas, which look a lot like lipomas but require more aggressive treatment.
Mast Cell Tumors
Mast cell tumors are reported to be the most common skin tumor found in dogs (representing approximately 20% of skin tumors). In dogs, these tumors are often malignant and require aggressive treatment. Mast cell tumors produce histamine so it is not unusual for these tumors to seem as if they itch or for affected dogs to have gastrointestinal disease associated with histamine release, such as gastric ulcers. In addition, a small number of dogs will develop bleeding disorders due to the histamine release from mast cell tumors.
I am pretty convinced that a mast cell tumor can look like any other skin tumor, or subcutaneous tumor, if it chooses to do so. These tumors are the major reason that we do aspirates from tumors we suspect are sebaceous hyperplasia, epidermoid cysts or lipomas. The “typical” mast cell tumor, though, is a lump that is in the skin itself. It will be partially hairless and the hair that is present will appear to be spiky (pinfeather appearance). Many of these tumors have an aggravated look because they release small amounts of histamine, which causes the skin to look inflamed or to be itchy. Most mast cell tumors are solitary lumps but we occasionally see dogs that seem to develop multiple mast cell tumors within a very short period of time. Mast cell tumors are malignant and can spread to almost any other tissue, but seem to prefer to spread to abdominal organs. There are two breeds that tend to have certain types of mast cell tumors. Boxers tend to have low grade tumors that respond well to removal. Shar peis tend to have highly malignant mast cell tumors.
The single most important thing to remember about mast cell tumors from a treatment aspect is that it is far better to identify them prior to surgical removal and then to plan surgery to ensure that adequate tissue is removed around the tumor to improve the chances for total removal. This generally means removing at least 3cm ( a fairly recent article suggests 2cm may be as effective) around all tumor edges. Under the tumor it is necessary to remove at least the first layer of tissue and if there appears to be any spread in this direction it is better to remove underlying tissue to the depth of the next fascial plane, even if that means removing muscle or other tissue that lies between the tumor and that fascial plane. It is not uncommon among general practice veterinarians to have a reluctance to remove this much tissue without a clear diagnosis, in advance, of mast cell tumor.
To give you some idea of exactly what this means, it is generally necessary to make incisions 2.5 times longer than their maximum width. If a mast cell tumor is 2cm in width and 3cm margins are created all around it, then the width of the incision is 8cm. The length of the incision would then need to be about 20cm to get good closure. For the metric challenged, 20cm is just a shade under 8 inches. My clients are often aghast at the length of incisions when we remove mast cell tumors, even though I try to explain in advance how long the incision will be.
I don’t want to delve extensively into the meaning of different grades of mast cell tumor. There is an older VetInfo that covers this topic in the subscriber area (http://www.vetinfo.com/subscriber/0800digest.html ). Just know that when you look at mast cell tumors in general, there is about a 50 to 60% chance that the initial surgery will successfully prevent tumor recurrence if it is done well, although this does vary widely by tumor grade.
In cats mast cell tumors are among the most common tumors but are much different in their clinical course. In cats most mast cell tumors are benign. It is generally recommended that these tumors be removed as they seem to be associated with intermittent itchiness in some cats and there is a small possibility of malignancy. However, the prognosis for a cat with a mast cell tumor is much better than for a dog.
Basal cell tumors
Cats are somewhat prone to basal cell tumors, particularly Siamese, Himalayan and Persian cats. These are usually smooth surfaced lumps, often hairless and most often occurring on the head, neck, chest and back. Basal cell tumors are usually benign and the few malignant tumors don’t tend to spread rapidly. Surgical removal is usually curative.
Dogs do not get basal cell tumors very often but these are some of the more spectacular tumors in dogs when they are left alone, if they have a tendency to grow (not all basal tumors get large). I have seen two or three dogs come into the clinic dragging a large basal cell tumor along the floor because it was so large. The tumors are generally solitary tumors, without hair, that are firm in texture. They may have a broad base but not infrequently they appear to be attached to the skin by a small base even when they are quite large. Even when these tumors are huge they are generally fairly easy to remove surgically and removal usually results in a complete cure.
Melanomas
In people melanomas are one of the more feared skin tumors. I do not know the statistics for people, but in dogs and cats about 85% of melanomas are benign, so these tumors are not as worrisome in dogs, even though they should be removed. Melanomas that occur in the oral cavity, at the junction of normal and specialized skin (lips, nose, anus, etc.) tend to be malignant. For some reason this doesn’t apply to eyelids, even though they do have similar characteristics. Melanomas tend to be darkly pigmented lumps, usually hairless, dome shaped and firm to touch. In some cases malignant tumors will not produce the typical black pigment and so will not appear to be black. Malignant melanomas tend to ulcerate more frequently then benign tumors, as well.
It is not possible to differentiate a malignant melanoma from a benign melanoma prior to surgical removal (and sometimes after removal) so it is recommended that all melanomas be removed. A possible exception to this rule would be melanomas affecting the oral cavity, as surgical removal rarely results in a complete cure unless radical surgery, such as removal of one half of the jaw, is possible to get wide tissue margins. Melanomas that occur in nail beds are also particularly likely to be malignant and amputation of the entire toe or even the affected limb should be considered if a surgical cure is to attempted.
This is another tumor with some breed differences in biologic behavior. Melanomas affecting Doberman pinschers tend to be benign, although they still should be removed as some are not. Unfortunately for miniature Schnauzers, most melanomas in this breed are malignant and should be treated more aggressively.
To be Continued Next Month – there are too many skin tumors to cover in one issue. Even then, please remember that this will not be a complete list of skin tumors. There are many other skin tumor types and some other lesions that resemble tumors but actually aren’t.
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The opinions expressed in this newsletter are those of Michael Richards, DVM.,
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