The Dog Cancer Survival Guide (112 page)

When you need to look up the definition of a word or phrase, find it listed in
Chapter 5
, which begins on
page 46
.

All of the above sarcomas occur in parts of the body that are on the outside – flesh, muscles, connective tissues, etc. Other sarcomas, those of the internal organs and the oral cavity, are considered separately because they are so different in metastatic rates, treatment and prognosis. The more common internal sarcomas include: oral tumors, nasal tumors, osteosarcoma and hemangiosarcoma, all of which are discussed in other chapters.

Locally invasive STS, on the other hand, have poorly defined margins and often infiltrate surrounding tissues with tentacles of cancer cells. In general, they tend to recur, especially if surgery is conservative, and especially if it features narrow margins.

When they metastasize, they generally do so through the bloodstream, typically to the lungs and liver. Regional lymph node metastasis is uncommon.

Low and intermediate grade STS tend to have a low metastasis rate of 10-20% and most commonly spread to the lungs. High grade STS is more metastatic, with 40-50% of these tumors spreading to the lungs and liver.

Genetic and environmental factors may be involved in STS; the cause of these tumors still remains poorly understood. (While cats can develop sarcomas at the site of injections, this is rare in dogs.)

STS tends to occur in larger breeds, without an apparent specific genetic cause. One study found that Rhodesian Ridgebacks and mixed breed dogs have a higher risk of STS, while another found that Flat-Coated Retrievers were more at risk, specifically for malignant fibrous histiocytoma. Older dogs tend to be more at risk, also.

Most dogs do not feel sick at the time of diagnosis and are brought in because their owners noticed masses that were getting larger. These masses tend to grow slowly and not be painful. They can occur anywhere on the body and, depending upon their location, may cause different symptoms. For example, a mass near a joint may cause pain when moving, or a mass on the ear might cause your dog to paw and scratch at his head.

Although STS are usually firm to the touch and fixed in place, they can also feel soft and have lobules (several rounded structures). STS can be confused with benign lipomas (fatty tumors).

The unknown origin of these tumors makes prevention unlikely, which is why it is so important to consistently and regularly monitor your dog for new skin or surface masses. The sooner a mass is found, the sooner it can be diagnosed and treated, if necessary.

Knowing the tumor type and the grade of STS is very important. It tells you what the likely prognosis is, and also makes treatment decisions easier. While some sarcomas, osteosarcoma, for example, do not need an aspirate or biopsy before starting treatment, STS always needs to be diagnosed by aspirate or biopsy, before surgery is performed.

All skin and subcutaneous masses need to be aspirated to make sure they are not mast cell tumors, benign fatty lipomas or abscesses (infections). When the aspirate confirms STS, a biopsy may not be needed before attempting surgery. Unfortunately, STS cells do not aspirate well – they can refuse to be drawn up the needle – so this test may not confirm STS. Sometimes dead tissue or inflamed tissue is drawn up, leading to a misdiagnosis of inflammation. If a tumor looks like STS and the aspirate only indicates inflammation, STS is still suspected and a biopsy is definitely in order.

Biopsies can be very useful, even if an aspirate confirms STS, and they are vital if it is not confirmed with an aspirate. A biopsy provides information that cytology/aspirates do not: tumor type and grade. STS tends to be locally invasive, and you don’t want the biopsy to tract (or leave a path of) cancer cells through surrounding tissues, so the biopsy must be carefully planned, so as to take a sample from an area that will be removed later in a curative surgery or radiated during radiation surgery.

The temptation is strong to remove the entire visible tumor during the biopsy: such a surgery rarely “gets it all out.”A second, curative surgery will likely be needed because STS generally requires aggressive surgeries with very wide margins in order to achieve clean margins. Knowing the tumor type (with a biopsy first) provides the best opportunity that only one curative-intent surgery will be needed.

In addition to the aspirate and/or biopsy, blood work and urinalysis are recommended to get information about your dog’s general health.

To complete staging for STS, a lymph node aspirate should be performed on nodes that drain the tumor location. Even though lymph node metastasis is rare, it is not unheard of, and knowing your dog’s status can help with prognosis and treatment planning. This is a minimally invasive test and it’s worth doing. Keep in mind that you cannot tell if STS has spread to lymph nodes by noting their size or appearance.

To check for spread to the lungs, three-view thoracic X-rays should be taken prior to curative surgery. Because metastasis to the lungs significantly decreases survival time (in one study the median survival time with metastasis was about eight months), this knowledge can help you to plan your treatments.

Abdominal ultrasounds can be used for general health screening, and can also show spread to the lymph nodes, liver or other organs. This is especially important for STS that is on the abdomen or the back half of the body, because tumors in these locations often spread to internal organs.

When the mass is close to bone, X-rays should be taken of the area to see whether the bone has been invaded. Computer tomography (CT) scans or MRI scans can show details that help plan surgeries and radiation, helping the surgeon determine just how far the tumor extends into surrounding tissues. Tumors are rarely contained in the mass we feel with our fingers or see with the naked eye – knowing as much as possible about their spread can help plan a surgery and determine whether additional therapy (radiation or chemotherapy) will be needed.

Certain cellular markers in the tumor sample, when measured, can help predict your dog’s prognosis when it comes to STS. I studied these markers during my oncology residency, and I bring them to your attention, because knowing your dog’s levels can really help you make treatment decisions.

Our study demonstrated a link, between the marker Ki-67 (which rises when a lot of cells are dividing) and the grade of an STS tumor. For example, dogs with a low Ki-67 score had a median survival time of over forty months; dogs with a high Ki-67 score, on the other hand, have a median survival time of twenty-two months. Dogs with higher scores were also twelve times more likely to die, due to their STS, than dogs with low scores.

AgNOR, a type of protein, is another useful cell proliferation marker. Dogs with a high AgNOR were almost six times more likely to die due to their STS disease than dogs with low AgNOR values.

Another helpful test to use when trying to predict the behavior of a STS is the intratumoral microvessel density (IMD). This measures how much angiogenesis, or new blood vessel formation, characterizes the tumor. In a follow-up study, we found that the more angiogenesis is happening, the more aggressive the tumor is and the more likely it is to metastasize. The IMD is typically run as a panel with the proliferation markers, and is commercially available in only a few labs. If needed, your vet can contact a specialty lab.

Measuring these proliferation markers can help you to know whether your dog is in a high-risk category that might benefit from more aggressive treatments. These markers can be measured on a biopsy sample that is specially stained and examined at a laboratory. Biopsy tissue is typically kept for a year or more at most labs, so these tests do not need to be run right away. If needed, they can be run after the biopsy results are sent back and analyzed. I recommend you discuss these markers with your oncologist.

The prognosis for STS tumors is variable, depending upon the tumor type, location, stage, grade and rate of cell division.

Here is the official staging for STS tumors:

Stage IA:
The primary tumor is less than 5 centimeter, grade 1 or grade 2, with no lymph node or distant metastasis.

B:
The primary tumor is larger than 5 centimeter, grade 1 or grade 2, with no lymph node or distant metastasis.

Stage IIA:
The primary tumor is less than 5 centimeter, grade 3, with no lymph node or distant metastasis.

B:
The primary tumor is greater than 5 centimeter, grade 3, with no lymph node or distant metastasis.

Stage IIIA:
Any size and any grade of tumor, with lymph node metastasis.

B: Any size and any grade tumor with distant
metastasis.

There is also a newer, modified staging system, which places less emphasis on tumor size and greater focus on the depth of invasion; I find it no more useful than the older system, however.

In general, the higher the stage is, the worse the prognosis. STS is highly treatable in low and intermediate grades, with survival times of up to five years with a combination of surgery and radiation.

Although not considered routine by most oncologists, measuring the proliferation and angiogenic markers listed in the previous section may also be helpful in assessing your dog’s prognosis.

The most effective treatment for STS is a wide, surgical excision of the tumor. As I mentioned above, careful planning for this surgery can help ensure a complete removal the first time and prevent recurrence. Since STS tends to recur, achieving clean margins in the first surgery is vital.

A three-centimeter margin around the removed tumor, including a full tissue layer from underneath, is the minimum margin needed. In some cases an even wider margin may be needed, which is why knowing the tumor type and grade is so important. Your surgeon can plan the best surgery when he has CT scans or other detailed images to review. There is a 15% recurrence rate for STS overall, usually within one year; dogs with dirty margins have a recurrence rate of 30%, which is why a post-surgery biopsy of the removed tissue is critical to confirm complete excision.

It is very important your vet reads this biopsy report closely and notes how wide the margins are. STS is very aggressive locally, and unless a wide, two or three-centimeter margin is clean, I am not confident that the procedure is a complete resection. (Tissue shrinks a little after it is removed from the body and before it is processed at the lab, which is one reason to err on the side of wider than necessary.) I mention this because I have personally seen biopsy reports call STS tumors clean even with very narrow one or two millimeter margins. This is careless medicine, and one of my pet peeves, because incomplete resections are ten times more likely to recur.

The biopsy will tell you the tumor’s mitotic index rate and grade, also. The mitotic index rate – how fast the cells are multiplying – tends to be higher in tumors that are likely to metastasize. A high mitotic rate often correlates with a higher-grade tumor, and these worsen the prognosis, likelihood of metastasis and survival times.

Many STS tumors occur on limbs, which presents a particular problem for the surgeon: there may not be enough skin and tissue to get clean and wide margins. This means that recurrence is much more likely and later surgeries will be progressively more complicated and difficult. Some owners choose to amputate in these situations, because – although it is a radical surgery – amputation achieves the necessary wide margins and prevents recurrence.

Other owners choose a course of post-surgery radiation therapy to prevent recurrence. While this approach saves the limb and can be very effective, tumor recurrence is more likely than it is with amputation (and amputation may be necessary later, in any case). Even so, this is a popular option because it avoids amputation and offers the likelihood of long-term control.