Tumors in Dogs

MENINGIOMA: A DIAGNOSIS WITHOUT CLEAR DIRECTION

Kayla, a 6-year-old spayed female German Shepherd owned by a local veterinarian, presents with a history of recent onset seizures. Although Kayla shows no other signs and her neurologic examination is otherwise normal, a prefrontal meningioma is certainly a consideration. MRI confirms this diagnosis within a few days, and Kayla heads to the surgery table. The tumor is approached through her frontal sinus and a large portion of her right olfactory tract is removed so that there are clean margins. Kayla’s recovery is uneventful and she goes on to run, play and be the wonderful family member she was before this ordeal first started. She never has another seizure and she lived for three years eventually succumbing to suspected tumor regrowth at the age of nine.

Of course, we best remember our successes, but the decision making process surrounding this most common of brain tumors is not so straightforward.

At least 85% of the older dogs that we see with recent onset seizures have a brain tumor. The vast majority of those are meningiomas and those patients that present without other neurologic signs, are most likely to have a prefrontal neoplasm that must be irritative but not affecting critical neural pathways. Most of these dogs present with seizures. These seizures can most often be terminated in the short-run with corticosteroids.

We know that the Golden Retriever is over represented with this particular neoplasm and more often than not, their meningiomas are in this prefrontal location. Although brachycephalic breeds have a higher incidence of brain tumors, they do not often have meningiomas. Boxers often present with brain tumors at a younger age (five to eight years) and typically present with additional signs of intracranial dysfunction such as “head-pressing”, circling, compulsive pacing and behavioral changes as well as seizures.

Although all meningiomas arise from arachnoid cells, there are several types of meningiomas. It is difficult to distinguish between them based upon MRI findings. In cats, meningiomas tend to compress the brain as they grow and their growth is assumed to be quite slow because the severely compressed cortex looks rather healthy once the neoplasm is removed. In dogs, it is far more likely that the meningioma will actually invade the brain tissue itself and for that reason, the tumor is often much more difficult to remove. Although we often state that canine meningiomas are slow growing, the evidence that this is true is anecdotal at best.

As in all things oncologic, there are several treatment options available. Radiation, as the sole therapy, has limited use with this particular tumor type in my opinion. In the few that I have followed, radiation shrinks the tumor somewhat (about 10%) and then it begins to grow again within a very short period of time.

We have placed dozens of dogs on CCNU using a modification of the protocol first described by Dr. Lisa Fulton and me in 1982. Whether this is a good primary protocol is unknown. We do not have any cases where there is a biopsy diagnosis and a follow-up autopsy that shows there is complete tumor remission. Nor, do we have any cases where the follow-up MRI shows complete regression of the tumor. We do, however, have numerous cases that have survived for the entire treatment course of twelve months and beyond. We have a handful of cases where the owner reports modest to complete recovery but, when the MRI’s were repeated, the tumor is unchanged or mildly smaller in size.

Surgical excision of this neoplasm is quite popular among those neurologists that perform surgery. Wide excision, with tumor-free margins, should theoretically be curative. This has not turned out to be the case and in several patients tumor re-growth has been discovered at some distance from the original surgical site. Whether this represents seeding from the original surgical removal, or is the result of the meningeal tissue being specifically prone to forming tumors at this time in the animal’s life, is totally unknown. An even more bothersome aspect of these meningeal re-growths is that the tumor can re-grow at a truly impressive rate. We have removed neoplasms that are only 1 cm3 in volume and tumor re-growth twelve weeks later reveals a neoplasm at the surgical site that is ten times larger. Does this reflect an enhanced growth rate related to some factor involved in the original surgery or was this a fast-growing neoplasm from the onset? These questions remain unanswered.

Over the years and after hundreds of meningioma surgeries, I, in conjunction with the neuropathology department at The University of Pennsylvania, have gone back over dozens of meningiomas to see if there is some association between the behavior of the neoplasm and the histopathologic type of meningioma removed. To date, no hint of an association has been discovered. It is quite remarkable that a meningioma in the same location, presenting with the same clinical signs and the exact same MRI findings, is likely to be one of several different types of meningiomas recognized by the WHO (World Health Organization). The WHO recognizes nine different types of meningioma.
1) Meningotheliomatous
2) Fibrous
3) Transitional
4) Psammomatous
5) Angiomatous
6) Papillary
7) Granular Cell
8) Myxoid
9) Anaplastic

We have also seen no correlation between tumor size and the success rate of any of the treatments. This surprisingly includes cases involving the surgical removal of very large tumors sometimes yielding the best patient response and longest survival times.

We currently are following an 11 year old, German Shepherd that presented with a single generalized seizure. An MRI revealed a pea-sized prefrontal meningioma-like lesion. We discussed surgical removal, but the owner elected to wait and see what happens without treatment of any kind. We repeated the MRI on this dog six months later and the tumor was unchanged in every way. We hope to follow this dog with additional MRI studies and are hopeful that, ultimately, the diagnosis will be confirmed on autopsy.

We have a handful of dogs whose tumor was removed a second time, after confirming they have regrown. We have had a small number of owners ask that radiation or chemotherapy be performed on animals that have had a surgical excision. It is hard to know the impact of these follow-up treatments, when we include a substantial population whose tumor never returns once it is removed. A corollary to this includes the fact that on presentation, the average age of these dogs is ten and they therefore may succumb to other age-related diseases before we actually know how the tumor or the surgical site might behave.

With all of this experience and with so few really dependable facts, the discussion with the pet owner about their best choices is both challenging and extensive. As with any treatment plan, the veterinarian wants to know the percentages when discussing complications and successful outcomes with the client. Although our experience in treating prefrontal meningiomas at VRA extends over three decades and includes hundreds of cases, dependable facts that apply to this very common brain neoplasm have neither surfaced in our collective literature nor our collective experience.

Categories: Canine Brain Tumor Surgical Considerations