What is Anal Gland Adenocarcinoma in Pets?
ANAL GLAND ADENOCARCINOMA OVERVIEW
Anal gland adenocarcinoma (AGA) (also known as apocrine gland anal sac carcinoma adenocarcinoma or anal sac adenocarcinoma) is an uncommon cancer that arises from the apocrine glands in the walls of the anal sacs.
This type of cancer tends to occur in middle aged to older dogs with many reports showing Cocker spaniels to be a breed at increased risk. Springer spaniels and Cavalier King Charles spaniels were also shown to be at increased risk. AGA occurs in both males and females. This cancer tends to be highly invasive within the tissues surrounding the tumor and also has a tendency to metastasize. High calcium levels (paraneoplastic hypercalcemia) are seen in approximately 25% of patients with AGA.
WHAT ARE THE SYMPTOMS OF AGA?
A lump next to the anus is sometimes seen, but the first symptoms are usually related to difficulty defecating. Stools may appear narrower or ribbon-like and pets may be noted to be straining when defecating or urinating.
In some patients, the only symptoms noted are related to a high calcium level. Most commonly, increased drinking and urinating are seen in patients with high calcium levels. Untreated, this can progress to severe kidney damage which may be irreversible if caught too late.
How is the diagnosis made?
Rectal palpation will confirm the presence of a mass. A tentative diagnosis is usually initially made with fine needle aspiration and cytology. A definitive diagnosis requires a piece of tissue be taken for histopathology (biopsy).
STAGING THE DISEASE:
Once a tentative or definitive diagnosis is made, it is extremely important to determine if the cancer has spread (metastasized). AGA can go to any tissue in the body, but tends to spread first to the regional lymph nodes (primarily sublumbar nodes). A much smaller percentage of patients can present with metastasis to the lungs, liver, or other sites.
Immunohistochemistry on biopsy samples (specifically E-cadherin expression) has been reported as prognostic indicator and may be recommended. A complete clinical work-up includes: Complete blood count (CBC), serum chemistry panel (including calcium levels), urinalysis, thoracic radiographs (3 views)and abdominal ultrasound (especially sublumbar/iliac lymph nodes).
Can my pet still be treated if the cancer has spread?
If the spread of disease is limited to the sublumbar nodes, treatment is still possible. If the cancer has spread to distant sites such as the lungs or liver, definitive treatment such as surgery or radiation therapy is generally not recommended. In patients with distant spread, treatment is aimed at palliation (making patients more comfortable).
Treatment of anal gland adenocarcinoma:
The recommended treatment for anal gland tumors is complete surgical excision followed by chemotherapy (e.g. carboplatin or mitoxantrone) in order to prevent or slow the onset of metastasis. For cases in which complete surgical excision cannot be obtained, follow-up radiation therapy is recommended to control local disease. Because sublumbar lympadenopathy occurs in more than 50% of patients, it is recommended that sublumbar lymphadenectomy (removal of sublumbar nodes) be performed.
Potential complications of surgery in this area include fecal incontinence. Our surgeon is very experienced with removal of anal tumors. In addition to our expertise in the procedure itself, we very carefully evaluate patients prior to making a decision to go to surgery to decrease the risks as much as possible. When patients present with tumors too large to safely remove, radiation therapy plus chemotherapy are recommended. In some cases, surgical removal is possible following radiation and chemotherapy.
In general, most studies support the use of a combination of therapy (surgery, radiation therapy, chemotherapy) over any one modality alone. A definite survival advantage was seen in patients whose treatment protocols included surgery. Therefore, treatment recommendations may include: Surgical excision of primary tumor and sublumbar lymph nodes if enlarged. If surgical margins are clean (tumor-free microscopically), chemotherapy with mitoxantrone or carboplatin (once every 3 weeks for 5-6 total treatments post-operatively). If margins are incomplete (cancer seen microscopically at tumor margins on biopsy) then radiation therapy in addition to the chemotherapy described above are used. More recently, the chemotherapy agent melphalan has been described as having efficacy and the newer tyrosine kinase inhibitors (e.g. Palladia) may prove beneficial.
What is the prognosis for AGA?
A cure is possible, but this is usually only seen when tumors are found at early stages and treated aggressively with complete surgical excision. The important factors that tend to predict prognosis are the size of tumors (dogs with tumors greater than 10.7 cm2 had median survivals of 9.7 months vs. 19 months for tumors smaller than 10.7 cm2) and hypercalcemia(patients with elevated calcium levels had median survival times of 8.5 months vs. 19.5 for patients with normal calcium levels). In one study, patients treated with surgery, radiation therapy and chemotherapy had median overall survival times of 32 months. Therefore, prolonged survival times can be seen with treatment. It is important to remember that every patient is an individual and recommendations may vary depending on your pet’s stage of disease and overall general health.
When tumors are too large for treatment or they have spread to distant sites, therapy is aimed at improving the quality of life of affected pets as much as possible. Symptoms of hypercalcemia can be controlled with fluid therapy and prednisone or calcium lowering drugs (e.g. bisphosphonates). Symptoms of pain associated with difficulty defecating can often be helped with stool softeners (e.g. Metamucil) and pain medications (e.g. non-steroidal anti-inflammatory drugs and/or tramadol are commonly used).