The anal sacs are little blind-ending skin pockets situated near the anus (at 4 and 8 hours of the anus). In the wall of the anal sacs glands are present of which the content fills the anal sacs.

Other tumors can be located near the anus, such as benign tumours of the hepatoid perianal glands, lymphoma, tumour of the soft tissue, squamous cell carcinoma, melanoma, leiomyoma, transmissible venereal tumour (kind of tumour that can be sexually transmitted) and mastocytoma.

Behaviour

This tumour type is in general malignant. The tumour grows slowly, but invades the local surrounding tissues and spreads rather fast. Most sources mention that in 50% of dogs metastases are present at the diagnosis, even when the tumour only measures 0,5 cm. These metastases are mostly found in the local lymph nodes, but can also appear in the liver, the bone, the spleen, the lungs and less frequently in the adrenal glands, the pancreas, the kidneys and/or the space in between the lungs (mediastinum).

This tumour type causes an increase in calcium blood levels in up to 27% of dogs, which can have a negative impact on the kidney function.

Prevalence

Relatively rare. When a tumour is situated near the anus, there is a 17% risk this is a tumour of the anal sac. The tumours of the anal sac represent 2% of the possible (sub)cutaneous tumours. Male dogs have the same risk as female dogs of developing this tumour type. The tumour mainly occurs between 9 and 11 years of age. In general the tumour is only situated in 1 anal sac and thus on only one side of the anus.

Breed predisposition

These tumours are mostly seen in Spaniels, particularly in Cocker Spaniels.

Symptoms

The presence of the primary tumour (where the tumour originates) can cause several symptoms: discomfort around the anus, swelling, bleeding, dragging the posterior on the floor. The metastases in the lymph nodes can become so large that they can obstruct the pelvis and/or cause pressure on the intestines (and with that a feeling of urgency to defecate) and/or pain and constipation. The consequence of high calcium blood levels in dogs with this tumour type can manifest itself in a clear increase of the frequency in which a dog drinks and urinates. Other symptoms consist of loss of appetite, becoming apathic and vomiting. When the tumour is removed, the calcium levels return to normal. When these symptoms reappear this can be an indication that metastases are present or the tumour has relapsed. Metastases towards the bone are rare, but when this is the case they may cause pain and/or fractures.

Physical examination

Via a rectal examination it is possible to inspect the anal sacs and to feel whether the lymph nodes are swollen. If abnormal swellings are present, the diagnosis must be confirmed via cytology -the tumour is pierced with a needle and the needle content inspected under the microscope- or via histology (biopsy).

Bloodwork

When the blood levels of ionized calcium are increased this can be an indication that an anal sac adenocarcinoma is present. High calcium blood levels can cause renal issues, which has an influence on anesthesia options and the quality and longevity of life.

Imaging

Abdominal or chest radiographs can indicate the presence of metastases towards the lungs or swollen lymph nodes in the abdomen. Via ultrasound and/or possibly a CT/MRI scan one can precise how (much) lymph nodes are affected (when using only radiography this can be under/overestimated). A biopsy is needed to confirm the tumour type.

Large studies that prove which treatment is the most appropriate one for which tumour stage are currently lacking. The ideal combination of treatments is unknown.

If the goal is to offer a temporary improvement of symptoms, then the following treatment options can be used: surgery, palliative radiotherapy, chemotherapy and/or tyrosine kinase inhibitors (kind of growth inhibitors). The efficacy of these options have not yet been definitively proven.

For the primary tumour surgery with conservation of the function of the anal sphincter is advised. Supplementary radiotherapy can result in a good local control for any remaining cancer cells.

If metastases are present chemotherapy based on carboplatin or mitoxantrone, toceranib phosphate, non-steroidal anti-inflammatory drugs, metronomic chemotherapy can be given and/or bisphosphonates for the treatment of excess calcium.

Surgery

This tumour type invades the local tissues and demands a wide excision. The total excision of this tumour can be rather difficult due to the proximity of the anal sphincter. Up to half of the sphincter can be excised without causing nerve damage. When more than half needs to be removed or when the tumour is situated on the left as well as on the right side of the anus, surgical excision can lead to nerve damage. The consequences include the loss of sphincter control and thus loss of control of defecation.

The swollen lymph nodes can also be removed, certainly when they cause obstruction of the pelvis and/or cause an excess of calcium blood levels. When the tumour can only be removed with little margin, the risk of recurrence is high. This is also the case when the distinction between healthy and tumoural tissue is difficult. Complications occur in about 10% of surgically treated cases. The most common complications include bleeding during the operation, infection, incontinence (fecal (incontinence occurs temporarily in 2.5% of cases, permanently in 0.8%) or urinary), squeezing to defecate and the formation of fistulas.

Radiotherapy

This treatment is advised when the tumour cannot be removed in its entirety. Up to 38% of dogs had a reduction in tumour size after radiotherapy and in 62% of dogs with clinical symptoms there was an improvement or disappearance of symptoms after radiotherapy.

Chemotherapy

So far, little is documented about the effect of chemotherapy as monotherapy or as a supplementary treatment for anal sac adenocarcinomas.

No impressive results were reported for chemotherapy (mitoxantrone, carboplatin, cisplatin, actinomycin D, melphalan) in monotherapy and their role in combination with other treatments is unknown.

Toceranib phosphate (tyrosine kinase inhibitor) is associated with a mild response (decrease in tumour volume).

Non-steroidal anti-inflammatory drugs have no proven effect so far, but their general affect against the formation of new blood vessels and tumour development could decrease the tumour growth.

When the tumour is in an early stage, the prognosis is moderate to good.

When the entirety of the tumour can be removed via a surgical excision and there are no metastases at the moment of diagnosis, the prognosis is good.

However, when there are metastases or it isn’t possible to remove the entirety of the tumour, surgery and supplementary radiotherapy will be necessary to treat the tumour and the metastases. When the affected lymph nodes can be surgically removed, 1,5 to 3 years survival rates have been reported and even cured dogs.

Depending on the excision of the tumour and the presence of metastases the life expectancy will differ. When there are numerous metastases, the mean survival is less than a year. Factors associated with a bad prognosis are big tumours, an advanced tumour stage (high tissue grade), fast multiplication of cancer cells, a lack of certain hormonal receptors, damaged overlying skin. Little is known at this time about the results with radiotherapy and chemotherapy.