Anal sac adenocarcinomas (ASACA) arise from the apocrine glands of the anal sacs in dogs. They are one of the most common perianal cancers in dogs. Several breeds are predisposed to ASACA, including German Shepherds, Cocker spaniels, and English Springer spaniels. There are two anal sacs in all dogs, which sit at the 5 and 7 o’clock positions on either side of the anus. These can be palpated on rectal examination, and if a mass is present it can usually be felt. Most dogs with ASACA only have it in one anal sac, however ~14% will have it in both, so it is important that both sacs are carefully palpated.
ASACA usually presents as a mass which is either seen by chance or found because it causes a change in defecation habits (constipation, straining, pain on defecation, etc). 16-53% of dogs with ASACA will have increased blood calcium levels due to hormone production by the cancer. Dogs with high calcium levels can show signs such as increased thirst and urination, lethargy, poor appetite, muscle weakness and vomiting.
ASACA can be highly metastatic. Depending on the study, 26-96% of dogs will have metastasis at the time of diagnosis. The risk of metastasis increases as the primary tumour enlarges. Metastasis is most commonly to the “sublumbar lymph nodes”. These are a cluster of lymph nodes that arise in the caudal abdomen under the spine near the pelvic canal. There can also be metastasis to other parts of the body such as the liver, spleen, lungs, and bones, although most commonly the lymph nodes are the first site of metastasis.
- Fine needle aspirate of the mass: this gives a reliable diagnosis in most cases of ASACA.
- Calcium measurement to check for hypercalcaemia. This should be checked even if there are no outward signs of high calcium, as some dogs can have high calcium without symptoms.
Recommended to plan treatment and give prognostic information
CT scan of the abdomen/pelvis
This is the most sensitive test to identify any enlarged lymph nodes. If lymph nodes are enlarged it indicates possible metastasis. Removing enlarged lymph nodes surgically has been shown to improve outcome, so identifying any enlarged lymph nodes is important. The CT scan also assesses all the other abdominal organs and bones to check for signs of any other metastasis.
Abdominal ultrasound of the abdomen can be used instead of CT, however this is not able to see as many lymph nodes as CT, so some enlarged lymph nodes may not be seen. Ultrasound can also assess the other abdominal organs for metastasis however cannot assess the bones.
CT scan of the lungs
This is to assess the lungs for any signs of metastasis. Alternatively an X-ray can be performed however this is less sensitive than the CT.
Routine blood tests
These screen for any concurrent underlying diseases that may impact our treatment choices. High calcium can cause kidney problems, so if calcium levels are increased then blood tests are essential to check for this.
This is primary treatment for ASACA and is recommended in most cases. This involves resection of the primary mass around the anus. Complete excision can be challenging, especially when tumors are large and/or highly invasive into surrounding tissues. If possible, any enlarged lymph nodes should also be removed especially in cases with elevated calcium levels and/or with signs of constipation/obstruction. This usually improves the symptoms and has been shown to improve survival times.
Not all cases require chemotherapy. However chemotherapy is also often recommended given the high rate of metastasis of ASACA. Chemotherapy can be used before surgery, after surgery, or if surgery is not possible. Chemotherapy options include intravenous treatments or oral medications. Non-steroidal anti-inflammatory drug (NSAIDs) are often used for their anti-inflammatory and analgesic properties as well as possible anti-tumor activity. If the ASACA is inoperable then a tyrosine kinase inhibitor (Palladia) can be used. Around 70% of dogs will respond to this treatment, with the tumour reducing in size or remaining stable in size for a period of time.
Radiotherapy is used for some cases of ASACA instead of or in combination with surgery and chemotherapy. Currently radiotherapy is not available in New Zealand and pets have to fly to Australia for this.
Palliative care is aimed at maximizing quality of life. Treatment is aimed at reducing any symptoms caused by the mass(es). Usually this involves using laxatives to reduce constipation and anti-inflammatories to reduce discomfort. Many dogs can live relatively normal lives for months will palliative treatment alone.
Treatment of hypercalcaemia
Hypercalcaemia can cause severe illness which sometimes requires emergency treatment before surgery. If a dog is very unwell from the high calcium we will generally hospitalise them for treatment with IV fluids and other medications to support their kidneys until the calcium can be reduced.
Removing the cancer tissue surgically usually resolves the hypercalcaemia. If surgery is not pursued, then medications can be used to normalize the calcium. The most common is a drug called zoledronate which is given as an IV infusion once every 3-6 weeks. The calcium levels will normalise in most dogs with this treatment.
Overall survival times for dogs treated with surgery are 1.5-2.5 years. Regrowth of the tumour or of lymph nodes can occur after surgery, sometimes after many months or years. IF this occurs, we can sometimes treat this with a repeat surgery, which improves the survival time. The survival times for dogs treated with chemotherapy alone are around 9-18 months.
Important prognostic factors include:
- The size of the primary tumour
- If there is lymph node metastasis
- If surgery is possible for the primary mass and any enlarged lymph nodes
- If there is distant metastasis (eg to the lung, liver, or spleen)