CONDITION
Anal Sac Adenocarcinoma
Anal sac adenocarcinoma is a cancer that arises in the tissue of the anal sacs — the small scent glands that sit just inside the anus in dogs and cats. It tends to grow locally and can spread to nearby lymph nodes and, in some cases, to more distant sites. Owners often notice scooting, licking at the rear, difficulty passing stools, or swelling to one side of the anus, though in some animals the first sign is detected during a routine examination. This condition is more common in older dogs, particularly certain breeds, and is rare in cats. The signs can be subtle early on and may overlap with more common anal sac problems, so investigation typically involves imaging and sampling of any masses or enlarged lymph nodes. This page explores what signs may be observed, what is happening in the tissue, how the condition is investigated and staged, and what treatment approaches exist. Each section is written to help you understand the landscape, not to direct a specific course.
Why this matters now
Anal sac adenocarcinoma tends to appear in older dogs, most commonly between eight and twelve years of age, though cases outside this range do occur. Certain breeds — including spaniels, German Shepherds, and some terrier breeds — appear in the literature more frequently, though the condition is recognised across all breeds and mixed breeds. In cats the condition is rare enough that age and breed patterns are less clear. There is no strong evidence linking diet, environment, or earlier anal sac disease to the development of this cancer.
The tumour typically begins in one anal sac and grows outward over weeks to months, sometimes becoming palpable before it causes obvious signs. Many dogs develop spread to the regional lymph nodes — those alongside the colon and deeper in the pelvis — relatively early in the course. In a proportion of cases the cancer reaches more distant sites, most often the lumbar spine, liver, or lungs, though this pattern varies widely between individuals. Some dogs live with localised disease for many months; others show rapid progression.
Signals & patterns
Early signals
Scooting or dragging the rear
The dog may slide along the ground on its bottom, often interpreted as a response to irritation or discomfort around the anus. This sign is common in many anal sac problems, so it does not indicate cancer on its own, but it can prompt closer attention.
Licking or chewing at the rear
Repeated attention to the area around the tail base or anus may reflect local discomfort. The behaviour can be intermittent and may be dismissed as a habit until a lump or swelling is found.
Swelling to one side of the anus
A firm lump may become visible or palpable beside the anal opening, often asymmetrical. This swelling may feel fixed rather than fluctuant, and it may not respond to the expression of the anal sacs in the way a simple impaction would.
Straining or flattened stools
As the mass enlarges, it can press on the rectum from outside, narrowing the space through which stool passes. Owners may notice ribbon-like stools or see the dog strain without producing much.
Later signals
Weight loss or reduced appetite
As the disease progresses or spreads, some dogs lose weight or show less interest in food. This change can be gradual and may coincide with other signs of advancing illness.
Lameness or stiffness in the hindquarters
Spread to the lumbar spine or pelvis can cause pain or altered gait. The dog may be reluctant to jump, climb stairs, or settle comfortably, and the lameness may not respond to rest.
Swelling around the vulva or scrotum
In some cases, enlarged lymph nodes deeper in the pelvis obstruct lymphatic drainage, leading to visible swelling of the perineum or external genitalia. This is often a later sign.
Increased thirst or urination
A paraneoplastic syndrome associated with this cancer can disturb calcium regulation, leading to elevated blood calcium. This in turn may cause the dog to drink and urinate more than usual, sometimes prompting investigation before other signs are obvious.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a detailed history and physical examination, including careful palpation of the area around the anus and assessment of the regional lymph nodes via rectal examination. If a mass is detected, imaging is used to assess its extent and to look for spread to lymph nodes and distant sites. Sampling of the mass and any enlarged lymph nodes, along with blood tests to assess calcium levels and organ function, helps to build a picture of the disease and inform the conversation about management.
Physical examination and rectal palpation
Blood biochemistry
Imaging (radiography and ultrasonography)
Fine-needle aspiration or biopsy
Advanced cross-sectional imaging
Options & trade-offs
Management of anal sac adenocarcinoma often involves a combination of approaches tailored to the individual dog, the extent of disease at presentation, and what feels workable for the household. Some animals are managed with surgery, some with medical therapies, and some with a mix of both alongside monitoring. The landscape is broad, and different combinations suit different circumstances.
Surgical removal
The primary tumour and, in many cases, the regional lymph nodes are removed during an operation. The anal sac and surrounding tissue are dissected carefully to preserve the function of the anal sphincter and nearby nerves. If the sublumbar lymph nodes are enlarged, they may also be excised, though this adds complexity and risk to the procedure.
Trade-offs: Surgery can achieve local control and in some cases prolongs survival, particularly when the disease is confined to the anal sac and local nodes. However, the operation carries risks including faecal incontinence, nerve damage, and complications related to anaesthesia, and it does not address distant metastases if present. Recovery takes several weeks, and some dogs experience ongoing discomfort or changes in defaecation.
Chemotherapy
Drugs that interfere with cell division are given intravenously or by mouth over a series of sessions, typically spaced two to three weeks apart. Protocols used in anal sac adenocarcinoma are adapted from those used in other carcinomas, and the aim is to slow progression, reduce tumour size, or manage metastatic disease.
Trade-offs: Chemotherapy can be delivered without anaesthesia and may be used alone or alongside surgery. Side effects tend to be mild in dogs compared to humans, but gastrointestinal upset and temporary drops in white blood cell counts can occur. Response is variable, and not all tumours shrink; some stabilise for a period, while others continue to grow.
Radiotherapy
Focused radiation is delivered to the primary tumour and regional lymph nodes over multiple sessions, typically requiring general anaesthesia or heavy sedation for each fraction. The treatment damages the DNA of rapidly dividing cells, which can slow or halt local tumour growth.
Trade-offs: Radiotherapy can be effective for local disease control, particularly in dogs where surgery is not feasible or in cases of incomplete surgical removal. Access is limited to referral centres, and the protocol requires repeated anaesthetic episodes over several weeks. Late side effects, such as tissue fibrosis or changes in bowel function, can develop months to years afterward.
Medical management of hypercalcaemia
If the tumour is producing parathyroid hormone–related protein and causing elevated blood calcium, intravenous fluids are given to promote calcium excretion, sometimes alongside drugs that reduce calcium release from bone or lower its reabsorption in the kidneys. This approach addresses the metabolic consequences of the cancer rather than the tumour itself.
Trade-offs: Correcting hypercalcaemia can improve comfort, appetite, and kidney function in the short term, and may be an important part of stabilising the dog before other treatments or as a standalone measure in advanced disease. However, calcium levels often rise again once the tumour continues to grow, and repeated hospitalisation may be needed.
Monitoring and symptomatic support
In some cases, particularly when the tumour is slow-growing or the dog is otherwise well, management focuses on observing the disease and addressing symptoms as they arise. This might include stool softeners, analgesia, or adjustments to diet and activity to maintain comfort and quality of life.
Trade-offs: This approach avoids the risks and side effects of surgery, chemotherapy, or radiotherapy, and some dogs remain comfortable for months without active treatment directed at the cancer. The tumour will continue to grow, and the window for other interventions may narrow over time. This approach suits animals where aggressive treatment is not desired or where comorbidities make other options higher-risk.
Common misconceptions
"Anal sac adenocarcinoma only causes problems when the tumour is large and visible."
Many dogs develop spread to lymph nodes or distant sites while the primary mass is still small and may not even be causing noticeable signs at the anus. Hypercalcaemia can occur early in the course and may produce signs such as increased thirst or lethargy before any local swelling is apparent. The relationship between tumour size and biological behaviour is not straightforward.
"If the calcium level is normal, the cancer is not serious or is caught early."
Not all anal sac adenocarcinomas produce parathyroid hormone–related protein, so a normal calcium does not indicate less aggressive disease or earlier stage. Some dogs with widespread metastases maintain normal calcium throughout, while others develop hypercalcaemia with relatively localised tumours. Calcium is one marker among many, and staging relies on imaging and tissue sampling.
"Once the tumour is removed surgically, the dog is cured."
Surgery removes the visible tumour and affected lymph nodes, but microscopic disease may already be present elsewhere, and recurrence at the surgical site or progression at distant sites can occur months or years later. Long-term outcomes vary widely, and many dogs benefit from monitoring and sometimes additional treatment after surgery. The term cure is used cautiously in cancer medicine, and survival is often described in terms of time frames and quality of life rather than permanent resolution.
Related conditions
Transitional Cell Carcinoma
Transitional cell carcinoma and anal sac adenocarcinoma are both locally invasive cancers that arise in structures near the pelvic region and can cause urinary or defecatory signs that may initially be mistaken for inflammation or infection. Both tumours tend to spread to regional lymph nodes and may be staged and managed using similar imaging and sampling approaches.
Oral Melanoma
Oral melanoma and anal sac adenocarcinoma are both aggressive epithelial tumours that share a tendency for early spread to lymph nodes and distant sites, and both are more common in older dogs. The clinical staging, prognostic assessment, and some treatment considerations may overlap, though the primary sites and specific behaviours differ.
Prostate Disease
Prostate disease in male dogs can cause signs that overlap with anal sac adenocarcinoma, including difficulty passing stools, straining, and swelling near the pelvis. Both conditions may be detected during rectal examination, and distinguishing between prostatic enlargement and a mass arising from the anal sac tissue can require imaging and biopsy.
Hyperthyroidism in Cats
Hypercalcaemia—elevated calcium in the blood—can occur as a paraneoplastic syndrome in dogs with anal sac adenocarcinoma, whilst it is also a common metabolic consequence of hyperthyroidism in cats. In dogs with anal sac tumours, the calcium elevation arises from tumour-secreted factors and may cause signs such as increased thirst, reduced appetite, or changes in kidney function.
If your dog has been investigated for a mass or signs around the anus, understanding how the staging process unfolds and what each test reveals can help you think through the questions that matter to you. The longevity and healthspan section of this resource explores how age, concurrent conditions, and individual variation shape the course of many cancers, and the pages on hypercalcaemia and metastatic disease offer additional context. This is a landscape where small details — the position of a node, the trend in calcium over weeks — can carry weight, and these are often useful points to revisit in conversation as the picture evolves.
Last reviewed: 30 June 2026 · Dr Alastair Greenway MRCVS