Anal sac apocrine gland adenocarcinoma arises from the specialised scent glands within the anal sacs, two small pouches located on either side of the anus. These glands normally produce a pungent, oily secretion used for territorial marking. The cancer is notable for two features: it frequently spreads early to the sublumbar (internal pelvic) lymph nodes — these are already involved in 50–80% of cases at diagnosis — and it commonly produces a protein called parathyroid hormone-related protein (PTHrP) that causes dangerously elevated blood calcium levels (hypercalcaemia) in 25–50% of affected dogs. This paraneoplastic hypercalcaemia can cause kidney damage, weakness, and loss of appetite, and is sometimes the first clinical sign that prompts investigation. Spaniel breeds appear to be at higher risk. Treatment typically involves surgical removal of the affected anal sac, often combined with chemotherapy.