The cancer antigen 15.3 (CA-15.3) is an important tumour marker for the evaluation of patients with a history of breast cancer. An increase of CA-15.3 can be a sign of breast cancer recurrence and warrants further investigation. However, CA-15.3 is not specific and can be elevated in several oncological and benign conditions. This case describes a megaloblastic anaemia due to folic acid deficiency and elevated CA-15.3 in a patient with a history of breast cancer. No signs of breast cancer recurrence were found, and serum CA-15.3 levels normalised after supplementation of folic acid. Benign causes of CA-15.3 elevation should be considered when evaluating a patient with a history of breast cancer.
(BELG J MED ONCOL 2022;16(6):360–2)