This is suggestive of second primary malignancy of endometrium - need tissue diagnosis to confirm . I need some more information to advise on how to proceed .Please share if any tissue diagnosis (biopsy)reports available . If proven to be cancer second primary , We will treat it as a new disease ,then it’s better to go-ahead with surgical removal of uterus . After that we have to proceed based on the current status of her breast cancer also , if the disease is controlled, better to go with a PET CT scan to evaluate the current status of disease and decide what more treatments are required based on the current status . And if the breast disease is controlled or not in a very advanced extensive disease status , then this second primary disease should be treated fully with combined adjuvant treatment as indicated . Also please share if there is any family visitors of malignancy in 1st or 2nd degree relatives.