Patients are treated on an individual basis. My grandmother was older than that, she had a mastectomy and radiotherapy.
The cancer metastasised into her spine. She had to spend the last months of her life in a nursing home.
My mother is 72. She has had two different cancers, one in each breast. Her mastectomies were 5 years apart. It has now metastesised into her liver. We don't know how long she has left, we know it will be months, but not if it will be years. She is having chemotherapy.
These are cases in different parts of the country.
OK now your mum.
Yes tamoxifen should stop it spreading. Treatment options are based on what the patient wants and how curable it is. If it cannot be cured then is it better to prolong life, which may have a poor quality, or inmprove quality of life but shorten the life. Obviously the best (other than a cure) is a good qualitiy of life for as long as possible.
There is a formula for treatment called The Nottingham Prognostic Indicator - copy and paste from Cancer help UK.
NPI = (0.2 x tumour diameter in cms) + lymph node stage + tumour grade, where
The lymph node stage can be 1 (if there are no nodes affected), 2 (if up to 3 glands are affected) or 3 (if more than 3 glands are affected)
The tumour grade is scored as either 1, 2, or 3
The formula gives scores which fall into three bands
A score of less than 4 ? this suggests a good outcome with a high chance of a cure
A score of between 4.01 to 5.4 ? this suggests an intermediate level with a moderate chance of cure
A score of more than 5.4 ? this suggests a lower chance of cure
Good luck to you and your mum, maybe you could go to an appointment with her to see why they are suggesting the current treatment.