Hello there,I hope the surgery that has been done to your mother is Modified Radical Mastectomy with Axillary lymphnode dissection and the stage as u mentioned is a T2N2 There is no wrong in doing a PET scan or there is no issue of not doing a PET scan also at this point of time.We can do low cost investigations such as CT chest ,abdomen and pelvis.Ideally to see whether the disease has spread to bones ,we can use a Bone scan which is ideal for detecting bone spread(metastases) in a breast cancer patient.Moreover PET is not a reliable investigation for brain spread since glucose uptake and  metabolic activity in brain can hide the disease( U need a separate MRI Brain to confirm brain spread) As such it is not mandatory to do a PET unless the doctor feels he/she warrants it TRUST YOUR DOCTOR!! Anyway for a breast cancer patient in this stage,she requires all treatment including chemotherapy , radiation and further hormonal therapy depending on ER,PR,HER2Neu status.