As per my understanding your mother is suffering from T-cell lymphoma stage 3. As per literature the survival rate for lymphoma stage III, is of 5 years in 83% of patients. But still she needs to be under observation of an oncologist. Further investigations, treatment all depends on her general condition and stage and type of cancer. Routine cytology with PET scans, and others may be required. But line of investigations is all planned according to patient's response to treatment and doctor's decision. It varies from case to case. Consult an oncologist. You may check this link and get in touch with experts having relevant qualifications - 10 Best Oncologist In India.