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Given her age and history of lymphoma, this lady has heart failure until proven otherwise. Chemotherapy is a risk factor for developing heart failure. EKG is not a good test for heart failure. She needs an echocardiogram to evaluate how well her heart is pumping.
Other causes of feet swelling include poor circulation( the swelling tends to be minimal) or kidney disease( typically other parts of the body are swollen as well). Lymph node enlargement can cause swelling, but not pitting edema.
I would recommend pursuing an echocardiogram to rule out heart failure. Meanwhile, low salt diet and feet elevation would be helpful to her.