An abnormal growth of tissue resulting from an uncontrolled multiplying of cells that serves no physiological function. Tumors can be either malignant (cancerous) or benign (non-cancerous). Malignant tumors include newly developed or ""de novo"" solid tumors as well as recurrent tumors that had previously been successfully treated. Posttransplant lymphoproliferative disorders (PTLD) are abnormal accumulations of immune cells brought about by the immunosuppression therapies used to reduce organ rejection.