Biomarkers, Tumor/blood; Blood Protein Electrophoresis; Electrophoresis, Agar Gel; Immunoglobulins/blood; Liver Transplantation; Lymphoproliferative Disorders/blood; Neoplasm Proteins/blood; Postoperative Complications/blood; Adult; Antibodies, Monoclonal/therapeutic use; Antibodies, Monoclonal, Murine-Derived; Antineoplastic Agents/therapeutic use; Diagnostic Imaging; Female; Follow-Up Studies; Humans; Immunosuppression Therapy/adverse effects; Incidence; Lymphoproliferative Disorders/diagnosis; Lymphoproliferative Disorders/epidemiology; Lymphoproliferative Disorders/etiology; Lymphoproliferative Disorders/therapy; Male; Middle Aged; Paraproteins/analysis; Postoperative Complications/diagnosis; Postoperative Complications/epidemiology; Postoperative Complications/etiology; Postoperative Complications/therapy; Predictive Value of Tests; Prospective Studies; Risk; Rituximab; Sensitivity and Specificity