**1. Introduction**

Multiple myeloma (MM) is a neoplastic plasma cell disorder, characterized by the uncontrolled, clonal proliferation of plasma cells in the bone marrow. It is the second most common hematologic malignancy after non-Hodgkin's lymphoma accounting for approximately 1% of neoplastic diseases, and the most common primary tumor of the skeleton [1]. MM is almost always preceded from a premalignant precursor condition (monoclonal gammopathy of undetermined significance, MGUS), which then develops into asymptomatic or smoldering myeloma (SMM) and, finally, into symptomatic disease [2]. Bone involvement in the form of focal osteolytic lesions—the hallmark radiographic sign of MM—represents a marker of disease-related end-organ damage, necessitating immediate initiation of treatment [3]. Bone disease is a major cause of morbidity and mortality for patients suffering from MM. Since practically all patients develop bone involvement during the course of the disease [4], its reliable identification represents a pivotal diagnostic challenge. Historically, skeletal damage has been assessed by conventional, whole-body X-ray survey (WBXR), which was the standard imaging approach for

MM. Nevertheless, this modality carries several limitations, including a low sensitivity—requiring a more than 30% bone demineralization before an osteolytic lesion becomes evident—its failure to detect extramedullary disease (EMD), which is a significant adverse prognostic factor of MM, and its poor performance in treatment response assessment [5]. The drawbacks of planar radiography have been overcome in recent years with the development and introduction in clinical practice of myeloma of novel imaging modalities, namely whole-body computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography/computed tomography (PET/CT). These techniques offer a higher sensitivity than WBXR, leading to its gradual substitution by them.

It is undisputable that the role of PET/CT with the radiotracer 18F-fluorodeoxyglucose (18F-FDG) in MM has been upgraded with an increasing amount of literature highlighting its value in diagnosis, prognosis and treatment response evaluation of the disease. According to the latest update of the International Myeloma Working group (IMWG), the detection of one or more osteolytic lesions on CT or PET/CT fulfills the criteria of bone disease and, therefore, of symptomatic MM requiring treatment [4].

This review article provides an overview of the position of PET/CT in MM management with focus on the most widely used tracer 18F-FDG. In addition, the main data published on new PET tracers targeting different molecular pathways involved in MM pathogenesis are presented.
