Plasma cell neoplasms (PCN) are clonal proliferations of immunoglobulin producing plasma cells. Extramedullary tumors form a small percentage of plasma cell tumors. Although 80–90 % of extramedullary lesions occur in head and neck region, they account for 1 % of all head and neck tumors. Furthermore, extramedullary plasmacytoma of the endolarynx and thyroid cartilage is very rare [1]. Computerized tomography (CT) widely used to detect thyroid cartilage involvement in multipl myeloma. Previous studies have shown that Positron emission tomography (PET)/CT with F-18 fluorodeoxyglucose (FDG) is useful in detection of both osseous and extraosseous myeloma lesions. However, to our knowledge there is no thyroid cartilage involvement shown with PET/CT. Our present case reports thyroid cartilage involvement of Multiple Myeloma (MM) visualized on F-18 FDG-PET/CT imaging. Three male patients were referred to our clinic (aged 58, and 64) for detection of disease involvement and evaluation of treatment response after the appropriate medical treatment. Diagnosis of MM was made for all three patients on the basis of the presence of a markedly elevated serum IgG level, an abnormal serum and urine electrophoresis showing a monoclonal kappa cell population with an M spike, and results of a bone marrow biopsy that revealed normocellular marrow with a monoclonal kappa plasma cell population. A conventional contrast-enhanced helical CT scan revealed lytic- expansile masses originating within the thyroid cartilage and disseminated lytic-expansile bone lesions. The patients underwent PET/CT imaging 60 min after 10 mCi FDG injection. MIP image of PET/CT demonstrated multiple bone lesions with increased 18 FDG uptake. PET/CT revealed hypermetabolic lesions involving the bone in all of the patients with increased 18 FDG uptake. Active 18 FDG uptake of the thyroid cartilage (SUV-max value between 4 to 8) was also detected in all patients. Extramedullary involvement of MM as presenting mesenteric mass in one patient and hyoid bone inolvement in two patients with increased 18 FDG uptake were detected. Thyroid cartilage involvement of the cases was shown in Figs. 1, ,22 and and33. Fig. 1 Diffuse involvement of cervical vertebrae and thyroid cartilage Fig. 2 Increased FDG uptake in thyroid cartilage in multiple myeloma Fig. 3 Involvement of cervical vertebrae and thyroid cartilage Extramedullary tumors form a small percentage of plasma cell tumors, 80–90 % of which occurs in head and neck [2]. Laryngeal involvement accounts for 6–18 % of all extramedullary plasmacytoma, including epiglottis, glottis, false cords, aryepiglottic folds, and subglottic structures. Involvement of the cricoid cartilage occurs in patients with overt MM as well as in patients with plasmacytoma [1, 3]. Involvement of the thyroid cartilage in PCN is exceedingly rare with only 7 previously reported cases. In 3 of these cases, the neck mass was the initial presenting feature of PCN. In 4 of the seven cases, hoarseness was a presenting feature of the thyroid cartilage lesion [4]. There are two theories explaining mechanisms of cartilaginous involvement in MM. First theory suggests that cartilage may be invaded by an adjacent plasmacytoma [2]. Second theory suggests that cartilage particularly in older patients, may undergo osseous metaplasia with formation of a true central marrow space, and plasmacytomas could originate directly within this marrow. Osseous metaplasia has been previously documented in both the cricoid and the thyroid cartilage and plasmacytoma occurring within an ossified cricoid cartilage has been confirmed pathologically [1]. Although the presence of both red marrow and plasmacytoma within a cartilage would suggest an etiologic relationship, since MM usually develops within bone marrow, documentation of this is difficult, given the rarity of cartilaginous plasmacytomas with either MM or extramedullary plasmacytoma. CT and magnetic resonance imaging (MRI) is useful for the detection of extramedullary sites of the disease. Molecular imaging modalities such as F18-FDG PET or FDG- PET/CT are whole body imaging techniques capable of furnishing merged functional and morphologic information. FDG-PET or PET/CT could detect the treatment response earlier than other imaging methods. Sensitivity and specificity of FDG-PET or PET/CT in the assessment of bone myelomatous involvement at initial diagnosis are very high, 85 and 92 %, respectively [5]. In addition, PET-FDG or PET/CT could help to identify eventual extra-medullary sites of disease not allowed by MRI: therefore, it is more sensitive than MRI for the localization of extramedullary sites of disease. Zamagni et al. have demonstrated that FDG-PET detects improvement of bone disease in more patients (65 %) than MRI does (35 %) [5]. In conclusion, FDG-PET/CT is more sensitive than conventional imaging modalities for the localization of extramedullary sites of the disease.