FDG-positron-emission tomography for staging and therapeutic assessment in patients with plasmacytoma

Plasmacytoma can be confined to bone (solitary bone plasmacytoma) or occur in extramedullary sites (extramedullary plasmacytoma).1–4 Diagnostic criteria are a biopsy-proven lesion of bone or soft tissue with evidence of clonal plasma cells, normal bone marrow with no evidence of clonal plasma cells and absence of end-organ damage.5,6 In a recent study, Zamagni et al. have shown that although MRI of the spine and pelvis still remains the gold standard imaging technique for the detection of bone marrow involvement in multiple myeloma, FDG-PET/CT provides additional and valuable information for the assessment of myeloma bone disease in areas not covered by MRI.7 The purpose of the present study is a prospective comparison of FDG-PET/CT and MRI for the appraisal of the staging and the therapeutic assessment of patients with plasmacytoma.

Twenty-four patients (17 males and 7 females; median age 60 years, age range 35–78) with pathologically documented plasmacytoma were included from June 2003 to June 2007 at the University Hospital of Nantes (France). Patients underwent whole body FDG-PET/CT (also covering skull, upper and lower limbs) and bone marrow MRI at the initial staging and therapy afterwards. The physicians for the therapeutic assessment knew the primary site of the disease. For both ethical and practical reasons, not every suspected involved lesion was evaluated by histology. When histological data were not available, the gold standard, therefore, resulted from an exhaustive analysis of patient data and follow-up.

Twenty-three patients were assessed for pre-therapeutic staging and 14 for therapeutic staging. At baseline staging, 54 tumor sites were confirmed according to the gold standard: 50 in bone areas and 4 in soft tissue areas (head and neck, breast and pelvis).

For the baseline staging, overall 460 regions in 23 patients were analyzed by FDG-PET/CT and 120 regions in 20 patients compared to MRI. FDG-PET/CT showed abnormal uptake in 57 sites (53 in bone areas and 4 in soft tissue areas): 53 foci considered as true-positive (TP) and 4 foci as false-positive (FP). FDG-PET/CT missed 1 lesion (iliac bone) confirmed by biopsy. In the 120 regions explored by MRI, 29 lesions were confirmed as being plasmacytoma, according to the gold standard. MRI detected 32 abnormal images, 27 images considered as TP and 5 as FP. MRI missed 2 lesions. FDG-PET/CT correctly classified the 5 FP and the FN. In those areas, FDG-PET/CT showed 3 foci considered as FP and missed 1 lesion, all correctly classified by MRI. Se, Sp, PPV and PNV were calculated. Results are shown in Table 1.

The accuracies of MRI and FDG-PET/CT in detecting lesions were concordant in 12/20 patients (60%) and discordant for the remaining 8/20 (40%). Finally, in 10/20 patients (50%), FDG-PET/CT detected plasmacytoma lesions in 18 areas (bone: 8 patients, soft tissue: 2 patients), which were outside the scope. Figure 1 shows a lombar spine plasmacytoma detected by FDG-PET/CT and MRI.

For therapeutic evaluation, 14 FDG-PET/CT, corresponding to an overall 260 regions, were analyzed; 10 FDG-PET/CT (61 regions) compared with MRI. Five patients have been treated by radiotherapy, 6 by chemotherapy and 3 by high-dose chemotherapy and autologous stem cell transplantation. A complete response was observed in the 5 radiated patients and in 2 patients treated by chemotherapy, only a partial response in 4 and none in 3 for the other patients. FDG-PET/CT showed persistent abnormal uptake in 18 sites, all in bone, and disease was confirmed for all these sites. Among the 61 regions compared using both FDG-PET/CT and MRI, the findings were concordant for 9 positive and 45 negative areas and discordant for 7 regions in 3 patients. As a matter of fact, MRI falsely classified 6 positive lesions, each of them being correctly classified by FDG-PET/CT (Figure 1). Se, Sp, PPV and PNV are given in Table 1.

Table 1.FDG PET/CT and magnetic resonance imaging performance for staging and therapeutic assessment.

FDG-PET/CT is a metabolic imaging technique where the scope covers the whole body (from skull to lower limbs). Major advantages of FDG-PET/CT are the ability to perform full body examinations, the potential to detect medullary and extramedullary lesions in one single examination and the possibility of distinguishing new active disease from scar or necrotic tissue. Studies comparing FDG-PET with conventional techniques for plasmacytoma detection have been limited due to the rareness of the disease. To the best of our knowledge, the present study is the first one conceived to assess FDG-PET/CT prospectively in a relatively large series of plasmacytomas for initial staging and therapeutic appraisal. In addition to this, we compared the performance of FDG-PET/CT to that of MRI in most cases. Our results show that FDG-PET/CT has a high performance for initial staging of plasmacytoma. Its performance seems equivalent to that of MRI in spine and pelvic bone. Thus, FDG-PET/CT allows investigation with a larger scope, e.g. soft tissues, skull, ribs and limbs. During initial staging, MRI missed 18 lesions located outside of the scope. Moreover, FDG-PET/CT shows a higher performance than MRI for therapeutic assessment. Indeed, MRI failed to differentiate scars from persistent active lesions, as previously observed for multiple myeloma.7 Our study found a higher performance for plasmacytoma assessment than that published for multiple myeloma.7–10 This is probably due to the generally larger size of usual plasmacytoma lesions. Furthermore, we used a hybrid system of PET integrated with CT, which allows for an exact detection of small and/or slightly active lesions. Such lesions are not easily recognized nor differentiated from soft tissue lesions, when using PET alone. In addition, by means of fused images, where each hypermetabolic lesion was concordant with the morphology of a lesion on the corresponding CT image, we could directly confirm the reliability and specificity of our FDG-PET/CT findings in all patients.

Figure 1.Images performed on a 49-year old male with a lombar plasmacytoma. Before treatment, plasmacytoma was detected by FDG PET/CT (A) and MRI (B). After external radiotherapy, FDG PET/CT (C) revealed no abnormal uptake. MRI (D) showed a persistent abnormal spine signal. Follow-up confirmed the absence of recurrence.

In conclusion, PET-CT proves to be a technique that can be used for the widespread screening of plasmacytoma lesions, both in bone and soft tissues in clinical practice. It can probably also be used to assess therapeutic efficiency.