Relationship Between Fluorine-18 Fluorodeoxyglucose PET/CT Uptake and the Plasma Cell Inltration Rate in the Bone Marrow of Patients With Multiple Myeloma

Background: The present study investigated the relationship between maximum standardized uptake values (SUV max ) on Fluorine-18 Fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) and the plasma cell (PC) proliferation percentage in the bone marrow (BM) of patients with multiple myeloma (MM). Material and methods: Patients clinically and pathologically diagnosed with MM between January 2012 and October 2019 were selected from the database of Dokkyo Medical University Hospital (DMU Hospital), Japan. Patients with other malignancies and those who received chemotherapy and radiation therapy or hematopoietic treatments in the previous 4 weeks and 2-3 months prior to PET/CT scans were excluded from the present study to avoid false uptake. We examined 60 patients (female: male; 27:33; 1:1.22) aged between 37 and 91 years; mean ± SD; 67.3 ± 10.2 years. Hematological and biochemical tests with results on hemoglobin (Hb), hematocrit (Ht), C reactive protein (CRP), beta-2 microglobulin (b2m), creatinine (Cre), albumin (Alb), calcium (Ca), white blood cells (WBC), and red blood cells (RBC) were recorded and evaluated. The percentage of PC in BM biopsy samples was calculated. On PET/CT images, the SUV max of the region of interest (ROI) of the right anterior iliac crest, the biopsy site, was measured. Relationships were examined using Pearson’s correlation coecient with SPSS. Results: The types of MM diagnosed were IgGk in 31 patients (51.6%), IgGλ in 19 (31.6%), IgAλ in 5 (8.3%), IgAk in 2 (3.3%), IgDλ in 2 (3.3%), and IgDk in 1 (1.6%). According to the Durie-Salmon staging system, 4 patients had stage I, 6 stage II, and 33 stage III, while stages were not recorded in the remaining 17 patients. A positive correlation was observed


Background
Multiple myeloma (MM) is the second most common blood cancer worldwide, accounting for 1% of all cancers, and is characterized as a malignant hematological disorder with the uncontrollable monoclonal proliferation of malignant plasma cells (PC) [1,2]. According to Global cancer statistics 2018, MM represented 0.9% of all new malignancies as well 1.1% of the leading causes of cancer deaths worldwide [3].
The International Myeloma Working Group (IMWG) consensus recommend the mandatory use of FDG PET/CT to con rm a suspected diagnosis of solitary plasmacytoma and provide crucial prognostic information [25,26]. The European Society of Medical Oncology (ESMO) and European Myeloma Network (EMN) guidelines also recommend low-dose whole-body CT and FDG PET/CT based on their availability [5,8].
The diagnosis and classi cation of MM previously required the presence of end-organ damage, known as the CRAB criteria, which include an elevated (C) calcium (Ca) level, (R) renal dysfunction, (A) anemia, and destructive (B) bone lesions. The Durie-Salmon Staging System (DSSS) shows the relationship between the extent of myeloma and associated damage, and the revised system in 2006 integrated new imaging techniques, such as FDG PET/CT and MRI. The International Staging System (ISS) is based on serum beta-2 microglobulin (b2m) and serum albumin (Alb) levels, and the 2015 revised system includes the chromosomal abnormality t(14;16), t (4,14), del17p and serum lactate dehydrogenase (LDH) levels.
The 5-year survival rate of MM in the United States (US) was 54%, approximately 75% among those diagnosed in the early stages, and 51% in those diagnosed at the late stage with metastasis to distant parts of the body. The survival rate of MM has steadily been increasing in the last few decades due to advances in treatment and management [27].
Increased PC in ltration rate brings higher FDG uptake in BM, hence in this retrospective study, we investigated this relationship based on our data. The result should be essential in the follow up evaluation of MM patients, where biopsy may not be necessary.

Materials And Methods
This retrospective study was approved by the Ethics Committee of Dokkyo Medical University. Between January 2012 and October 2019, all patients with biopsy-proven MM were retrospectively evaluated. We examined 60 patients (females:males; 27:33; 1:1.22) aged between 37 and 91 years; mean ± SD; 67.3 ± 10.2 years. We excluded patients with other malignancies and those who received hematopoietic treatments or chemotherapy and radiation therapy in the past 4 weeks and 2-3 months prior to PET/CT scans. Since this was a retrospective study, hematological and biochemical test results (hemoglobin (Hb), hematocrit (Ht), C reactive protein (CRP), b2m, creatinine (Cre), Alb, Ca, white blood cells (WBC), and red blood cells (RBC)) were obtained prior to or after 2 weeks, whichever was the nearest to the PET/CT scan date; however, some results had longer date gaps. The demographic and clinical properties of patients are shown in Table 1.

18F-FDG PET/CT imaging
Prior to PET/CT, all patients fasted overnight (for at least 6 hours) and were intravenously administered FDG based on their body mass. Blood glucose levels were measured before the FDG injection, it should be equal or less than 200 mg/dl. Patients rested in a warm room during the FDG uptake time of 1 hour.
Whole-body imaging was performed using a Siemens Biograph mCT Flow PET/CT scanner or Siemens Biograph mCT Horizon PET/CT scanner at the PET Center, DMU Hospital, Tochigi prefecture, Japan. The PET acquisition time was 2.2 min per eld of view. CT scan data were obtained with a peak voltage of 120 kV and tube current of 70 mAs.
Regarding the semi-quantitative evaluation, the SUV max of the ROI, namely, the right anterior iliac crest, was calculated using semiautomatic image registration software package. The right anterior iliac crest was selected to standardize the calculation of FDG uptake by BM because it was the biopsy site and concordant with previous studies [12,28,29]. Figures 1 and 2 demonstrates the PET/CT scan image of MM patients.

PC counting
All H&E-stained specimens were examined under a light microscope (BX53 Olympus, Japan) and digital photos were taken using CellSens Standard imaging software. PC were counted using the application QuPath 0.2.2 [30]. The in ltration rate of PC represents the number relative to all nucleated hematopoietic cells in BM. PC biopsy samples are shown in Figures 3 and 4.

Statistical analysis
Statistical analyses were performed using SPSS 27 (SPSS Inc, Chicago, III., USA) [31]. The relationship between FDG uptake and PC in ltration as well other hematological and biochemical parameters was analyzed using Pearson's correlation coe cient. The results obtained were considered to be signi cant at a P value less than 0.05 (p<0.05).

Results
The average SUV max of ROI was between 1.34 and 7.76 (mean 2.72 ± 1.33) and the percentage of PC in the BM of the right anterior iliac crest was between 11.45 to 52 (mean 29.8 ± 9.7). The relationship between FDG uptake and PC in ltration in BM was positive (r=0.7, p<0.001) ( Figure 5). As a limitation of this retrospective study, hematological and biochemical parameter dates differed and, thus, FDG uptake and hematological and biochemical parameters did not correlate. However, we selected 21 cases in which PET/CT scans and parameter results occurred within one week and calculated the relationship between them. A positive correlation was observed between FDG uptake and b2m (r=0.596, p=0.004) (Figure 6).

Discussion
The present results revealed a correlation between FDG uptake by BM on PET/CT and PC proliferation in patients with MM.
MM is a PC proliferative disorder with high morbidity and mortality rates.
In comparisons among FDG PET/CT, 11 C-Methionine PET/CT, and 68 Ga-Pentixafor PET/CT, the latter two were found to be promising for the detection of intramedullary and extramedullary lesions in patients with MM [34][35][36][37]. Nanni et al. [22] compared whole-body X-rays (WBXR), magnetic resonance imaging (MRI), and FDG PET/CT in patients with MM. The ndings obtained showed that the sensitivity of FDG PET/CT for detecting small lytic bone lesions was higher than that of WBXR, but similar to that of MRI for the detection of bone diseases in the spine and pelvis. However, MRI may be superior for diagnosing in ltrative patterns in the spine [19,20]. Another study reported that FDG PET/CT was superior to technetium sestamibi myocardial perfusion imaging (technetium-99m-MIBI) and MRI for the detection of focal lesions [16].
Chanty et al. [38] reported that for the initial diagnosis of MM, FDG PET/CT was 40-60% more accurate at detecting solitary plasmacytoma than plain X-rays. Ben S et al. retrospective study found that PET/CT scan results affected the planned management of two-thirds of patients with PC disorders including MM [7]. In a study by Dimitrakopoulou-Strauss, PET/CT prior to and after the rst course of chemotherapy appeared to be useful for identifying patients who will respond to treatment [39], furthermore, studies have also shown that PET/CT scans are helpful in assessing responses to treatment [40] and allow for better management of patients with MM [41].
In the present study, a positive correlation was observed between PET/CT FDG uptake and the PC in ltration rate (r=0.7, p<0.001). As a limitation of this retrospective study, hematological and biochemical parameter dates differed and, thus, no correlation was observed between FDG uptake and hematological and biochemical parameters. However, we selected 21 cases in which PET/CT scans and parameter results occurred within one week and calculated the relationship between them. A positive correlation was observed between FDG uptake and b2m (r=0.596, p=0.004). Therefore, the time gap between PET/CT and hematological and biochemical parameter tests is crucial for assessing the signi cance of this relationship.
Signi cant prognostic laboratory parameters, such as b2m, CRP, and LDH, correlated with the number of focal FDG-avid lesions on PET/CT. Furthermore, the suppression of FDG prior to transplantation was previously identi ed as an independent favorable prognostic factor [46]. In newly diagnosed MM patients, the presence of at least 3 focal lesions and SUV>4.2 or extramedullary disease predicted poor progression-free survival [47]. PET/CT may be used to predict the outcomes of patients with new, relapsed, or refractory MM [44,[46][47][48][49][50][51] Based on these ndings, increased FDG uptake on PET scan indicates a higher percentage of PC in BM; therefore, BM biopsy may not be necessary in the follow-up period because SUV max values in the BM of MM patients with recurrence will be elevated.

Conclusion
An increased uptake of FDG on PET/CT correlated with the percentage of PC in BM. Therefore, the percentage of PC in BM and FDG PET/CT are crucial for the diagnosis, staging, treatment evaluation, and follow-up of MM patients.

Declarations
Ethics approval and consent to participate This retrospective study was approved by the Ethics Committee of Dokkyo Medical University, Tochigi, Japan.

Consent of publication
Not applicable.

Availability of data and material
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request

Competing interest
The authors declare that they have no competing interests.

Funding.
This work did not receive any nancial support.
Authors' contribution TG, YN, YK develop the concept and design the study. YN supervised the study. TG, KI, YM acquired data.
TG, YN, YK, HA analyzed and interpreted the data. KI advised on the plasma cell counting. DK, SH advised on statistical analysis. All authors read and approved the nal manuscript.