The potential role of osteoporosis in unspecific [18F]PSMA-1007 bone uptake

Unspecific bone uptake is one of the main limitations of PET imaging with some PSMA-targeting radiopharmaceuticals, especially with [18F]PSMA-1007. We explored the potential association between osteoporosis and the occurrence of unspecific [18F]PSMA-1007 bone uptake investigating markers which might correlate with bone mineral density. We retrospectively analyzed treatment-naïve patients with a confirmed diagnosis of prostate adenocarcinoma who underwent staging [18F]PSMA-1007 positron emission tomography (PET). Qualitative image analysis was performed independently by three experienced nuclear medicine physicians. Patients were divided in two groups according to the presence/absence of unspecific bone uptake. Clinical information, blood count parameters (assessed within 3 months to the PET scan), body mass index (BMI), and bone density as estimated by computed tomography were collected. The Kruskal–Wallis and t-test were used to compare parameters. We analyzed 77 patients: 29 of them (38%) had unspecific bone uptake at [18F]PSMA-1007 PET, most commonly in the pelvic bones (69%) and ribs (62%). We did not find any significant difference in clinical parameters in the two groups. In patients with unspecific bone uptake, white blood cell, and neutrophil counts were significantly higher; in the same group, we observed lower values of BMI and bone density, although not statistically different. We observed unspecific bone uptake on [18F]PSMA-1007 PET in more than 1/3 of patients. In this exploratory analysis, we found a significant correlation between blood count parameters and unspecific [18F]PSMA-1007 bone uptake. We may speculate that [18F]PSMA-1007 unspecific bone uptake could be associated with osteoporosis. This hypothesis needs to be further investigated in larger populations and exploring more specific markers of osteoporosis.


Introduction
The advantages offered from PSMA-targeting radiopharmaceuticals over choline-based ones [1] together with the success of VISION and TheraP trials have definitely changed the management of prostate cancer.Although [ 18 F]PSMA-1007, [ 68 Ga]Ga-PSMA-11, and [ 18 F]DCF-PyL have shown a great overlap in the diagnostic setting, preventing the recommended use of one molecule in favor of others, [ 18 F]PSMA-1007 is burdened by a higher rates of uncertain findings [1][2][3][4][5][6][7][8].Specifically, unspecific bone uptake, reported in up to 72% of cases with [ 18 F]PSMA-1007 without any morphological CT-correlate [6], is one of the main limitations of PET imaging with this radiopharmaceutical and may lead to misdiagnosis, additional investigations, or delays in treatment.
Collectively, although the precise reason for this pitfall is still unknown, underlying benign bone and bone marrow disorders have been suggested as the most rational causes of unspecific bone PSMA uptake [3,9].Indeed, evidence does not convincingly support the hypothesis that [ 18 F] PSMA-1007 uptake in the skeleton is secondary to free fluorine-18 [8].Preclinical data in mice showed no significant differences in bone uptake between PSMA-1007 and PSMA-11 [10,11], and immunochemistry showed that normal bone marrow does not express PSMA [12].However, differences related to the chemical structure and pharmacokinetic properties of [ 18 F]PSMA-1007 compared to [68Ga]Ga-PSMA-11 have also been suggested as causes of the higher number of unspecific findings observed with [ 18 F]PSMA-1007 [13].
Biopsy of focal unspecific bone [ 18 F]PSMA-1007 uptake revealed Paget's disease, hyperplastic bone marrow [9], bland fibroblastic reaction, and 'woven bone,' as observed in fibrous dysplasia [3].These diagnoses might be associated with specific conditions (e.g., aerobic training, chronic anaemia, smoking, and obesity) and vary in incidence accordingly [14][15][16][17].However, these benign alterations are much rarer than unspecific findings observed on PSMA imaging; they are generally located in pelvis and legs and associated with morphological changes evident at radiological imaging or bone scintigraphy.
Therefore, unspecific bone PSMA uptake in the general population might be related to other, more prevalent conditions.Osteoporosis is the most common metabolic bone disease, with an overall prevalence in elderly men worldwide of 12.5% (95% confidence interval: 9.3-16.7%)[18].We aimed to explore the potential association between osteoporosis and the occurrence of unspecific bone [ 18 F]PSMA-1007 uptake investigating markers which might correlate with bone mineral density (i.e., blood count parameters [19], BMI [20], and bone Hounsfield units (HU) values on CT [21]).

Material and methods
We retrospectively analyzed treatment-naïve patients with a confirmed diagnosis of prostate adenocarcinoma who underwent [ 18 F]PSMA-1007 PET imaging for staging.All patients with confirmed bone metastases, synchronous malignancy, known bone disorders, and/or active inflammatory processes were excluded.The patient selection process is summarized in Fig. 1. [ 18 F]PSMA-1007 PET scans were acquired using the SIGNA PET/MR system, PET/CT Discovery-STE or Discovery-690 (GE Healthcare, Waukesha, WI, USA) according to the joint EANM and SNMMI procedure guidelines [22].Qualitative analysis of [ 18 F]PSMA-1007 PET/CT or PET/MR images was performed independently by three experienced nuclear medicine physicians, and non-specific bone lesions were defined according to specific criteria as described by Arnfield et al. [3].Selected patients were divided in two groups according to the presence/absence of unspecific bone uptake.For men with unspecific bone [ 18 F] PSMA-1007 uptake, anatomic distribution was recorded.Clinical information (age, Gleason score, and initial prostate specific antigen (iPSA)), blood count parameters assessed within three months to the PET scan, and BMI values were collected.Neutrophil-to-lymphocyte (NLR), platelet-tolymphocyte (PLR), and monocyte-to-lymphocyte (MLR) Fig. 1 Summary of the patient selection process ratios were calculated.In all patients who performed PET/ CT, bone HU values were measured as previously described [21].Briefly, we manually drew a 1-1.5 cm 3 spheric volumeof-interest (VOI) in the central trabecular portion of the L1 vertebral body, excluding the cortical bone.The correct positioning of the VOI was confirmed in the three projections (i.e., coronal, axial, and sagittal).In case of bone alterations in L1 (e.g., fracture, collapse, and hemangiomas), the VOI was drawn in L2.Regardless of the scanner employed (i.e., Discovery-STE or Discovery-690), CT images were performed with the following acquisition parameters: 120 kv, 30-400 smart mA, pitch 0.984:1 / 39.37, noise index 19.50, and rotation time 0.5 s.Descriptive statistics summarized baseline patients' characteristics and results.The median and mean value of blood count parameters, BMI, and bone HU values in the two groups were compared using the Kruskal-Wallis test and the t-test, respectively.p values < 0.05 were considered statistically significant.
The study was approved by the local Ethics Committee (approval number 81/INT/2022).

Results
A total of 77 patients with a median age of 67 years (range 51-87) who underwent [ 18 F]PSMA-1007 PET/CT (n = 66) or PET/MR (n = 11) between October 2021 and May 2023 were included.The most prevalent Gleason scores were 4 + 3 (29%), 3 + 4 (25%), and 4 + 5 (25%).Median PSA at imaging was 7 ng/mL (IQR 5.4-10).Patient characteristics are summarized in Tables 1 and 2. Forty-eight patients had no bone findings while 29/77 had unspecific bone uptake at [ 18 F] PSMA-1007 PET, most commonly in the pelvic bones (69%), ribs (62%), and vertebrae (17%).Figure 2 show two examples of patients without and with unspecific [ 18 F]PSMA-1007 bone uptake.No statistically significant difference in clinical parameters between the two groups was found.Median white blood cell and neutrophil counts were significantly higher in patients with unspecific bone uptake compared to those without (7100/mm 3 vs.6200/mm 3 , p = 0.0337 and 4600/mm 3 vs.3850/mm 3 , p = 0.0452).The same was observed comparing mean counts (7400/mm 3 vs.6400/mm 3 , p = 0.0135 and 4650/mm 3 vs.3900/mm 3 , p = 0.01).Median MLR was significantly lower in patients with unspecific bone uptake (0.27 vs. 0.35, p = 0.0369).Other parameters resulted similar in the two groups (Table 3).Eight patients had bone alterations  in L1 (four hemangiomas, one Schmorl's node, one enostosis, and two collapses); therefore, for them, the VOI was drawn in L2, instead of in L1 as for the other subjects; two of out these eight patients (one hemangioma and one collapse) belonged to the group with unspecific bone uptake.BMI and median bone HU values were lower in the group of patients with [ 18 F]PSMA-1007 unspecific bone uptake, although the difference did not reach statistical significance (24.8 vs. 26, p = 0.17 and 122 vs. 134, p = 0.2233, respectively).Figure 3 shows distribution of HU values according to age in the two groups.

Discussion
We found a correlation between blood count parameters and unspecific [ 18 F]PSMA-1007 bone uptake.Specifically, patients with unspecific [ 18 F]PSMA-1007-avid foci in the skeleton had significantly higher mean and median white blood cell and neutrophil counts compared to those without unspecific bone findings.These data are consistent with the literature.Recently, Li et al. [19] investigated a large cohort of osteoporotic patients and healthy  subjects (more than 1100 people in total) and reported higher white blood cell, neutrophil, and monocyte counts in cases than in controls.These results were confirmed even when the analysis was restricted to men (about 6% of this population).
In our cohort, monocyte counts were not significantly different between patients with and without unspecific [ 18 F] PSMA-1007 bone uptake.We also observed a significantly lower median MLR in patients with unspecific [ 18 F]PSMA-1007-avid lesions than in subjects without bone findings; however, data about MLR in osteoporosis are controversial [23,24].
Our preliminary results showed that patients with unspecific [ 18 F]PSMA-1007 bone uptake had a slightly lower BMI compared with patients with no bone alterations (24.8 vs. 26 kg/m 2 , p = 0.17).Although obesity is a major cause of several comorbidities, higher BMI levels are associated with increased bone turnover, probably due to increased mechanical stress on the skeletal system.This results in improved bone density and favorable microarchitecture on dual-energy X-ray absorptiometry and quantitative CT [20].Lee et al. identified a BMI range of 25.0 to 29.9 kg/m 2 as the lowest risk of osteoporosis in men in nationwide Korean population data, and an increase in BMI of 1 kg/m 2 reduces the risk of osteoporosis by 28% [25].
Median bone HU values in our cohort were lower in patients with unspecific [ 18 F]PSMA-1007-avid lesions than in those without bone findings (122 vs. 134, p = 0.2233), although the difference did not reach statistical significance.Vadera et al. [21] recently confirmed bone HU values as surrogate marker of osteoporosis in an adult British population (mean age of 65.8 years) mainly constituted by females (73%).L1 attenuation measurement differed among DEXAdefined normal bone density subjects, osteopenic and osteoporotic patients (178 HU vs. 143 HU, vs. 118 HU, respectively) and they identified a cut-off of 131 HU as the most balanced in terms of sensitivity and specificity (AUC = 0.74, sensitivity = 69%, specificity = 70%).Similar literature data which correlated bone mineral density measurements from CT-derived L1 trabecular attenuation values identified other thresholds [26,27].Values of L1 attenuation measures in our population were lower than those reported in literature, even when considering cohorts of men only (132 HU versus 157) [27].However, the method for HU assessment might be at least in part responsible for this difference, since region-of-interest manually and automatically drawn resulted in a certain intra-patient variability (average 21 HU higher in case of automatic assessment) [27].Additionally, the BLADE study evaluated body composition and bone mineral density in a cohort of prostate adenocarcinoma patients without bone metastases before and after gonadotropin releasing antagonist treatment, identifying a consistent relationship between these two parameters.Their cohort comprised a significant number of osteopenic and osteoporotic patients (respectively, 52% and 14% at baseline), and they found a local intra-patient heterogeneity of bone composition and, consequently, of bone mineral density in both axial and appendicular skeleton [28,29].Moreover, our data confirmed the age-related bone density loss measured by L1 trabecular attenuation at CT (Fig. 3).Jang et al. [27] demonstrated that age affects HU values with a fair constant and predictable loss averaging at 2.5 HU per year.Although our values were globally lower than those reported in literature, patients presenting unspecific [ 18 F] PSMA-1007 bone uptake were younger but with lower HU values than patients without bone findings at PET imaging.As mentioned, these differences were not statistically significant but might further support the hypothesis that loss in bone mineral density might have a role in unspecific [ 18 F] PSMA-1007 bone uptake.
Considering the significant findings on white blood cell and neutrophils and trends regarding HU values and BMI, we can speculate that osteoporosis might play a role in unspecific [ 18 F]PSMA-1007 bone uptake.The bone is a multi-functional organ that responds to a wide range of different mechanical, hormonal, and immune stimuli.Specifically, a close and complex interaction between the immune system and cells involved in bone remodeling has emerged, leading to the genesis of the field of osteoimmunology [30].Many factors, including immune cells, glutamate signaling, mesenchymal stromal cells, and endothelial cells, play a crucial role in bone homeostasis and thus in osteoporosis [19,[31][32][33].Unspecific [ 18 F]PSMA-1007 bone uptake might be the result of bone remodelling typical of osteoporosis.
The rate of unspecific [ 18 F]PSMA-1007 bone uptake observed in our cohort was in line with the literature [3,5] even if higher rate of unspecific bone findings have also been reported [6]; moreover, the pelvis and ribs, which are frequently involved in osteoporosis, resulted the most commonly involved sites (69% and 62%, respectively).Age and, in particular, the age-related decline in sex hormones (e.g., estrogen and testosterone), is a major cause of osteoporosis and fractures in men [34].Rib fractures are associated with the classic risk factors for osteoporosis [35,36] and are the most common incidental clinical fractures in men.Subjects with diminished bone mineral density suffered rib fractures and low-energy trauma more frequently than those with normal bone mineral density (74% and 36% vs. 51% and 15%) [34].Similarly, 64% of all pelvic fractures are osteoporotic and the incidence increases with age, rising to 94% in patients aged > 60 years [37,38].
Our exploratory analysis has several limitations: first, the small sample size; second, our cohort was retrospective; third, in absence of available data on clinical risk factors (e.g., alcohol units/day intake, parent fractured hip, and falls over last 12 months), bone mineral density, and of bone biomarkers for osteoporosis assessment [39,40], we were unable to establish a diagnosis of osteoporosis.Therefore, we explored factors (i.e., blood count parameters, BMI, and bone density as estimated on CT) which might correlate with reduced bone mineral density but the lack of data on pre-existing conditions and co-morbidities prevented the investigation of possible confounding factors and we cannot exclude other causes of raised white blood cell and neutrophil counts.Finally, neither pathology nor follow-up data were available to confirm the unspecific nature of [ 18 F]PSMA-1007 bone uptakes interpreted as such; even if rarely few of these findings might be metastases [41].
Results of our exploratory analyses regarding blood counts, BMI, and bone density as estimated on CT support the hypothesis that unspecific [ 18 F]PSMA-1007 bone uptake might be secondary to osteoporotic bone remodeling.However, lack of gold standard parameters for diagnosing osteoporosis (e.g., dual-energy X-ray absorptiometry score) as recommended by guidelines [40] prevents further speculations, and other conditions altering bone homeostasis cannot be excluded as a cause of this phenomenon.Moreover, bone metastases, albeit rare, cannot be a priori excluded and bone uptake mediated by non-PSMA-receptor-mediated mechanisms might be another option worth considering [7,41].Some features (e.g., site and number) have been suggested as predictors to correctly interpret bone PSMA uptake findings [3,9,42].A comprehensive evaluation of baseline patient's characteristics, clinical data, and imaging-including the CT or MRI component of hybrid images-site, number, pattern, and intensity of radiopharmaceutical uptake, is mandatory when reporting PET scans to correctly interpret findings.
With PSMA-targeting imaging on the rise and the evermore-wide use of fluorinated compounds for their many advantages, unravelling the biological phenomena behind unspecific bone uptake in [ 18 F]PSMA-1007 is a highly relevant unmet clinical need.

Fig. 2
Fig. 2 Two examples of patients with (A-E) and without (F) unspecific bone [ 18 F]PSMA-1007 uptake.Maximum intensity projection image (A) of a 65-year-old patients presenting with a prostate adenocarcinoma Gleason score 4 + 3 and iPSA = 5 ng/mL, showing intense bifocal uptake at the level of the prostate gland and multiple areas of bone uptake in the ribs and pelvic bones.Axial fused PET/CT images

Fig. 3
Fig. 3 HU distribution in different age groups, in patients without unspecific [ 18 F]PSMA-1007 bone uptake (full box plot) and in patients with unspecific [ 18 F]PSMA-1007 bone uptake (outlined box plot)

Table 1
Summary of patient characteristics

Table 2
Summary of patient characteristics for each group

Table 3
Comparison of median and mean blood count values between the two groups