The image data of 129 cases were retrospectively reviewed. Elevated uptake of 68Ga-FAPI in/around the bone or joint was found in 82 cases (63.6%) which were enrolled in this study. The patients (47 male and 35 female) had a mean age of 56.65 ± 10.15 years (range, 26–80 years). A total of 73 patients underwent PET/MR and 9 patients underwent PET/CT. Table 1 presents the general information of the enrolled patients.
Table 1
General information of the enrolled patients
Characteristics
|
No.
|
%
|
Gender
|
|
|
Male
|
47
|
57.3%
|
Female
|
35
|
42.7%
|
Age (y, mean ± SD, range)
|
56.65 ± 10.15 y, (26–80 y)
|
Reasons of PET imaging
|
|
|
Malignant tumor
|
70
|
85.4%
|
Fibrosis related diseases
|
12
|
14.6%
|
Scanner
|
|
|
PET/MR
|
73
|
89.0%
|
PET/CT
|
9
|
11.0%
|
In 82 cases, a total of 295 68Ga-FAPI-avid bone/joint lesions were identified, including 94 malignant bone lesions in 15 cases and 201 benign bone/joint lesions in 76 cases, of which 9 cases had both malignant and benign lesions (Fig. 1 and Fig. 2). All malignant lesions were diagnosed as bone metastases. The benign diagnoses were as follows: osteofibrous dysplasia (n = 13, 4.4%), degenerative bone disease (n = 48, 16.3%), periodontitis (n = 33, 11.2%), arthritis (n = 56, 19.0%) and other inflammatory or trauma-related lesions (n = 51, 17.3%).
As shown in Fig. 3, the mean SUVmax of bone metastases was significantly higher than that of benign diseases (7.14 ± 4.33 vs. 3.57 ± 1.60, p < 0.0001), but overlap occurred in some cases. Two representative cases (Fig. 1 and Fig. 2) had both bone metastases and benign lesions, which showed similar 68Ga-FAPI uptake. The differences in SUVmax among subgroups of benign disease were significant (p < 0.0001). The uptake of 68Ga-FAPI in periodontitis (4.45 ± 1.17) was much higher than that in other entities. However, paired comparisons of SUVmax in osteofibrous dysplasia (3.48 ± 1.56), degenerative bone disease (3.53 ± 1.68), arthritis (3.24 ± 1.48), and other inflammatory or trauma-related lesions (3.41 ± 1.76) showed no significant differences.
Table 2 lists the detected lesion numbers. Most cases (62/82, 75.6%) had 1–3 positive lesions in/around the bones and joints, and 34.2% (28/82) had only a single lesion. While 24.4% cases had > 3 lesions, with > 20 positive lesions presenting in two cases. Among the cases with metastases, 46.7% had 1–3 bone metastases, and the remaining 53.3% of cases had > 3 bone metastases. In cases of benign bone/joint diseases, 81.5% of cases had 1–3 lesions, and 19.5% had > 3 lesions.
Table 2
Distribution of number of lesions and percentage
No. of lesions
|
All lesions
|
Benign diseases
|
Metastases
|
Frequencies
|
Percentage
|
Frequencies
|
Percentage
|
Frequencies
|
Percentage
|
1
|
28
|
34.1
|
28
|
36.8
|
4
|
26.7
|
2
|
21
|
25.6
|
20
|
26.3
|
2
|
13.3
|
3
|
13
|
15.9
|
14
|
18.4
|
1
|
6.7
|
4
|
2
|
2.4
|
5
|
6.6
|
1
|
6.7
|
5
|
2
|
2.4
|
3
|
3.9
|
2
|
13.3
|
6
|
5
|
6.1
|
3
|
3.9
|
1
|
6.7
|
7
|
1
|
1.2
|
0
|
0
|
0
|
0
|
8
|
1
|
1.2
|
0
|
0
|
0
|
0
|
9
|
2
|
2.4
|
0
|
0
|
1
|
6.7
|
10
|
3
|
3.7
|
2
|
2.6
|
0
|
0
|
16
|
0
|
0
|
0
|
0
|
1
|
6.7
|
17
|
1
|
1.2
|
1
|
1.3
|
0
|
0
|
18
|
1
|
1.2
|
0
|
0
|
0
|
0
|
19
|
0
|
0
|
0
|
0
|
1
|
6.7
|
20
|
2
|
2.4
|
0
|
0
|
1
|
6.7
|
Total
|
82
|
100.0
|
76
|
100.0
|
15
|
100.0
|
Table 3 summarises the distributions of different types of diseases. Vertebral bodies and posterior elements were the most frequently (n = 82, 27.8%) involved sites. The shoulder joint (n = 45, 15.3%) was the second most common site, followed by the alveolar ridge (n = 33, 11.2%), the pelvis (n = 31, 10.5%), ribs (n = 27, 9.2%) and clavicle (n = 24, 8.1%).
Table 3
Locations of each type of diseases
Locations
|
No. of patients
(%)
|
No. of lesions (%)
|
Malignant, n(%)
|
Benign Lesions, n (%)
|
Metastases
|
Osteofibrous dysplasia
|
Degenerative bone disease
|
Periodontitis
|
Arthritis
|
Others*
|
Skull
|
5 (6.1)
|
5 (1.7)
|
3 (3.2)
|
1 (7.7)
|
|
|
|
1 (2.0)
|
Alveolar ridge
|
25 (30.5)
|
33 (11.2)
|
|
|
|
33 (100.0)
|
|
|
Sternal angle/ Sternocostal joint
|
3 (3.7)
|
6 (2.0)
|
|
|
|
|
|
6 (11.8)
|
Sternum
|
1 (1.2)
|
2 (0.7)
|
2 (2.1)
|
|
|
|
|
|
Spine
|
|
|
|
|
|
|
|
|
Vertebral body and posterior element
|
30 (36.6)
|
82 (27.8)
|
40 (42.6)
|
1 (7.7)
|
39 (81.3)
|
|
|
2 (3.9)
|
Intervertebral disc
|
5 (6.1)
|
6 (2.0)
|
|
|
4 (8.3)
|
|
|
2 (3.9)
|
Spinal facet joint
|
1 (1.2)
|
9 (3.1)
|
|
|
|
|
|
9 (17.6)
|
Ribs
|
12 (14.6)
|
27 (9.2)
|
20 (21.3)
|
4 (30.8)
|
|
|
|
3 (5.9)
|
Appendicular skeleton and joint
|
|
|
|
|
|
|
|
|
Clavicle
|
15 (18.3)
|
24 (8.1)
|
2 (2.1)
|
1 (7.7)
|
|
|
|
21 (41.2)
|
Scapula
|
1 (1.2)
|
1 (0.3)
|
1 (1.1)
|
|
|
|
|
|
Pelvis
|
19 (23.2)
|
31 (10.5)
|
24 (25.5)
|
2 (15.4)
|
4 (8.3)
|
|
|
1 (2.0)
|
Tendon attachment in ischial tuberosity
|
4 (4.9)
|
6 (2.0)
|
|
|
|
|
|
6 (11.8)
|
Humerus
|
2 (2.4)
|
2 (0.7)
|
|
1 (7.7)
|
1 (2.1)
|
|
|
|
Femur
|
5 (6.1)
|
5 (1.7)
|
2 (2.1)
|
3 (23.1)
|
|
|
|
|
Shoulder joint
|
32 (39.0)
|
45 (15.3)
|
|
|
|
|
45 (80.4)
|
|
Hip joint
|
8 (9.8)
|
11 (3.7)
|
|
|
|
|
11 (19.6)
|
|
Total No.
|
82
|
295
|
94 (31.9)
|
13 (4.4)
|
48 (16.3)
|
33 (11.2)
|
12 56(19.0)
|
51 (17.3)
|
* Others: inflammatory or trauma-related abnormalities |
Bone metastases were mainly located in the vertebral bodies and posterior elements (n = 40, 42.6%), pelvis (n = 24, 25.5%), ribs (n = 20, 21.3%), with the rest distributed among the skull (n = 3, 3.2%), sternum (n = 2, 2.1%), clavicle (n = 2, 2.1%), scapula (n = 1, 1.1%), and femur (n = 2, 2.1%). Figure 1 and Fig. 2 show two typical cases with multiple bone metastases (red arrows).
For the subgroup of benign diseases, osteofibrous dysplasia occurred at many sites, including the skull, clavicle, ribs, vertebral bodies, pelvis, and long bones, featuring ground-glass opacities with well-defined borders and intact overlying bone on CT images, and these lesions had not changed significantly for years (Fig. 1d and Fig. 4). Elevated uptake in the alveolar ridges was diagnosed as periodontitis (Fig. 5). The majority of degenerative bone diseases occurred in the spine, usually at the edge of the vertebral body, including osteodysgenesis, osteophyte and schmorl’s node (n = 39, 81.3%, Fig. 2d; Fig. 6d) and intervertebral discs (n = 4, 8.3%). The most representative arthritis occurred in the shoulder (n = 45, 80.4%, Fig. 1b; Fig. 6b) and hip (n = 11, 19.6%, Fig. 6e). Other inflammatory or trauma-related abnormalities mainly appeared at the acromial or sternal end of clavicle (n = 21, 41.2%), spinal facet joints (n = 9, 17.6%, Fig. 6c), sternal angle / sternocostal joint (n = 6, 11.8%), and tendon attachment in ischial tuberosities (n = 6, 11.8%, Fig. 1c). Figure 6 presents one representative case with 17 foci in bones and joints, all diagnosed as benign entities. One case showed healed fractures in three consecutive ribs (Fig. 7).
In areas with coexisting malignant and benign lesions, the proportion of the diseases was different. In the pelvis and ribs, malignant foci predominated (25/31, 20/27). In vertebral bodies and posterior elements (malignant n = 40; benign n = 42), skull (malignant n = 3; benign n = 2) and femur (malignant n = 2; benign n = 3), the ratios were closer to 1, which suggests that more attention should be paid to distinguishing between benign and malignant in these sites.