Staging PET/CT (sPET)
Among 70 sPET results, the lymphoma clinical stage was concordantly evaluated in 69 cases (98.6%). In one patient (1.4%) with HL, the Q.Clear algorithm increased the stage from 1 to 2; the upgrade had no significant influence on the management. The precise distribution of stages is presented in Figure 1.
Interim PET/CT (iPET)
A total of 70 PET/CT scans were performed to evaluate the response to chemotherapy. As assessed by the DS score, the results were concordant in 59 cases (84.3%), i.e., the same DS was obtained with both reconstruction methods.
As presented in Table 2, the analysis of PET/CT images with Q.Clear and OSEM showed a discordance of the DS in 11 cases (15.7% cases), and the differences were statistically significant (p<0.001). In 3 patients (4.3%), Q.Clear reconstruction resulted in change in the DS from 3 to 4, which subsequently led to an upgrade to the positive PET group.
Table 2. Deauville scores obtained using Q.Clear and OSEM– interim PET
Deauville score
|
Q.Clear
|
1
|
2
|
3
|
4
|
5
|
OSEM
|
1
|
-
|
-
|
-
|
-
|
-
|
2
|
-
|
23
|
3
|
-
|
-
|
3
|
-
|
-
|
21
|
3
|
-
|
4
|
-
|
-
|
-
|
13
|
5
|
5
|
-
|
-
|
-
|
-
|
2
|
Despite conversion to the positive PET group by Q.Clear reconstruction, the treatment strategy in these patients with HL was continued as initially planned.
Each of these three patients underwent another PET/CT examination for the final evaluation of treatment response (ePET). In two of them, a complete metabolic response was confirmed since DS=2 was scored with the use of both reconstruction methods. In the third patient, ePET showed pathological right external iliac lymph nodes with increased 18F-FDG uptake in both reconstruction algorithms. Detection of the new lymph nodes was classified as progression of the disease, and the patient was qualified for another course of chemotherapy. Therefore, positive iPET with Q.Clear could have correctly converted one patient out of 70 to the worse prognosis group.
End of treatment PET/CT (ePET)
Among 70 ePET scans performed after completed treatment, concordant results with both algorithms were observed in 59 cases (84.3%). Discrepancies in the DS after using both reconstructive algorithms occurred in 11 cases (15.7%). The detailed DS scores obtained are presented in Table 3.
Table 3. Deauville scores obtained using Q.Clear and OSEM after completed treatment
Deauville score
|
Q.Clear
|
1
|
2
|
3
|
4
|
5
|
OSEM
|
1
|
-
|
-
|
-
|
-
|
-
|
2
|
-
|
27
|
3
|
-
|
-
|
3
|
-
|
-
|
25
|
6
|
1
|
4
|
-
|
-
|
-
|
1
|
1
|
5
|
-
|
-
|
-
|
-
|
6
|
The observed DS discordances between Q.Clear and OSEM were statistically significant (p<0.001). In 7 patients (10.0%), the use of Q.Clear caused conversion to the positive PET group. Two of these patients, who had been initially diagnosed with stage III lymphoma, were qualified to undergo selective radiation therapy due to positive PET results with remaining high activity in the axillary lymph nodes. In both cases, the follow-up PET/CT examination 3 months after radiotherapy did not show increased 18F-FDG uptake in these lymph nodes.
In another patient with elevated 18F-FDG uptake in unilateral inguinal lymph nodes (DS = 4 according to Q.Clear and DS=3 according to OSEM), the decision was made to perform a follow-up PET/CT instead of treatment escalation. The scan obtained 6 months later showed similar uptake in these nodes. The histopathological verification of the nodes confirmed benign inflammatory infiltration with no signs of lymphoma involvement.
In another 64-year-old patient with NHL, a round iliac lymph node with increased 18F-FDG uptake was detected on ePET. Using Q.Clear, the SUVmax was 3.0, which was higher than the liver SUVmax = 2.6. The scan was interpreted as positive (DS = 5 because of a new lesion that was negative on previous scans), and the patient was qualified to undergo the next treatment regimen, which led to metabolic and morphologic regression of the node. The positive reaction to treatment indirectly confirmed the involvement of the node. However, if OSEM was used, the scan would have been interpreted as negative since the SUVmax value of this node was lower than that of the liver (2.2 vs. 2.8, respectively), which would have led to a conclusion of a negative scan (DS = 3). Adequate images are presented in Figure 2.
After analysis of the retrospective results of all DS scores (i.e., both i-PET and e-PET), it was observed that PET performed with the Q.Clear reconstruction algorithm caused an increase in the DS in 22 cases (15.7%). Concordant results were observed in 118 cases (84.3%). The differences in the DS were statistically significant (p<0.001). In 10 patients (7.1%), the increase in the DS caused conversion to the positive PET group. The difference was also statistically significant (p=0.007), and in 4 patients, it had an effect on treatment strategy: 1 patient was referred for a new chemotherapy course; in the other 2 patients, selective radiotherapy was performed, and 1 patient had a biopsy of lymph nodes.
Detection of relapse (r-PET)
In the retrospective analysis of 70 rPET scans, all the results were concordant. Scans assessed with the Q.Clear as well as OSEM reconstructive algorithms showed a relapse in 13 cases (18.6%) and complete remission in 57 patients (81.4%).
Reference regions and target lesion
Additionally, the SUVmax values of reference regions (MBPS and liver) and of target lesions obtained with both reconstruction algorithms were compared at each stage of lymphoma management. In summary, the SUVmax of the MBPS, liver and target lesions of 280 PET/CT examinations were evaluated. Using the Q.Clear algorithm, the SUVmax values of the MBPS were higher in 90 cases (32.1%), equal in 75 (26.8%) and lower in 115 scans (41.1%) than those determined using OSEM. For the liver reference region, the SUVmax values measured with Q.Clear were higher in 75 cases (26.8%), equal in 63 (22.5%) and lower in 142 patients (50.7%). In cases of target lesions evaluated in 223 PET scans, the SUVmax measured with Q.Clear was higher in 198 patients (88.8%) than that determined with OSEM and equal in 25 (11.2%) patients; no cases of a lower SUVmax measured with Q.Clear were recorded.
We evaluated the percentage of small target lesions (defined as smaller than 25 mm) in the series of scans obtained at different stages of lymphoma management in our cohort. Number of small target lesions in the subgroups are: s-PET 6 out of 70 (8.6%), i-PET 48 out of 70 (68.6%), e-PET 47 out of 70 (67.1%) and r-PET 4 out of 13 (30.1%). The SUVmax values in each group are presented in Table 4.
Table 4. SUVmax values measured in smaller (<25 mm) and larger (≥25 mm) target lesions using both reconstruction algorithms
|
Target lesion
<25 mm
Q.Clear
|
Target lesion
<25 mm
OSEM
|
Target lesion
≥25 mm
Q.Clear
|
Target lesion
≥25 mm
OSEM
|
SUVmax ratio*
|
SUVmax
range
|
mean
SUVmax
|
SUVmax
range
|
mean
SUVmax
|
SUVmax
range
|
mean
SUVmax
|
SUVmax
range
|
mean
SUVmax
|
<25 mm
|
≥25 mm
|
s-PET
|
3.8-13.0
|
6.9
|
2.9-9.6
|
5.2
|
3.7-23.6
|
10.4
|
3.2-20.7
|
9.1
|
1.33
|
1.14
|
i-PET
|
1.2-13.2
|
3.2
|
1.1-10.5
|
2.5
|
1.4-18.8
|
2.9
|
1.1-16.9
|
2.6
|
1.28
|
1.11
|
e-PET
|
1.2-14.2
|
2.8
|
1.1-11.8
|
2.4
|
1.3-10.4
|
2.7
|
1.3-7.6
|
2.4
|
1.16
|
1.13
|
r-PET
|
4.5-6.6
|
5.2
|
3.1-4.8
|
3.9
|
4.2-13.8
|
8.6
|
3.8-10.9
|
7.6
|
1.33
|
1.13
|
*SUVmax ratio – the mean SUVmax measured using the Q.Clear divided by the mean SUVmax of the lesion measured using the OSEM reconstruction algorithm