Characteristics of patients
The characteristics of 71 patients with secondary HLH are shown in Table 1. Their median age was 38 years (range, 14-79 years) and 37 (52.1%) were male. Among them, 22 (31.0%) were diagnosed as M-HLH. Eleven of M-HLH were detected with B cell neoplasms including 6 diffuse large B-cell lymphoma, 2 classical Hodgkin lymphoma, 1 follicular lymphoma, 1 non-Hodgkin B cell lymphoma and 1 B-cell acute lymphocytic leukemia and 4 of them had EBV infection simultaneously. The rest of M-HLH were identified with T/NK cell neoplasms including 6 NK/T-cell lymphoma, 2 peripheral T-cell lymphoma, 2 unspecified T-cell lymphoma and 1 anaplastic large cell lymphoma and 7 of them had EBV infection concurrently. In 49 (69.0%) of NM-HLH, there were 24 EBV infection, 12 other infections, 5 adult Still’s disease, 1 systemic lupus erythematosus, 1 undifferentiated systemic rheumatic disease and 6 with unspecific underlying causes.
Comparison of baseline 18F-FDG PET/CT findings and laboratory data between M-HLH and NM-HLH
The comparisons of 18F-FDG PET/CT and laboratory features were listed in Table 2 and 3. Regarding gender and age, it seemed that M-HLH occurred more commonly in males and older adults. The percentage of males and the median age in the M-HLH type were larger than those in the NM-HLH type [72.7% vs. 42.9%, p=0.020; 49 (32-58) vs. 33 (25-52), p=0.041]. Most of PET/CT variables had a significant difference between the two groups. For instance, the asymmetrical distribution of hypermetabolic lymph nodes (p=0.002), bone lesion (p<0.001) and focal liver lesion (p=0.015) were more frequently presented in the M-HLH group. Moreover, the patients with M-HLH tended to have a greater value in the SUVmax of lymph nodes and the SUVmax, volume, TLG of spleen and liver compared with the NM-HLH (p<0.05). Similarly, the SUVmax of bone lesions in the M-HLH was significantly higher than in the NM-HLH at a level of p<0.1. There were no differences between the groups in terms of hypermetabolic lymph nodes, the long and short diameter of the lymph node with the highest FDG uptake and the SUVmax of bone marrow background. As for the laboratory parameters in the cases with M-HLH, white blood cell, absolute neutrophil and platelet counts were lower, whilst the levels of C-reactive protein, lactate dehydrogenase, soluble CD25, beta-2-microglobulin, IL-8 and IL-10 were higher in comparison with the NM-HLH category (p<0.05).
Diagnostic performance of visual analysis of 18F-FDG PET/CT images
Patients were considered “positive” if 18F-FDG PET/CT images showed hypermetabolic lymph nodes and/or focal increased FDG uptake in bone, liver and/or spleen. This approach was applied to diagnose M-HLH with great sensitivity (90.9%, 20/22) but poor specificity (36.7%, 18/49) and its accuracy was only 53.5%. The false-negative PET/CT images of two M-HLH were shown in Fig. 2a and b. It was noteworthy that multifocal of increased FDG uptake in bone marrow was usual in patients with EBV infection (positive predictive value (PPV) for malignancy: 53.3%, 8/15) (Fig. 2c and d), but was a malignant sign in cases without EBV infection (PPV: 87.5%, 7/8). Only one patient without malignancy and EBV infection had focal hypermetabolic bone lesions (Figure 2e). She had cytomegalovirus (CMV) infection, and the SUVmax of bone lesions was lowest at 3.9.
In this study, focal spleen lesions were observed in 13 patients and the potential causes were malignancy (7/13), EBV infection (5/13) and unknown (1/13). Notably, there were 6 cases with both malignancy and EBV (2 classical Hodgkin lymphoma, 2 NK/T-cell lymphoma, 1 diffuse large B-cell lymphoma and 1 follicular lymphoma) and 1 case with malignancy alone (peripheral T-cell lymphoma). Focal liver lesions were discovered in 6 patients and the possible causes were malignancy (5/6) and EBV infection (1/6). There were 2 cases with both malignancy and EBV (1 classical Hodgkin lymphoma and 1 NK/T-cell lymphoma) and 3 cases with malignancy alone (2 diffuse large B-cell lymphoma and 1 peripheral T-cell lymphoma).
Multivariate logistic regression models
18F-FDG PET/CT and laboratory parameters were analyzed separately and conjointly by multivariate logistic regression. Lastly, two PET/CT (SUVmax of hypermetabolic lymph nodes and SUVmax of bone lesions) and three laboratory variables (lactate dehydrogenase, C-reactive protein and sCD25) were selected to build the predictive models. The Odd ratios and p values of these variables were shown in Table 4. Two best logistic models were picked up to construct nomograms. One nomogram was only based on two PET/CT parameters (SUVmax of hypermetabolic lymph nodes and SUVmax of bone lesions) and the other included both types of features (SUVmax of hypermetabolic lymph nodes, SUVmax of bone lesions and C-reactive protein) (Fig. 3).
The diagnostic performance of each parameter was shown in Supplemental Table 1 (p<0.05). Although none of these parameters was individually suited to predict malignancy, there were interesting data shown in Table 5. The multivariate logistic model constructed by SUVmax-bone lesions and SUVmax-lymph nodes displayed acceptable diagnostic accuracy with an area under the ROC curve (AUC) of 0.867 (0.766-0.936) (Fig. 4a and c). At a cut-off of 27.0%, its sensitivity and specificity were 90.9% (70.8-98.9) and 81.6% (68.0-91.2), whilst the positive and negative predictive value were 69.0% (54.8-80.3) and 95.2% (84.1-98.7), respectively. Remarkably, the multivariate logistic model built by SUVmax-bone lesions, SUVmax-lymph nodes and C-reactive protein had the largest AUC (0.884, 95%CI: 0.791-0.978) (Fig. 4b and c). At a cut-off of 32.4%, its sensitivity and specificity were 86.4 (95%CI: 65.1-97.1) and 87.2% (74.3-95.2), while the positive and negative predictive value were 76.0% (59.6-87.2) and 93.2% (82.6-97.5) (Fig. 4b and c). The decision-curve analysis showed that those two models had greater clinical utility for prediction of malignancy than models established by laboratory variables alone (Fig. 4d).
Decision tree for diagnosing M-HLH
The decision tree showed that the model constructed by two PET/CT features alone (SUVmax-bone lesions and SUVmax-lymph nodes) had a predictive ability to identify malignancy, with a sensitivity of 86.4% (19/22), a specificity of 87.8% (43/49) and an accuracy of 87.3% (62/71) (Fig. 5 and Table 5). Eighty percent (12/15) of the patients with SUVmax-bone lesions≥7.0 and 70% (7/10) of the patients with SUVmax-bone lesions<7.0 and SUVmax-lymph nodes≥12.7 were M-HLH, while those with SUVmax-bone lesions<7.0 and SUVmax-lymph nodes<12.7 were almost NM-HLH (43/46, 93.5%).