Description of studies
A detailed PRISMA ﬂowchart of the study identification, screening, and exclusion process was shown in Fig. 1. The primary manual search yielded 426 potentially eligible literatures through searching of electronic databases and 1 record by manual search. After excluding duplicate studies (198 studies), 229 publications were kept for screening, of which 102 records were excluded according to the inclusion and exclusion criteria from database searching. Then, the remaining 127 articles were further assessed by abstract reviewing, and 67 studies were discarded either due to cell or animal studies data. Following careful review of titles and abstracts, 60 studies were considered in full-text articles and assessed for suitability. 20 studies were excluded for obvious irrelevance, 16 studies were precluded for other cancer studies, and 12 studies dismissed due to no related essay (Also see Additional file 1: Table S2). Finally, 12 studies were presented in this meta-analysis [43-53].
Characteristics of studies
The demographic information of all relevant studies was detailed in Table 1. According to this table, a total of 12 studies with 978 MM patients were included in this systematic review and meta-analysis, between 1999 and 2017. Most of the studies were conducted in people of the Asian race, tracked by 7 studies (58.4%) [47, 48, 50-54] and 4 studies (33.4%) in European countries [44, 46, 49, 55], one study in USA (8.2%) , and no study from African populations. Gender subgroups among 978 patients included 377 male and 307 female patients. The major clinicopathological features of the included studies are shown in Table 2. More than 80% of the MM patients were diagnosed by histopathological tests. PAS combined-staining with endothelial markers (CD31 or CD34) is a commonly used method for identification of tumor VM in paraffin-embedded tissue specimens (66.7%) in 8 studies [44, 48, 50-55] as well as PAS staining in 4 studies [43, 46, 47, 49]. Moreover, significant predictors of VM+ in both adjusted and unadjusted analyses were Clark level IV/V (84.4%). Finally, eleven studies reported the association between VM and clinicopathological parameters regarding OS [43-54], with the follow-up period ranged from 39-480 months.
All 12 papers were methodologically essayed according to NOS and QUADAS-2 quality evaluation standards of the Cochrane Reviewer handbook. Both systems’ tools focused on the study dependent on the methodology. Overall, the average NOS score was approximately 7.4 out of 12, which could be classified nearly in the high quality group. For each study, the NOS score is sorted in Table 1. Furthermore, QUADAS-2 results confirmed that significant bias was not detected in the present meta-analyses. Details of the quality evaluation of eligible studies according to the NOS score were summarized in the Additional file 1: Table S3. The reviewers' decisions about each risk of bias and applicability concerns graph were presented as percentages across selected studies. Figure S1 shows all parameters of QUADAS-2 assessment individually (Additional file 2: Figure S1). In this study, no significant bias and applicability concerns were found in all the selected studies.
Outcome of the meta-analysis
The relationship between VM+ and overall survival of MM patients was identified applying the pooled proportions test method. We used a random effect approach because the heterogeneity of the overall prognosis was relatively high, which is shown across the study (I2 = 79.8, p-value < 0.001). Based on heterogeneous cross of 12 studies, VM was associated with poor prognosis in 38% of MM group compared to the VM-group (P = 0.35, 95% confidence intervals (95% CIs): 0.27-0.42, p-value < 0.001). Therefore, these results suggested that VM+ indicated a poorer prognosis for MM patients (Fig. 2).
The effect of heterogeneity on the diagnostic threshold was evaluated based on the Spearman correlation coefficient. Fig. 3 presents the forest plots of pooled sensitivity and specificity, with their 95% CIs for individual studies. According to the results, the overall pooled sensitivity of VM+ tumor was 0.82 (95% CI: 0.79-0.84, Fig. 4a), while the specificity of VM+ tumor was 0.69 (95% CI: 0.66-0.71; Fig. 4b), among the 12 included studies. Furthermore, the overall pooled results for PLR, NLR, and DOR were 2.56 (95% CI: 1.94-3.93), 0.17 (95% CI: 0.07-0.42), and 17.75 (95% CI: 5.30-59.44), respectively.
Associations between VM+ and the possible demographic and clinicopathological features of MM patients are sorted in Table 3. According to the results, none of the above covariates contributed to the heterogeneity (all p-values > 0.05). Therefore, according to those covariates, the pooled sensitivity, specificity, PLR, NLR, DOR, and AUC were measured for significant sub-analysis parameters. We detected statistically significant relationships between VM and sample size, VM and race, as well as between VM expression and staining method (Fig. 5). As shown in Fig. 5a and Table 3, VM+ is a potentially accurate prognostic biomarker in CD31-/PAS+ (P = 0.24, 95% CI: 0.15-0.35) compared to CD34-/PAS+ (P = 0.39, 95% CI: 0.27–0.42) and PAS+ staining subgroups (P = 0.40, 95% CI: 0.30–0.52). As a result, the CD31-/PAS+ staining methods are relatively accurate diagnostic methods for detection of the VM, with 75% sensitivity and 70% specificity. The subgroups analysis was performed based on sample size (≤100 vs. >100; Fig. 5b). The proportion of population with a high sample size (3 studies with more than 100 MM cases) was 0.41 (95% CI: 0.28–0.56; p-value = 0.12); while that of a sample size with less than 100 MM patients (9 studies) was 0.31 (95% CI: 0.23-0.41; p-value < 0.001). Meanwhile, the highest specificity, NLR, and AUC in sample sizes less than 100 suggested that VM is more accurate in diagnosis of smaller sample sizes. Interestingly, our results show that the overexpression of the VM was a high risk prognosis factor in Asia populations (7 studies with 503 cases; P = 0.32; 95% CI: 0.23–0.42; p-value < 0.001; Fig. 5c). As seen in Table 3 and Fig. 5C, the pooled sensitivity and specificity were higher in the Asian patients compared to Caucasian patients (85% vs. 69% and 78% vs. 68%, respectively). Moreover, we could not find any significant correlation between the VM+ melanoma samples with gender, age, Clark level, and location of sampling (Data not shown).
Publication bias and sensitivity analysis
The publication bias and sensitivity were analyzed using Funnel plots and empirically utilizing regression tests according to Begg’s test. The analysis was conducted by excluding a single study at a time. A symmetric inverted funnel shape in this study implies a ‘well-behaved’ data set, in which publication bias is improbable. Following exclusion of the ten studies, there was no obvious statistical evidence for publication bias in our meta-analysis (t = 1.41; p-value = 0.19) (Fig. 6). Hence, the results of the current meta-analysis were credible and stable, due to no noticeable publication bias inﬂuencing overall results.