A wide spectrum of entities may give rise to musculoskeletal soft tissue masses in pediatrics, including neoplastic and non-neoplastic masses. MRI plays a significant role in characterization of soft tissue tumors, yet conventional techniques lack specificity for proper differentiation between benign and malignant masses . DWI is a noninvasive method for investigation of tumor histology and used effectively for differentiation between benign and malignant masses .
In this study, there was a statistically significant difference between the mean ADC value of benign and malignant masses (P = 0.001*), and the mean ADC value for benign masses was 1.495 ± 0.55 SD × 10−3 mm2/s versus (0.449 ± 0.27 SD) × 10−3 mm2/s for the malignant masses. This is in agreement with previous studies, and one study stated that the mean ADC value of benign masses was 2.31 ± 1.29 × 10−3 mm2/s versus 0.90 ± 0.70 × 10–3 mm2/s for malignant masses (P < 0.001) . Other studies reported that there was a statistically significant difference between the mean ADC value of benign and malignant soft tissue tumors (P < 0.05) [5, 14].
The reported significant difference in our study also matched with that in the study of Zou et al.  that showed a significant decrease in the mean ADC value of patients with malignant soft tissue tumors in comparison with the ADC values obtained in patients with benign soft tissue tumors (P < 0.001). Their reported mean ADC value of benign soft tissue tumors was 1.73 × 10–3 mm2/s, while the mean ADC value of malignant soft tissue tumors was 0.8 × 10–3 mm2/s. Also our results matched with the previous study where the reported mean ADC value for malignant soft tissue tumors was 0.90 ± 0.32 × 10–3 mm2/s with a statistically significant difference between benign and malignant masses (P < 0.001) . The same significant difference between benign and malignant soft tissue tumors was also recorded in the previous study (P < 0.0001) . Other study showed that malignant rhabdomyosarcoma had a characteristically low ADC value of 0.71 ± 0.15 × 10−3 mm2/s with a statistically significant difference between benign and malignant masses (P < 0.05) . Another study stated that there was a statistically significant difference between the mean ADC value of benign and malignant masses (P < 0.001) (mean ± SD, 1.43 ± 0.56 × 10−3 mm2/s for benign masses versus 0.74 ± 0.18 × 10−3 mm2/s for malignant masses) .
Among benign masses, as they are composed primarily of free water, we found that the highest mean ADC value recorded was that of Baker’s cyst cases (2.6440 ± 0.109 SD) × 10−3 mm2/s. This is in agreement with results of other study  in which the mean ADC value of benign cystic masses was the highest among benign cases (2.61 ± 0.35) × 10−3 mm2/s. Also our results are in agreement with Khedr et al.'s  where the highest ADC value among benign cases was recorded in cases of ganglion cyst (2.8 ± 0.23) × 10−3 mm2/s and cystic neurofibroma (2.5 ± 0.04) × 10−3 mm2/s. It was reported that in cases in which a cyst is suspected, ADC mapping could prove a useful complement to anatomic imaging. It has been found that the use of a mean ADC value greater than (2.5 × 10−3 mm2/s) yielded a sensitivity of 80% and a specificity of 100% in the diagnosis of benign cystic lesions, indicating that no soft tissue neoplasms are missed with DWI and ADC mapping .
Many previous studies reported that not all benign soft tissue tumors have a large extracellular space and not all malignant soft tissue tumors are more cellular than benign tumors . In our study, there was an overlap in the recorded mean ADC values between some benign and malignant masses. The pediatric patients with benign masses demonstrating restricted diffusion like malignancy were patients with abscess (mean ADC value = 0.6810 ± 0.11314 × 10–3 mm2/s) and patients with PVNS (mean ADC value = 1.0580 × 10–3 mm2/s). Our results matched with the previous study where the mean ADC value for soft tissue abscess was 0.877 × 10–3 mm2/s. This could be explained by the high viscosity pus within abscesses that contains inflammatory cells, cellular debris, bacteria and proteins slowing water diffusion .
In addition, several theories are plausible for low ADC values in PVNS. The presence of hemosiderin and other blood products in these masses likely affects the ADC measurements. Even if great care was taken to avoid sampling of clearly hemorrhagic components, it is possible that microscopic areas of hemorrhagic products were present also in the sampled pixels and affected ADC values . Other potential reasons for low ADC measurements in PVNS include the nature of intralesional matrix or presence of hypercellular components, nodulated and villose proliferation of the synovium with thickened synovial rinds of hemosiderin-laden tissue [9, 22].
A large number of studies have yielded conflicting results and have found different cutoff ADC values between benign and malignant masses although the most discriminating ADC value appears to be near (1 × 10–3 mm2/s). The reason of this conflicting result in previous studies can be explained by the variations in histopathological types of neoplasms, differences in DWI techniques used (equipment, b values and DWI sequences). However, all previous studies confirmed the fact that aggressive tumor usually records a very low ADC .
Regarding the ROC analysis in this study, the cutoff point for the mean ADC value was 0.88 × 10–3 mm2/s with sensitivity of 100.0%, specificity of 92.3% and diagnostic accuracy of 93.3%. This recorded cutoff point was lower than that of previous study  in which the cutoff value was 1.235 × 10–3 mm2/s; however, our study recorded higher sensitivity, specificity and accuracy in comparison with their results that were 73% sensitivity, 91.7% specificity and 80.3% overall accuracy. Other study reported that malignant tumors tend to exhibit a lower mean ADC value than that of benign soft tissue tumors and proposed using a threshold mean ADC value of 1.34 × 10−3 mm2/s . The ROC analysis in another study  yielded nearly the same area under curve AUC that was 0.89 × 10−3mm2/s and a fairly close cutoff value for mean ADC which was ≤ 1.03 × 10–3 mm2/s, but with relatively lower sensitivity (90%) and specificity (91%) than those of our study.
The limitations of this study were that the number of pediatric patients especially those with malignant tumors was limited in this study. We did not perform a histogram analysis or calculate perfusion effects. At last, we did not obtain inter-observer variations of measurement of ADC values.