Objective This study aimed to explore the characteristics of ⁹⁹Tc^m-methylene diphosphonate (MDP) accumulated in extraosseous soft tissue tumors in children via bone scintigraphy and determine the correlation between tracer uptake and tumor histopathology.

Methods A total of 33 pediatric patients with ⁹⁹Tc^m-MDP uptake in extraosseous soft tissue tumors, who underwent bone scintigraphy in Xinhua Hospital Affiliated to Shanghai Jiaotong University School of Medicine and Tenth People's Hospital of Tongji University from December 2008 to December 2018, were retrospectively identified. The patients included 21 male children and 12 female children, ranging in age from 3 months to 10 years. On the basis of histopathology, the extraosseous soft tissue tumors in children were categorized into blastomas, mesenchymal tumors, and germ cell tumors. The highest degree of radioactivity accumulation in focal lesions was classified as “+,” “++,” and “+++” based on the results of bone scan. The features of ⁹⁹Tc^m-MDP uptake in different pathological types of tumors and the correlations between ⁹⁹Tc^m-MDP uptake in soft tissue tumors and tumor pathology were investigated. Rank sum test was performed.

Results Among 33 children, there were 26 cases of blastomas (78.8%, including 15 cases of neuroblastoma, 6 cases of Wilms' tumor, 4 cases of hepatoblastoma, and 1 case of pancreatoblastoma). Five cases were mesenchymal tumors (15.2%, including 3 cases of pulmonary metastases from osteosarcoma, 1 case of malignant extrarenal rhabdoid tumor, and 1 case of left thigh vascular tumor). Two cases were germ cell tumors (6.0%). No statistical significant difference was found in the degree of ⁹⁹Tc^m-MDP uptake in 33 pediatric patients with three different types of soft tissue tumors (H=1.993, P=0.369). The main imaging features of 33 cases of extraosseous tissue tumors included calcification, necrosis, and other features. Six cases of ⁹⁹Tc^m-MDP uptake were “+” (6/33, 18.2%), 11 cases were “++” (11/33, 33.3%), and 16 cases were “+++” (16/33, 48.5%). No significant difference in the degree of MDP uptake was found in 33 patients with different reasons causing ⁹⁹Tc^m-MDP accumulation (H=2.313, P=0.315). In terms of different pathological types of tumors, six cases of ⁹⁹Tc^m-MDP uptake were “+” (18.2%, including 5 cases of blastoma and 1 case of mesenchymal tumor). Eleven cases were “++” (33.3%, including 9 cases of blastoma and 2 cases of mesenchymal tumor). Sixteen cases were “+++” (48.5%, including 12 cases of blastoma, 2 cases of mesenchymal tumor, and 2 cases of germ cell tumor).

Conclusions During the bone scan, pediatric extraosseous soft tissue tumors with ⁹⁹Tc^m-MDP accumulation mostly indicate malignancy, and neuroblastoma was the most common type. No significant correlation was found in the degree of neoplastic ⁹⁹Tc^m-MDP uptake and the histopathological types in child.