Objective  To elucidate the biological behavior of malignant transformation (MT) of fibrous dysplasia (FD) and investigate its risk factors for diagnostic identification.

  Methods  A retrospective analysis of 394 FD cases from March 2006 to March 2017 in Beijing Ji Shui Tan Hospital was performed. Seven cases had been histopathologically confirmed as malignant (study group). According to age, location, and other epidemiological data, we performed a 1:2 case-matched comparison between the patients with malignant FD and 14 patients with benign disease (control group). Clinical features, visual analog scale (VAS) score, tumor volume, imaging characteristics, alkaline phosphatase (AKP) levels, lactate dehydrogenase (LDH) levels, oncologic results, and function data were analyzed.

  Results  The 7 MT cases included 3 males and 4 females. Mean follow-up time was 175 (3-396) months, and mean and median follow-up time were 25 (3-51) months after MT. Mean and median age were 45.6 and 47 (24-60) years, respectively. Among the 7 cases, 6 involved the femur and 1 involved the tibia. Two cases involved a single lesion whereas the remaining 5 involved multiple lesions. There were 5 recurrent cases and 2 initial cases. The mean MT period from initial surgery were 207 (37-377) months. VAS scores in the study group were significantly higher than that in the control group (Z=-3.317, P=0.001); the VAS scores decreased significantly after operation (Z=-2.384, P=0.017). Preoperative AKP levels were different between the study and control group; the levels were significantly higher in the former group than in the latter (Z=2.314, P=0.021). However, postoperative AKP levels were similar in both groups (Z= 0.821, P=0.821). LDH levels were not significantly different between the two groups, either preoperatively (Z=1.269, P=0.205) or postoperatively (Z=0.075, P=0.940). As for the study group, AKP levels decreased significantly after surgery (Z=-2.366, P=0.018); LDH levels were also lower after surgery than before (Z=-2.028, P=0.043). CT enhancement values were higher in the study group than in the control group (Z=-3.659, P < 0.001). Univariate analysis indicated that preoperative VAS score, AKP level, cortical damage, presence of soft tissue mass, and CT enhancement value were clinical risk factors for determination of MT of FD. Histopathological analysis revealed 4 cases of osteosarcoma, 2 of low-grade spindle cell sarcoma, and 1 of undifferentiated pleomorphic sarcoma. Three patients had received adjuvant chemotherapy, 2 cases involved pulmonary metastasis, and 1 patient had died. The mean Musculoskeletal Tumor Society (MSTS) scores for the study and control groups were (95.0±3.9)% and (86.0±10.9)%, respectively (F=5.689, P=0.029).

  Conclusions  Malignant transformation of fibrous dysplasia is rare. The preoperative VAS score, AKP level, cortical damage, presence of soft tissue mass, and CT enhancement value may be helpful for clinical screening of malignant transformation. An adequate surgical margin is required for treatment of this lesion.