[Malignant transformation of polyostotic fibrous dysplasia in long bone: a clinicopathological analysis of four cases].

<b>Objective:</b> To investigate the clinicopathological and genetic characteristics of malignant transformation of polyostotic fibrous dysplasia (FD) in long bone. <b>Methods:</b> A retrospective analysis of clinical characteristics and morphological features was conducted from 4 cases of malignant transformation of FD diagnosed at Beijing Jishuitan Hospital from January 2016 to December 2023. Hotspot mutations for GNAS gene were tested in 4 cases by Sanger sequencing, in which both FD and malignant tissues were detected in 3 cases respectively. <b>Results:</b> There were 2 female and 2 male patients, aged 46 to 53 years [mean (49±3.2) years], and the course of the disease spanned from 2 months to 36 years. The tumor involved the femur (<i>n</i>=2), tibia (<i>n</i>=1) and humerus (<i>n</i>=1). Three of them were diagnosed with FD before surgery. Single photon emission computed tomography showed multiple increases in bone metabolism, CT showed poorly margin, cortical destruction and soft tissue mass with uneven enhancement. Three cases had both FD and sarcoma components, while the remaining case exhibited exclusively sarcoma. The sarcomas displayed significant morphological variation, with 1 case diagnosed as osteosarcoma and 3 cases classified as low to high grade spindle cell sarcoma. Immunohistochemical results did not provide any indications for clear classification. Sanger sequencing demonstrated GNAS mutations of p.R201H (c.CGT>CAT, <i>n</i>=2) and p.R201C (c.CGT>TGT, <i>n</i>=2). All 4 cases were followed-up for 18 to 76 months, and received chemotherapy after surgery; 2 cases maintained disease-free, one case was diagnosed with invasive breast cancer through a core needle biopsy 3 months after chemotherapy, and another one was found to relapse 18 months after surgery. <b>Conclusions:</b> Some cases of polyostotic FD occur in association with café-au-lait macules and/or endocrine hyperfunctioning in McCune-Albright syndrome (MAS); polyostotic FD and MAS have more malignant potential than monostotic FD, but they are not the risk factors for FD malignancy. GNAS mutations may be involved in the occurrence and development of FD. The histologic types of malignant transformation of polyostotic FD in long bone are diverse, the sarcoma components of FD also present the GNAS mutation, suggesting potential involvement in the pathogenesis of FD malignancy.