Malignant Solitary Fibrous Tumor of the Humerus: A Case Report of an Extremely Rare Primary Bone Tumor

Primary intraosseous solitary fibrous tumor: an extremely rare case report and brief review of the literature

Solitary fibrous tumor (SFT), a rare mesenchymal neoplasm of fibroblastic origin, was initially discovered in the mediastinal pleura and then described in many extra-pleural sites. The reports of primary solitary fibrous tumor of bone are extremely rare and only a few cases have been previously mentioned in the literature, most of which in flat and short bones.

Here we present the case of a 53-year-old female, who was referred to the emergency department of a peripheral hospital after an accidental fall. Imaging studies revealed an intertrochanteric fracture with an underlying intramedullary lytic lesion. A biopsy was performed and a diagnosis of Ewing sarcoma was initially suggested. She arrived at our hospital where we reevaluated the case. The biopsy was reviewed and a diagnosis of intraosseous SFT was proposed. She underwent en-block resection of the proximal right femur. Primary SFTs of the bone are, like in our case, easily misdiagnosed due to the low specificity of the imaging studies and the extreme rarity of the localization. An accurate diagnosis and early resection are very important and with careful long-term follow-up is essential, particularly in those who with malignant behavior, for the early detection of possible recurrence or metastasis.

Solitary fibrous tumors: Clinical and imaging features from head to toe

Solitary fibrous tumors (SFTs) are rare fibroblastic mesenchymal tumors that are usually benign with variable malignant potential. They can develop in any organ due to their spindle cell origin. The exact etiology of solitary fibrous tumors is unknown. The majority of SFTs are benign with 10-30% of them exhibiting aggressive and malignant features. The aggressiveness of this type of tumor is not associated with its histological features, which makes surgical resection the treatment of choice. We will review the clinical and radiological features and possible differential diagnoses of SFTs according to their anatomical sites following the World Health Organization 2020 classification.

Clinical, Histological, and Molecular Features of Solitary Fibrous Tumor of Bone: A Single Institution Retrospective Review

Simple Summary

Solitary fibrous tumors arising from the bone are an extremely rare event and only few cases have been previously described in the literature. It is characterized by a prominent, branched vascularization, with a thin and dilated vascular texture defined as “staghorn” and by the presence of the NAB2-STAT6 gene rearrangement, present in about 90% of cases and considered a pathognomonic feature. In the present study, we described our series of 24 cases of primary solitary fibrous tumor of the bone to find any clinical and molecular prognostic factors and to compare them with those currently used for soft tissue solitary fibrous tumor and to evaluate the risk stratification system proposed by Demicco, in order to understand whether this system was able to correctly predict the risk of local and distant metastatic relapse even in the case of solitary fibrous tumor of the bone.

Abstract

Primary solitary fibrous tumor (SFT) of the bone is extremely rare, with only few cases reported in the literature. We retrieved all cases of primary SFT of the bone treated at our institution and we assessed the morphology and the immunohistochemical and molecular features to investigate the clinical outcome of primary SFT of the bone and any clinical relevance of clinical and histological criteria of aggressiveness currently adopted for the soft tissues counterpart. Morphologically, 15 cases evidenced high cellularity, cytologic atypia, and foci of necrosis and were associated with more than 4 mitotic figures/10 HPF. Immunohistochemical analysis showed an expression of CD34 and of STAT6 immunopositivity in 95% and in 100% of cases, respectively. The presence of NAB2-STAT6 chimeric transcripts was found in 10 out of 12 cases in which RT-PCR analysis was feasible, whereas TERT promoter mutations analysis was feasible in 16 cases and only a C-to-T substitution in a heterozygous state was found in one DNA sample for the C228T genetic variant. P53 variants were assessed in 12 cases: 11 (91.6%) cases showed a variation, while in one case, no alteration was found. Disease-specific survival was 64% at 5 years and 49% at 10 years. Statistical analysis showed no correlation between survival and all the clinicopathological and molecular parameters evaluated. In conclusion, at difference to SFT of soft tissues, aggressive behavior of primary SFT of the bone seems to be independent from mitotic count or any other clinicopathological and molecular features.

Solitary fibrous tumor of bone developing lung metastases on long-term follow-up

Solitary fibrous tumors are rare mesenchymal neoplasms of fibroblastic or myofibroblastic origin. Primary solitary fibrous tumors arising in bone are extremely rare and rarely metastasize. We present a case of solitary fibrous tumor where the diagnosis was delayed due to a failure to recognize the subtle, lytic lesion underlying a fracture of the left humerus. The patient underwent proximal humeral replacement and was followed closely with imaging of humerus and chest. A small lung metastasis was found on CT scan 38 months later and was resected. Two additional small metastases were found and resected 62 months after initial tumor resection. The purpose of this case report is both to highlight the radiologic challenges which can lead to overlooking a lytic lesion underlying a fracture and to show the importance of long-term follow-up in patients with solitary fibrous tumor.

Rare Primary Malignant Bone Sarcomas

Simple Summary

Primary malignant bone tumors are infrequent cancers. More than 90% of these neoplasms are classified as osteosarcomas, Ewing sarcomas or chondrosarcomas, and their clinical presentation, diagnosis, and treatment principles are well-established. The entities described in this article, are ultra-rare varieties of bone sarcomas, and there clinical and histological characteristics are not well known. Therefore, they are very difficult to be diagnosed and there is a lot of uncertainty on their treatment. Because of their rarity, it is also extremely difficult to perform clinical research on these cancers. This article creates more awareness of these very rare bone tumors. It explains how to recognize and diagnose each entity and it summarizes the medical scientific literature that is available on these cancers. Increasing awareness and clinical research for these cancers are key elements to improve the prognosis for patients with these diseases in the long term.

Abstract

Rare primary malignant bone sarcomas (RPMBS), other than osteosarcoma, chondrosarcoma, chordoma, and Ewing sarcoma, account for about 5–10% of primary bone tumors and represent a major diagnostic challenge. These tumors include spindle cell and round cell sarcoma entities, hemangiopericytoma-like and vascular tumors. Additionally, several histotypes, traditionally described in the soft tissues, such as myxofibrosarcoma, synovial sarcoma, and malignant peripheral nerve sheath tumor of bone, have been reported in patients with primary bone tumors. While wide surgical resection is the mainstay of local treatment, systemic therapy of these rare entities is controversial. Patients with undifferentiated spindle cell or pleomorphic high-grade tumors of bone, are usually treated with osteosarcoma-like chemotherapy, while patients with round cell and undifferentiated round cell tumors (URCTs), may respond to sarcoma treatment regimens for Ewing sarcoma patients. Studies on analogies and differences among these ultra-rare tumors have seldom been reported. This review describes relevance, clinical aspects, diagnostic procedures, staging, treatment recommendations, and current research in this composite tumor group.