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Myositis ossificans, a benign tumor composed of spindle cells and osteoblasts, can clinically and radiologically mimic osteosarcoma. While recognition and accurate diagnosis of myositis ossificans can be a challenge, this is critical as it may allow a conservative surgical approach to maximize functional outcomes. Herein, we present a patient with surface myositis ossificans confirmed genetically by the presence of COL1A1::USP6 gene fusion, along with a literature review. Due to the enhanced visualization of the bone matrix, computed tomography (CT) imaging may be a superior imaging modality to magnetic resonance (MR) imaging. Staged biopsies with samples obtained from the periphery and center of the lesions may allow pathologists to discern the zonal distribution histologically. Furthermore, immunohistochemistry fluorescence in situ hybridization and molecular testing can aid in the distinction of myositis ossificans from mimics. Because of their resemblance to other bone tumors, these cases of myositis ossificans highlight the importance of a multidisciplinary approach integrating clinical, radiologic, and pathologic analysis and involving serial imaging, sampling, and judicious use of ancillary immunohistochemical and molecular testing., (© 2024 Scandinavian Societies for Pathology, Medical Microbiology and Immunology.)

Myositis ossificans (MO) is a rare benign cause of heterotopic bone formation in soft tissue that most commonly affects young adults, typically following trauma. We report the case of an 11-year-old girl who developed MO mimicking osteosarcoma in her right shoulder. Plain radiography and computed tomography showed poorly defined flocculated densities in the soft tissue and a periosteal reaction along the proximal humerus. On magnetic resonance imaging, the mass displayed an ill-defined margin and inhomogeneous signal change. Histologically, the mass had a pseudosarcomatous appearance. Based on these findings, the patient was initially misdiagnosed with osteosarcoma at another hospital. The diagnosis was difficult because the patient was 11 years old and had no trauma history, with atypical radiographic changes and a predilection for the site of origin for osteosarcomas. We finally made the correct diagnosis of MO by carefully reviewing and reflecting on the pathological differences between stages., (© 2015 Japan Pediatric Society.)

Soft tissue lesions can contain bone or cartilage matrix as an incidental, often metaplastic, phenomenon or as a diagnostic feature. The latter category includes a diverse group ranging from self-limited proliferations to benign neoplasms to aggressive malignancies. Correlating imaging findings with pathology is mandatory to confirm that a tumor producing bone or cartilage, in fact, originates from soft tissue rather than from the skeleton. The distinction can have dramatic diagnostic and therapeutic implications. This content focuses on the gross, histologic, radiographic, and clinical features of bone or cartilage-producing soft tissue lesions. Recent discoveries regarding tumor-specific genetics are discussed., (Copyright © 2015 Elsevier Inc. All rights reserved.)

Myossitis ossificans (MO) is an aberrant reparative process that causes benign heterotopic ossification in soft tissue. We report a case of MO presenting as a large mass located at the dorsal aspect of the distal thigh, with no history of trauma, with radiological and clinical features mimicking parosteal sarcoma. An incisional biopsy was performed and the mass was excised. The histological features identified the lesion as MO. In half of the cases, these ossifications may adhere to the periosteum. In these cases, the lesion is known as parosteal MO, which may be confused with a parosteal osteosarcoma. This parosteal MO seldom becomes malignant. We emphasize the importance of a differential diagnosis of MO, since these lesions may simulate tumours and lead to misdiagnosis.

Reactive lesions of bone and soft tissue can appear alarming on histologic examination because they are often cellular and have atypical (activated) cytologic features, such as distinct nucleoli and mild hyperchromasia, and mitotic activity. Reactive lesions of bone and periosteum also produce bone and cartilage matrix, resulting in confusion with osteosarcoma or chondrosarcoma. Careful attention to key cytomorphological features such as the pattern of bone formation, uniform appearance of cells, and absence of atypical mitoses should help identify the reactive nature of a lesion. Correlation with clinical and radiological findings is also imperative to avoid misclassification of the tumor because reactive lesions often arise at sites where osteosarcoma and chondrosarcoma are rare (e.g., the hand) and lack aggressive radiological features. In this review we discuss reactive lesions of bone that are commonly confused with malignant neoplasms and that the practicing pathologist is likely to encounter at some point. Several of these lesions have had characteristic chromosomal translocations documented in recent years, but continue to be included as reactive lesions based on their overall clinicopathological features.

An ossifying soft-tissue lesion is potentially malignant. The possibility of malignancy increases in the absence of significant trauma. Pseudomalignant myositis ossificans can be easily confused with malignant lesions. A 22-year-old man presented with an ossifying mass in the adductor compartment of the thigh. The mass was excised and histopathologically reported as a soft-tissue osteosarcoma, but was later confirmed as pseudomalignant myositis ossificans by a pathologist specialising in oncology. Careful consideration and correlation of clinical, radiological, and histopathological findings is required to ensure the correct diagnosis is made and both initial and ongoing management is appropriate.

The aim of the study is presentation of problems in osteosarcoma diagnosis. In 3 cases possibilities of mistakes especially in differential diagnosis with myositis ossificans are presented.

A 53-year-old woman had extraskeletal osteosarcoma that developed from a soft tissue bony mass present on the volar aspect of the left wrist for 4 years. Initially, the bony mass was soft and movable, but during the first year it became hard and fixed. The patient had no history of trauma. Because the lesion did not grow or cause any symptoms, the patient did not come to the hospital until 4 years after she first noticed the lesion. Radiologically, the bony mass had features characteristic of mature myositis ossificans, showing "eggshell" ossification. A nonmineralized soft tissue mass occurred between the surface of the radius and the bony shell. Histologically, a high-grade osteosarcoma was present between the surface of the radius and the well-differentiated bone tissue, which included fatty and hematopoietic marrow. All the findings indicated that our patient had an extremely rare case of malignant transformation of myositis ossificans.

Background: Extraskeletal osteosarcomas are rare malignancies that account for about 1% of all soft tissue sarcomas. Few large series have been reported., Methods: Clinical records and histologic slides of all patients with extraskeletal osteosarcomas treated at the Mayo Clinic between 1915 and 1988 were reviewed., Results: The study group consisted of 40 patients, most of whom presented in the sixth and seventh decades of life (mean age, 50.7 years). There was a male predominance (male-to-female ratio, 1.9:1). The lower limbs most commonly were involved (68%), usually the thigh and buttock regions. Ninety-three percent of tumors presented as an enlarging soft tissue mass, with a history of trauma in nine patients. In nine patients, the lesions were initially interpreted histologically as benign, most commonly as myositis ossificans. Histologically, all were high grade osteosarcomas. Multiple local recurrences (45%) are a feature of this tumor. All recurrences occurred within 3 years. Distant metastasis (65%) is also common and is usually to the lungs (81%). Radical resections appear to be the best option for local control, with resection of the pulmonary metastasis occasionally producing a cure. By univariate analysis of Kaplan-Meier survival curves, the patients with predominantly chondroblastic tumors fared better than those with predominantly osteoblastic tumors (P = 0.03). Analysis of survival differences of the three main subtypes together (osteoblastic, chondroblastic, and fibroblastic) was not significant. A small-sized initial lesion did not equate with better survival. Seventy-three percent died of the disease, with a mean follow-up of 5.9 years., Conclusion: Extraskeletal osteosarcoma is a high grade malignant tumor associated with a 5-year survival rate of 37% (95% confidence interval, 28%-59%). Local recurrences and distant metastasis are common and usually occur by 3 years after excision. Patients with the chondroblastic subtype survive longer than those with the osteoblastic subtype.

This study is based on 41 cases of parostesl (or juxta cortical) osteogenic sarcoma, which the authors prefer to define as periosteal osteosarcoma. The clinical features, radiological and histological characteristics are analysed. Treatment and prognosis are discussed. The treatment usually indicated is amputation, but in selected cases radical segmental resection is appropriate. The prognosis is comparatively favourable if the correct surgical treatment is carried out without delay.

A case of retroperitoneal extra-osseous osteogenic sarcoma is reviewed, as well as the criteria necessary for the diagnosis. The general characteristics and the differential diagnosis of the tumor are also presented.

Juxtacortical osteosarcoma should be considered a very rate distinctive entity under all malignant bone tumors. The tumor has a remarkable tendency to grow from the periostal tissues peripherally with a usually marked degree of ossification without primary medullary involvement. It's different und characteristic behavior in clinical, roentgenographic and microscopic findings from that of other types of bone-forming sarkomas is discussed by means of a just treated case. In contrast to osteogenic osteosarcoma the prognosis for early well treated juxtacortical osteosarcoma is much better.