Myositis ossificans of the anterior chest wall

Clinical History

A 36-year-old active man presents with a tender lump in the left anterior chest wall. There was no history of trauma. Initial MRI of the anterior chest wall was ordered.

Imaging Findings

MRI of the chest wall was performed to investigate the mass (Fig. 1). The lesion showed heterogeneously increased T2 signal with a hypointense rim (Fig. 1a). The mass was slightly hyperintense to muscle on T1 sequences (Fig. 1b). The mass measured 3.2 (AP) x 1.9 (craniocaudal) x 3.7 cm (transverse). Surrounding muscle was oedematous, but there was no evidence of invasion into subcutaneous soft tissues or underlying costal cartilage (Fig. 1). Given the nonspecific MRI features, biopsy was ordered to rule out a primary soft tissue tumour.

Ultrasound-guided biopsy was conducted and pathology reported the pathological and radiologic findings were in keeping with myositis ossificans. Follow-up CT imaging was recommended.

Follow-up CT imaging demonstrated the lesion within the musculature anterior to the costal cartilage (Fig. 2). The lesion was calcified, smaller than previously measured on MRI, and skin tenting was reduced. These findings were consistent with a diagnosis of myositis ossificans.

Discussion

Posttraumatic myositis ossificans (PTMO) is a benign cause of a soft tissue mass which may spontaneously resolve over time. PTMO is the most common subtype of myositis ossificans (MO) and comprises up to 75% of cases [1]. It often occurs in the 2nd or 3rd decade of life with a slight predilection for males [1]. Vastus lateralis and brachialis are the most commonly affected muscles, however, other sites such as pectoralis, gluteal, and intercostal muscles can be affected [2]. Despite its nomenclature, a history of trauma is absent in up to 40% of cases [3].

PTMO often presents as a tender swelling with or without cutaneous erythema, but it can also be clinically asymptomatic. Given the nonspecific clinical picture, physicians rely on diagnostic imaging to delineate MO from infection or malignancy. In our case, MRI of the anterior chest wall was initially ordered to investigate this soft tissue mass.

MRI allows for visualisation of acute lesions prior to calcification. Lesions often appear T1-isointense with or without peripheral enhancement [4]. T2 scanning shows intermediate to high signal and may have a hypointense rim [5]. MRI findings can often be nonspecific, particularly in the early stages when traumatic oedema is present. This was the case with our patient, and the lesion was therefore biopsied. Pathology reports definitively ruled out a malignant lesion and suggested the possibility of MO. CT imaging was recommended for definitive diagnosis.

CT provides valuable additional information in these cases due to its sensitive and accurate depiction of calcium deposition. CT findings vary depending on the maturity of the MO lesion. In early weeks there is soft tissue swelling, but no calcification or ossification. After several weeks, a centrifugal zonal maturing pattern develops and the periphery calcifies. Although uncommon, it is possible for homogenous calcification to occur. This is the most frequent reason for decreased diagnostic confidence [6]. CT imaging will also show local tissue compression without invasion of nearby structures [4]. Finally, CT is able to evaluate periosteal reactions, monitor skin tenting, and track lesion maturation [6]. These factors help to exclude a malignant process and confirm the diagnosis.

Management of MO is usually conservative given that lesions often regress spontaneously. Therefore, recognition of characteristic imaging features is important to avoid unnecessary surgical intervention. This case demonstrates a less commonly encountered site for MO and outlines imaging characteristics to guide definitive diagnosis.

Differential Diagnosis List

Final Diagnosis

Myositis ossificans of the anterior chest wall

URL:	https://eurorad.org/case/10841
DOI:	10.1594/EURORAD/CASE.10841
ISSN:	1563-4086