Proliferative myositis: expanding the differential diagnosis of a soft tissue mass in infancy

Abstract

Proliferative myositis, a pseudosarcomatous lesion of skeletal muscle, is quite rare in the pediatric population. While benign, it is not always recognized as such, and may be treated with an extensive resection that can result in permanent disfigurement. We report a case of an infant with the diagnosis of proliferative myositis, who to our knowledge is the youngest patient to be evaluated with magnetic resonance imaging (MRI). Although the MRI findings are non-specific, we highlight the importance of considering proliferative myositis in the differential diagnosis of a soft tissue mass, which ultimately might prevent an overly aggressive resection in a child.

Proliferative Myositis: A Comprehensive Review of 33 Case Reports

“…10,16 Other imaging modalities, preferably magnetic resonance imaging (MRI), may need to be performed. Although PM in this study appears as isointense comparing to the surrounding muscle on T1-weighted (T1w) image, 3,9,10,14,16,23,30 hypointense lesions have also been previously reported. 36,37 PM also appears as a hyperintense mass 9,10,12,14,16,28,29 with variable hypointense linear structures 12,16,25 on T2-weighted (T2w) image.…”

“…On gross inspection, a mass appeared white to grey. 3,20,21,27,29 Poorly-demarcated masses were more common than well-circumscribed ones. It typically replaced a variable proportion of a muscle resulting in induration of the involved muscle.…”

“…PM usually occurs in the middle-aged and senior adults, but very rare in childhood. 2,3 Only one affected infant was described during the review period. 3 The patient age ranged from 8 months to 78 years, with a median age of 54 years ( Figure 1A and Table 1).…”

“…38 Upon contrast injection, the lesion usually reveals homogenous enhancement 9,12 but heterogenous enhancement can also be seen. 3,4 Positron emission tomography (PET) was also performed in some previous studies and revealed PM as a "hot spot". However, PET is not recommended because of its inability to differentiate malignancy from inflammation.…”

“…2,6,15,19 These cells proliferate and spread skeletal muscle bundles apart, producing a checkerboard-like pattern. 2,3,7,11 Although surrounding skeletal muscle bundles are mostly left intact, some secondary regions of atrophy or focal necrosis may be noted. 2,11,14,23 The stroma varies from myxoid to collagenous.…”