Introduction

Proliferative myositis (PM) is a rare pseudosarcomatous process that falls within a spectrum of benign myofibroblastic tumor-like lesions including nodular fasciitis and proliferative fasciitis [1]. In some instances, proliferative fasciitis is considered a variant of PM [2]. In addition, nodular fasciitis-like areas can occur in PM [3]. It usually manifests as a very rapidly growing soft tissue mass in an adult patient (median age at onset 50 years) arousing the clinical suspicion of a soft tissue sarcoma. The most common locations are the head and neck and the upper extremities [4, 5]. Although histologic presentation of PM resembles other benign fibroblastic proliferations, its magnetic resonance imaging (MRI) appearance is closer to inflammatory myopathies, which leads to completely different imaging differential diagnoses and histological expectations [1]. Because of the very rapidly growing nature of PM, biopsy and histological analysis are usually sought to obtain a diagnosis. We report an unusual case of PM in which the histologic and MRI presentations have overlapping features of typical PM and reactive fasciitis.

Case report

A 52-year-old female aerobics instructor presented with a 3-week history of a painful swelling in the anterior medial aspect of her right arm without a history of trauma, fever, or neurovascular symptoms. Her medical history was notable for Lyme disease several years previously, hypercholesterolemia, and migraines, which had been medically treated. There was no history of immunosuppression. Physical examination revealed a tender mobile mass measuring 6 × 5 cm in the proximal anterior right upper extremity with a full range of motion at both the shoulder and elbow joints. Ultrasonography with color Doppler showed a solid nodular heterogeneous soft tissue lesion in the medial aspect of the right upper arm within the biceps brachii. Venous color Doppler revealed no evidence of venous thrombosis (not shown). Sonographic evaluation suggested the possibility of muscle tear, and MRI was advised for further evaluation.

The MRI consisted of multiplanar series in short tau inversion recovery (STIR), T2-weighted, and T1-weighted images before and after gadolinium injection, with and without fat suppression. The examination revealed a fusiform mass measuring 6 × 4 × 2 cm within the short head of the right biceps brachialis muscle. Its upper and lower contours were ill-defined. On T1-weighted MR images, the mass displayed signal isointensity to muscle, while STIR images revealed diffuse increased signal intensity with linear hypointense structures continuing muscle fascicles. Contrast-enhanced T1-weighted images showed marked enhancement of the lesion with central branched hypointensity on axial images and ill-defined enhancement in both the superior and inferior margins. Below the inferior pole of the mass an extended fascicular hyperintensity in STIR and enhancement in contrast-enhanced T1-weighed series was noted around the biceps, extending more posteriorly to the anterior part of the triceps fascia (Fig. 1). Because of the clinical history of the rapidly growing mass and its deep location, an aggressive process such as a soft tissue sarcoma was suspected and a biopsy was recommended for a definite diagnosis.

Fig. 1
figure 1

Initial MR examination. a Axial T2-weighted MRI view, b coronal STIR image, and (c and d) axial contrast-enhanced T1-weighted MRI view. a, b The mass appears inhomogeneously hyperintense in T2-weighted image (large black arrows). The hypointensities within the lesion have geometrical contours on axial images—consistent with “checkerboard-like pattern”—and are clearly continued with muscle fascicles at the upper and lower pole of the tumor in coronal image (small black arrows). The inferior margin of the lesion cannot be identified and seems to continue within the interfasciular space inferiorly (empty white arrow). c Contrast-enhanced T1-weighted MRI view showing marked inhomogeneous enhancement (dashed white arrow) and central branched area of decreased signal intensity tracing the interfasciular spaces (small white arrows). d Inferior to the tumor, reactive fasciitis of the biceps brachialis (plain white arrows) and the anterior medial part of the triceps (white arrowheads) is noted with increased signal in STIR and contrast enhancement in T1-weighted image

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Incisional biopsy was performed under IV sedation. Intra-operative findings confirmed the intramuscular origin of the lesion from the short head of the right biceps brachialis muscle. The involved muscle was firm and pale and the fascial planes appeared intact. A 1 × 1.5-cm wedge sample was excised for pathological examination. There were no post-procedure complications.

On cross section, the surfaces demonstrated fibrillary muscle fascicles interspersed with gray-white fibrous septa of varying widths. Frozen section and routine histological H&E stains revealed a proliferation of myofibroblastic cells with fairly bland nuclear features in a loose, somewhat myxoid pattern with inflammatory infiltrates reminiscent of fibroblasts growing in tissue culture. The process was more concentrated in the epimysium but infiltrated the perimysium and even the endomysium between and around fascicles of skeletal muscle and individual muscle fibers. The muscle infiltration was associated with myofiber atrophy and the presence of ganglion-like cells with vesicular nuclei containing large nucleoli and basophilic cytoplasm were seen. These features were consistent with PM (Fig. 2). No histological features of malignancy were found, and a diagnosis of PM was rendered.

Fig. 2
figure 2

a Hematoxylin and eosin(H&E)-stained sections demonstrate a loosely arranged, myxoid myofibroblastic proliferation containing lymphocytes (100×). b The feathery spindle cell proliferation demonstrates epimysial concentration with extension between perimysial connective tissue and separates individual myofibers (100×). c A few larger cells with basophilic cytoplasm, vesicular nuclei, and prominent nucleoli resembling ganglion cells are seen in the infiltrate at higher magnification (200×). d Atrophic individual myofibers (open arrows) are entrapped between the myofibroblastic infiltrate in a field without ganglion-like cells. There are no regenerative myofibers seen (200×)

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Seven weeks postoperatively, the patient reported resolution of her symptoms. Physical examination and MRI revealed almost complete resolution of mass with residual high SI in STIR and contrast enhancement at the medial aspect the biceps fascia and along the path of the biopsy (Fig. 3). The patient has remained asymptomatic ever since.

Fig. 3
figure 3

Follow-up MR examination, 7 weeks later. Axial (a) STIR and (b) contrast-enhanced T1-weighted MR image showing complete resolution of the intramuscular mass with persistence of slight fascial STIR hyperintensity and contrast enhancement at the anterior medial aspect of the biceps brachialis (plain arrows) and along the biopsy path (dashed arrow)

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Discussion

PM is a rare mass-forming myofibroblastic proliferation containing ganglion-like cells and arising in skeletal muscle [6]. Although the process is benign, the clinical presentation of a rapidly growing muscular mass usually arouses the clinical suspicion for soft tissue sarcoma leading to biopsy. Histological description of PM usually consists of fibroblastic proliferation along with basophilic giant cells, which expand around and between muscle fascicles; however, areas in which the myxoid stroma with inflammatory infiltrate is prominent and does not have abundant ganglion-like cells suggest nodular fasciitis [1]. Both nodular fasciitis and PM fall within the same category of benign myofibroblastic proliferations. In our case the presence of histologic features suggesting both entities simply suggests that this case is an intermediate form of PM and nodular fasciitis.

Previous radiological reports of PM described homogeneous signal intensity in T1-weighted images similar to muscle, with almost homogeneous enhancement in contrast-enhanced T1-weighted images. The most characteristic finding of PM is best visualized in STIR images and consists of preserved muscle fascicles without discontinuity through the hyperintense muscular mass [7–9]. Careful analysis of axial images reveals a characteristic appearance referred to as a “checkerboard-like pattern” related to the geometrical pattern of internal strands of fibroblastic proliferation, which are hyperintense compared to muscle fascicles [9]. The “checkerboard-like pattern” was first described in CT and the geometrical pattern of fibroblastic proliferations was described as linear hypodensities. Because of the particular appearance of preserved muscle bundles within the lesion, differential diagnosis before histological confirmation included other types of myositis (inflammatory, infectious, etc.) however, and because of the rapid growing of the mass, a sarcomatous process was also considered. This characteristic MR appearance may perfectly match the corresponding histologic presentation of fibroblastic proliferation interspersed with muscle fascicles. Although PM is a fibroblastic infiltration within the interfascicular spaces, it may present as a well-circumscribed fusiform mass limited by muscle fascicles [8–10] or it may be more extensive appearing as a muscle enlargement [5]. Fascial enhancement associated with PM was only noted in one previous publication but was limited to the adjacent fascia [11]. The enhancement observed in our patient was more striking and extended to the anterior compartment of the arm (Fig. 1). We believe fascial enhancement suggests the reactive nature of the proliferative process. Although a previous report of proliferative fasciitis did not include contrast MR study, the authors showed fascial thickening and signal abnormalities similar to our patient’s MRI findings [2]. Our case actually combines typical MR and histological findings of PM in addition to unusual extensive fascial enhancement, suggesting a reactive process which we think may be explained by the combined histological features suggesting nodular fasciitis. In fact, the histological examination showed areas suggesting nodular/reactive fasciitis and other areas more suggestive of PM. Nodular fasciitis closely resembles proliferative myositis. The difference between both entities mainly consists in the presence of ganglion-like myofibroblasts with vesicular nuclei and basophilic cytoplasm in PM. These cells are absent in nodular fasciitis.

Despite close histological resemblance between PM and nodular fasciitis and other benign fibroblastic proliferations, these latter entities are not usually considered in the differential diagnosis because of their substantially different locations and MRI findings [7, 8, 12]. The most important MR finding that discounts the possibility of other benign fibroblastic proliferations is the interspersed muscle fascicles within the tumor. Indeed, this latter finding has never been described in nodular fasciitis [1, 10, 13]. Additionally, although nodular fasciitis can be intramuscular, it is most commonly located in the subcutaneous tissue [1].

Because of the typical MRI appearance of interspersed muscle fascicles within the mass, the most commonly discussed differential diagnoses in PM are other causes of myositis, such as myositis ossificans. Radiological textbooks also usually discuss PM amongst other causes of myositis rather than as myofibroblastic proliferations [1]. Although myositis ossificans is the most commonly discussed diagnosis in previously published cases of PM [7, 8, 12], its MRI presentation is actually quite different. Indeed, the presence of continuous muscle fascicles within the tumor and the enhancement pattern—usually homogeneous—is very unusual in myositis ossificans [12]. The particular MR appearance in this patient could have suggested other causes of inflammatory myopathies such as focal myositis, diabetic myositis, or neurogenic myopathies [1]; however, the clinical presentation of a very rapidly growing mass ruled out these diagnoses.

In conclusion, we have presented an unusual case of PM with some histologic features of nodular fasciitis. Associated fascial enhancement, as observed in our patient, suggested a reactive process. We believe this MRI appearance should be known to radiologists in order to narrow the differential diagnoses of rapidly growing muscular mass.