Final diagnosis
Strain of the quadriceps muscle.

Differential diagnosis
Soft tissue sarcoma

Discussion
The quadriceps muscle, considered as a muscle group functioning over two joints and undergoing eccentric contraction, is at increased risk of injury in contact and strenuous sports. Strain injuries of this muscle are common and may appear with a triad of localized swelling and increased thigh circumference, thigh pain and loss of knee flexion.

A rectus femoris muscle tear, as a type of quadriceps muscle strains, in itself, may not cause a significant or measurable functional deficit. After an acute injury the knee flexion is limited and painful. However, a chronic tear may appear as a mass without functional deficit or pain.

In both, acute and delayed-onset muscle lesions, ultrasound and MR imaging may provide information not available at physical examination, localizing tissue abnormalities, confirming the presence or absence of focal hematoma and showing no evidence of fatty infiltration or fibrosis.

The T2-weighted scans show the lesion as a focus of abnormal high intensity signal in the muscle surrounding the tendon. Another pattern on T2-weighted images consists of the tendency for MR abnormalities to occur in concentric zones, with perifascial "rim sign", explained by haemorrhage or edema from muscle necrosis. A central circular and high intensity area is noted, probably due to an intramuscular fluid collection, surrounded by a concentric low intensity ring, and peripheral high intensity rim. Fibrotic tissue usually results in areas of low signal intensity in all sequences. The use of gadolinium-enhanced T1 weighted MRI is very useful for accentuating the lesion. The T1-weighted images after intravenous gadolinium may enhance the muscular component, with the appearance of a "bull's eye", with an enhancing mass centred about a hypo-intense structure corresponding to the intramuscular tendon. The uptake of gadolinium in the area of the muscle injury is due to the increased vascularity of the lesion.

Sonographic findings in muscular ruptures include direct and indirect signs such as the discontinuity of muscle fibers and the presence of an associated hematoma. In complete ruptures, sonograms show the "clapper-in-the-bell" sign. However, identifying the torn fragments of the injured muscle may be difficult. Real-time examination during contraction better demonstrates the gap between the muscle fragments. On sonograms of partial ruptures, the injured area shows jagged margins, and a surrounding hyper-echoic halo may be present. A clapper-in-the-bell sign can also be present and is pathognomonic of a rupture. A combination of longitudinal and transverse sonograms is required to determine the exact size and location of a rupture.

Our patient underwent MRI evaluation, which showed, as the physical examination, a mass at the rectus femoris muscle. A tissue biopsy was performed, with the result of chronic inflammation and reactive fibrosis. Similar changes were previously reported in animals with mechanically induced muscle strains.

Unlike soft tissue sarcoma these lesions are small, non-progressive and associated with chronic quadriceps muscle strains. Most soft tissue tumors are not usually confined to the region of the tendon. Although on axial sections, both the muscle strain and the soft tissue tumor may have a similar appearance, sagital and coronal planes may demonstrate the longitudinal pattern of the strain.

Without a clear history of trauma or infection, the presence of an unexplained soft tissue mass is a cause for concern. Ultrasound and MR imaging may help to differentiate a traumatic injury from a sarcoma. The radiologist should look for the diagnostic signs in different imaging techniques. However, in the absence of them, a biopsy of the lesion should be undertaken.