Fig. 2C & 2D: T1-weighted (TR/TE 510/15) (Fig. 2 C) and T2-weighted (TR/TE 2000/80) (Fig. 2 D) coronal images through the posterior thighs. The extension of the tumor along the preserved fascicles of the semimembranosus (SM) and semitendinosus (ST) muscles is clearly evident as is the preservation of these muscle bellies. The entire course of the long head of the biceps femoris is replaced (arrows).

Discussion: Three forms of skeletal muscle lymphoma are described: primary extranodal involvement; secondary involvement in patients with disseminated disease; or, secondary involvement by contiguous spread from osseous disease [1]. All are rare but primary involvement is exceptional. Large series have reported muscle involvement in 1.4% of patients but primary involvement is present in only 0.14% of lymphoma patients [1].

Muscle involvement by lymphoma may present as a discrete mass, or more commonly, as diffuse muscle enlargement [2]. Preservation of intermuscular fat planes has been described [2,3]. On unenhanced CT, lymphomatous muscle is hypodense to isodense relative to normal muscle. Intravenous contrast administration has been reported to increase the conspicuity of the involved muscle, either by decreased enhancement relative to normal muscle [2]or by diffuse enhancement of the lymphomatous muscle [3]. Adjacent edematous changes in fat and along fascial planes can be seen. [3].

The MR appearance of skeletal muscle lymphoma has only rarely been described [1,4]. Metzler et al. [4] described two cases, one with Hodgkin Disease involving the psoas muscles and vertebrae, and a second case with primary non-Hodgkin lymphoma of the calf. On T2-weighted images signal intensity is isointense to hypointense to fat, and hyperintense to muscle. Hyperintensity relative to muscle was also noted on the TR/TE 500/30 images. Muscle, fascial, and subcutaneous fat changes were described. Hosono et al.[1] described four patients with non-Hodgkin lymphoma involving skeletal muscle of the extremities, two cases of thigh involvement and two of arm involvement. These authors described enlargement of the involved muscles with preservation of adjacent fat planes, tumor extension along muscle fascicles, and homogeneous enhancement following intravenous gadolinium administration. Vessels appeared to be spared. Involved muscles were hyperintense to muscle on T2-weighted images and isointense to slightly hyperintense on T1-weighted images (imaging parameters were not defined, however).

The case we present demonstrates these typical imaging characteristics. The involved muscles are enlarged and diffusely infiltrated, with preservation of their normal contours. The signal intensity is isointense to muscle on T1 (SE 510/15) and isointense to fat on T2 (SE 2000/80). Tubular signal voids, presumably vessels with high flow, are noted coursing through the involved muscles.

The differential diagnosis of muscle enlargement is lengthy and includes myopathies, muscle infarction, and post denervation enlargement. The idiopathic inflammatory myopathies are characterized by proximal muscle weakness, occasionally with muscle tenderness and rash. MR imaging usually reveals nonspecific changes of edema in affected muscles, adjacent subcutaneous or intermuscular fat, and, occasionally along myofascial planes. Typically, these inflammatory changes are seen in a patchy distribution bilaterally in proximal muscle groups. With chronic inflammation, atrophy and fatty replacement may be seen[5]. Involvement of a single or several muscles in their entirety is unlikely.

Two forms of muscle involvement in sarcoidosis are described. The myopathic form appears normal at MRI. Nodular sarcoid, on the other hand, has a typical MR imaging appearance. Long intramuscular lesions are seen ,with stellate low signal intensity centrally ("dark star") and a high signal rim. This appearances creates a characteristic 3-striped pattern on T1 and T2-weighted, and gadolinium-enhanced images [6].

Skeletal muscle infarction is a rare complication in patients with diabetes [7] and sickle cell anemia [8]. One or several muscles may be involved. The imaging findings are nonspecific with low to slightly increased signal intensity to muscle on T1 weighted images and increased signal intensity on T2 weighted images[7, 8]. In these patients, differentiation from infection may be difficult.

Denervated skeletal muscle typically atrophies and is replaced by fat. Occasionally, however, the muscle enlarges and the patient can present with a mass or swelling. One or several muscles can be diffusely involved. The enlargement can result from true hypertrophy with increase in size of the muscle fibers, or from pseudohypertrophy, with enlargement by accumulation of fat and connective tissue within the muscle. Petersilge et al. [9] described the MR appearances of both. Muscles that have undergone true hypertrophy are enlarged with signal characteristics identical to normal muscle, while pseudohypertophy is seen as an enlarged muscle with increased signal which corresponds to fat.

References:
1. Hosono M, Kobayashi H, Kotoura Y, et al. Involvement of muscle by
malignant lymphoma: MR and CT appearances. J Comput Assist Tomogr
1995; 19:455-459.

2. Malloy PC, Fishman E, Magid D. Lymphoma of bone, muscle, and skin:
CT findings. AJR 1992; 159:805-809.

3. Grunshaw ND, Chalmers AG. Skeletal muscle lymphoma. Clin Radiol 1992; 45:399-400.

4. Metzler JP, Fleckenstein JL, Vuitch F, Frenkel EP. Skeletal muscle lymphoma:
MRI evaluation. Magn Reson Imaging 1992; 10:491-494.

5. Adams EM, Chow CK, Premkumar A, Plotz PH. The idiopathic inflammatory
myopathies: spectrum of MR imaging findings. Radiographics 1995; 15:563-574.

6. Otake S. Sarcoidosis involving skeletal muscle: imaging findings and relative
value of imaging procedures. AJR 1994; 162:369-375.

7. Nunez-Hoyo M, Gardner CL, Motta AO, Ashmead JW. Skeletal muscle infarction
in diabetes: MR findings. J Comput Assist Tomogr 1993; 17:986-988.

8. Feldman F, Zwass A, Staron RB, Haramati N. MRI of soft tissue abnormalities:
a primary cause of sickle crisis. Skeletal Radiol 1993; 22:501-506.

9. Petersilge CA, Pathria MN, Gentili A, et al. Denervation hypertrophy of muscle:
MR features. J Comput Tomogr 1995; 19:596-600.

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