Acute myonecrosis on MRI: etiologies in an oncological cohort and assessment of interobserver variability

Abstract

Objective To determine etiologies of myonecrosis in oncology patients and to assess interobserver variability in interpreting its MRI features. Materials and Methods Pathology records in our tertiary cancer hospital were searched for proven myonecrosis, and MRIs of affected regions in those patients were identified. MRI reports that suggested myonecrosis also were identified. Each MRI was reviewed independently by two of six readers to assess anatomic site, size, and signal intensities of muscle changes, and presence of the previously reported stipple sign (enhancing foci within a region defined by rim enhancement). The stipple sign was assessed again, weeks after a training session. Cohen kappa and percent agreement were calculated. Medical records were reviewed for contemporaneous causes of myonecrosis. Results MRI reports in 73 patients suggested the diagnosis of myonecrosis; pathologic proof was available in another two. Myonecrosis was frequently associated with radiotherapy (n=34 (45%) patients)); less frequent causes included intraoperative immobilization, trauma, therapeutic embolization, ablation therapy, exercise, and diabetes. Myonecrosis usually involved lower extremity, pelvis, and upper extremity; mean size was 13.0 cm. Stipple sign was observed in 55–95% of patients at first assessment (k=0.09–0.42; 60–80% agreement) and 55–100% at second (k=0.0–0.58; 72–90% agreement). Enhancement surrounded myonecrosis in 55–100% patients (k=0.03 – 0.32; 58–70% agreement). Conclusion Myonecrosis in oncology patients usually occurred after radiotherapy, and less commonly after intraoperative immobilization, trauma, therapeutic embolization, ablation therapy, exercise, or diabetes. Although interobserver variability for MRI features of myonecrosis exists (even after focused training), a combination of findings facilitates diagnosis and conservative management.

Muscle: Ischemia, Infarction, and Compartment Syndrome

“…Although increased, the T2 signal intensity typically remains below that of fluid. 43 Loss of normal intramuscular fatty septa is a constant imaging feature, best appreciated on T1-weighted images. 35 Increased T1 signal can be seen within involved muscles, thought to represent hemorrhage within areas of infarction (►Fig.…”

“…9). 34,35,43 Both diffuse and peripheral rim enhancement patterns have been described, although the enhancement pattern does not correlate with clinical outcome in these patients. 34 Some have theorized that diffuse enhancement represents ischemic muscle, whereas peripheral enhancement with central nonenhancement represents areas of infarction and necrosis (►Fig.…”

“…31,35 Subcutaneous edema is almost universally seen, and subfascial fluid is also a frequent imaging finding. [34][35][36][37][38]42,43 The sonographic appearance of diabetic myonecrosis has been described as a well-demarcated, predominantly hypoechoic intramuscular area with linear hyperechoic muscle fibers coursing through the lesion. In contrast to intramuscular abscess or necrotic tumor, no motion or fluid swirling is seen within the area of myonecrosis with the application of transducer pressure.…”

“…31,43,60 The first pattern is thought to represent reversible muscular ischemia, most frequently from overexertion, and it may be a severe form of delayed-onset muscle soreness. 60 The second pattern is thought to represent muscular necrosis, 31,60 and the presence of subcutaneous edema and subfascial fluid in most cases, 60 similar to myonecrosis, 34,35,37,38,42,43 would support this. The areas of intramuscular enhancement comprising the Fig.…”

“…The imaging differential diagnosis of myonecrosis and rhabdomyolysis includes intramuscular abscess, necrotizing fasciitis, pyomyositis, and inflammatory myositis. 35,42,43 Rarely, intramuscular lymphoma or metastatic disease can mimic myonecrosis and rhabdomyolysis. It is often difficult to distinguish between these entities on MRI, but several imaging and clinical features may be helpful (►Fig.…”