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Published online before print October 14, 2004, 10.1148/radiol.2333031410
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(Radiology 2004;233:763-767.)
© RSNA, 2004

Musculoskeletal Imaging

Soft-Tissue Lipomas: Accuracy of Sonography in Diagnosis with Pathologic Correlation1

Prasuna Inampudi, MD, Jon A. Jacobson, MD, David P. Fessell, MD, Ruth C. Carlos, MD, MS, Smita V. Patel, MB, BS, Lydia O. Delaney-Sathy, MD and Marnix T. van Holsbeeck, MD

1 From the Department of Radiology, University of Michigan Medical Center, 1500 E Medical Center Dr, TC-2910G, Ann Arbor, MI 48109-0326 (P.I., J.A.J., R.C.C., S.V.P.); Department of Radiology, William Beaumont Hospital, Royal Oak, Mich (D.P.F.); Radiology Associates of Muncie, Ball Memorial Hospital, Muncie, Ind (L.O.D.S.); and Department of Radiology, Henry Ford Hospital, Detroit, Mich (M.T.v.H.). Received September 1, 2003; revision requested November 11; revision received March 26, 2004; accepted April 16. Address correspondence to J.A.J. (e-mail: jjacobsn@umich.edu).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
PURPOSE: To retrospectively determine the accuracy of sonography in helping to distinguish soft-tissue lipomas from other soft-tissue masses by using histologic proof as the reference standard.

MATERIALS AND METHODS: Institutional review board approval was obtained, and informed consent was waived. Thirty-nine patients who underwent sonographic evaluation of a soft-tissue mass followed by biopsy or resection were retrospectively evaluated. Two musculoskeletal radiologists (readers 1 and 2) reviewed the sonographic images, characterized the masses, and rated the level of confidence in the diagnosis of lipoma by using a five-point scale. A level of confidence was also rated for the prospective sonographic report, which was reviewed and designated as reader 3. Receiver operating characteristic (ROC) curves, including 95% confidence intervals, were generated, and the area under the ROC curve (Az) was calculated for each reader. Sensitivity, specificity, and accuracy for each reader were calculated by using a confidence rating of 4 or 5 as positive for lipoma. Weighted {kappa} analysis was also performed to assess for interobserver variability.

RESULTS: Histologic examination yielded 25 lipomas and 14 nonlipomas. The echogenicity of lipomas ranged from hypoechoic to hyperechoic relative to muscle, although most were isoechoic or hyperechoic. Az values were 0.79 for reader 1, 0.56 for reader 2, and 0.77 for reader 3. There was no significant difference between the Az for each reader and for chance. Interobserver agreement was fair, with a {kappa} value of 0.35 among the three readers. Sensitivities were 52%, 40%, and 52%, and accuracies were 64%, 49%, and 64% for readers 1, 2, and 3, respectively.

CONCLUSION: Sonography demonstrates low accuracy in the diagnosis of soft-tissue lipomas.

© RSNA, 2004

Index terms: Lipoma and lipomatosis, 40.363 • Soft tissues, US, 40.1298 • Ultrasound (US), tissue characterization


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Lipomas are one of the most common mesenchymal tumors that can develop in any fat-containing region of the body, including the soft tissues, bones, retroperitoneum, mediastinum, or the gastrointestinal tract (1). Superficial soft-tissue lipomas commonly present at a clinical examination as soft, painless, well-delineated, and mobile masses. They may be multiple in up to 5% of individuals and are most common in patients older than 50 years (1). Lipomas have been evaluated initially with sonography because of its availability and low cost compared with those of magnetic resonance (MR) imaging (2). Some have advocated the use of sonography to confirm the presence of a true mass, whereas others have stated that sonography alone may establish the diagnosis of a lipoma (3). However, the sonographic appearances of lipomas, and in particular their echogenicity, have been shown to be variable (1,4–8). The echogenicity of lipomas may range from hypoechoic to anechoic, depending on the degree of connective tissue and other reflective interfaces present within a lipoma (1). Thus, the purpose of our study was to retrospectively determine the accuracy of sonography in helping to distinguish soft-tissue lipomas from other soft-tissue masses by using histologic proof as the reference standard.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Patients
A database search and review of sonographic examination reports were used (P.I., S.V.P., L.O.D.S.) to retrospectively identify patients who underwent sonography of a soft-tissue mass during a 4-year period (January 1995 through December 1999) at University of Michigan and Henry Ford Hospital. Institutional review board approval was obtained from both institutions, and informed consent was waived at both institutions. All patients were assessed to undergo subsequent biopsy or resection. Thirty-nine patients (16 male and 23 female; age range, 5 months to 73 years; mean age, 37.5 years) made up the study population.

Imaging
The sonographic imaging was completed independently by one of six musculoskeletal radiologists (J.A.J., D.P.F., M.T.v.H., with 3–23 years of experience) as part of a daily clinical practice at these two institutions by using 7–12-MHz linear transducers (HDI 3000 and 5000, Philips ATL, Bothell, Wash; 5200, Acoustic Imaging, Phoenix, Ariz; Sonoline Elegra, Siemens Medical Systems, Issaquah, Wash). Evaluation of the soft tissues was guided with use of physical examination findings, patient history, and information submitted on the examination requisition by the clinical service. In each case, transmission gel was used in place of a standoff pad. Imaging plane depended on the orientation of the soft-tissue mass but generally included short- and long-axis images relative to the mass.

Image Interpretation
Two musculoskeletal radiologists (J.A.J., D.P.F., readers 1 and 2), with 6 and 4 years of experience in musculoskeletal sonography, performed a blinded retrospective review of all available static images obtained from the 39 sonographic examinations. Each reader completed the retrospective review independently and without knowledge of clinical data, histologic results, or the prospective sonographic report. The lesions were characterized according to their echogenicity relative to the adjacent muscle (hypoechoic, isoechoic, or hyperechoic), margins (well or ill defined), shape (round, oval, or other), and location (subcutaneous, intramuscular, between tissue planes, or other). The degree of concordance between readers with regard to lipoma characterization was determined. Each reader also classified these masses as lipoma or not lipoma and rated the level of confidence by using the following five-point scale: 1, definitely not a lipoma; 2, probably not a lipoma; 3, uncertain; 4, probably a lipoma; and 5, definitely a lipoma. This subjective classification was based on the experience of the individual reader in musculoskeletal sonography, familiarity with the sonographic evaluation of lipomas, and knowledge of the literature on this topic. Factors that likely influenced this classification included echogenicity, shape, and margins of the mass. The prospective sonographic report was also reviewed by one of the authors (D.F.P.), without knowledge of clinical data or histologic results, and a level of confidence was rated with the five-point scale. This report was designated as reader 3. The rating was based on how strongly the diagnosis of lipoma or other mass was suggested in the dictated report.

Standard of Reference
Histologic diagnoses were prospectively provided by one of 20 staff pathologists at each institution as part of their daily practice, and these results were obtained by means of a retrospective medical record review. Of the 39 patients, 38 had undergone surgical excision of the soft-tissue mass. One patient had undergone percutaneous biopsy, which yielded fragments of tissue ranging from 1 to 20 mm. Gross pathologic and microscopic examinations were completed in 38 and 11 patients, respectively. A specimen composed of mature white adipose tissue at examination was consistent with diagnosis of lipoma.

Statistical Analysis
Receiver operating characteristic (ROC) curves, including 95% confidence intervals, were generated. The area under the ROC curve (Az) values were calculated for each reader by using JROCFIT, an online ROC analysis tool from Johns Hopkins University (9). Overlapping 95% confidence intervals represented no statistically significant difference between the ROC curves. Descriptive statistical analysis was performed as follows: Sensitivity, specificity, and accuracy for each reader were calculated by using a confidence rating of 4 or 5 as positive for lipoma. To assess for interobserver variability, weighted {kappa} analysis was also performed with Stata 6.0 software (Stata, College Station, Tex) (10). To assess for potential selection bias, we compared the age and sex distribution of patients with diagnosis of lipomas versus diagnosis of nonlipomas by using differences in proportion and the Student t test, respectively.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 
Histologic Diagnosis
The 39 masses were from 16 male and 23 female patients. There was no significant difference (P > .05) between patients with lipoma and patients with nonlipoma with regard to sex and age (Table 1). Histologic diagnosis of the 39 soft-tissue masses (Table 2) yielded 25 lipomas (Figs 1–3) and 14 nonlipomas (one liposarcoma, one lipoblastoma, two fat necrosis, two hemangiomas, two angiolipomas [Fig 4], one ganglion cyst, one epidermal inclusion cyst, one neurofibroma, one leiomyoma, one lymphedema, and one vascular malformation). Thirteen lipomas were located in the upper extremities, five were located in the lower extremities, five involved the back, one involved the anterior abdominal wall, and one involved the face.


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  		TABLE 1. Demographics of the Study Population

 

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  		TABLE 2. Histologic Results of 39 Soft-Tissue Masses

 


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  		Figure 1. Long-axis sonogram in a 16-year-old female with pathologically proved lipoma (arrows) of the back, which was characterized by both readers as well defined and hyperechoic relative to the adjacent musculature (M).

 


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  		Figure 2. Short-axis sonogram in a 53-year-old woman with pathologically proved lipoma (arrows) of the arm, which was characterized by both readers as well defined and isoechoic to the surrounding muscle (M).

 


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  		Figure 3. Short-axis sonogram in a 43-year-old woman with pathologically proved lipoma (arrows) of the lower leg, which was characterized by both readers as well defined and hypoechoic to the adjacent musculature (M).

 


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  		Figure 4. Long-axis sonogram in a 60-year-old man with pathologically proved angiolipoma (arrows) of the wrist, which was characterized by both readers as well defined and hyperechoic compared with the muscle (M).

 
Sonographic Features
Retrospective review of the static sonographic images revealed the following results: With regard to lipoma echogenicity, reader 1 classified 20% as hyperechoic, 60% as isoechoic, and 20% hypoechoic; reader 2 classified 52% as hyperechoic, 28% as isoechoic, and 20% as hypoechoic relative to the muscle (Table 3). Concordance between readers for the masses categorized by echogenicity was 10 (three hyperechoic, four isoechoic, three hypoechoic masses). With regard to lipoma margins, reader 1 classified 40% as well defined and 60% as ill defined, and reader 2 classified 48% as well defined and 52% as ill defined (Table 3). Concordance between readers for the masses categorized according to definition of border was 17 (10 ill defined, seven well defined). With regard to lipoma shape, reader 1 classified 60% as oval, 8% as round, and 32% as other; reader 2 classified 100% as oval (Table 3), with 15 being concordant. With regard to lipoma location, reader 1 classified 56% as subcutaneous, 20% as intramuscular, 20% as between tissue planes, and 4% as other; reader 2 classified 76% as subcutaneous, 24% as intramuscular, 0% as between tissue planes, and 0% as other (Table 3).


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  		TABLE 3. Retrospective Characterization of 25 Soft-Tissue Lipomas

 
With use of the ROC analysis, the Az values were 0.79 for reader 1, 0.56 for reader 2, and 0.77 for reader 3 (Fig 5). There was no significant difference between the Az for each reader (P > .05), and there was no significant difference between the Az for each reader and for chance (P > .05). The interobserver agreement among the three readers was fair, with {kappa} of 0.35. By using a confidence rating of 4 or 5 as positive for lipoma, summary statistics were also calculated for each reader. For reader 1, sensitivity was 52% (13 of 25), specificity was 86% (12 of 14), and accuracy was 64% (25 of 39). For reader 2, sensitivity was 40% (10 of 25), specificity was 64% (nine of 14), and accuracy was 49% (19 of 39). For reader 3, sensitivity was 52% (13 of 25), specificity was 86% (12 of 14), and accuracy was 64% (25 of 39).



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  		Figure 5. Graph of the ROC curves for diagnosis of lipoma. The Az values for each reader were not significantly different from each other or from chance. The interobserver agreement among the three readers was fair ({kappa} = 0.36).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
 
Lipomas are one of the most common benign mesenchymal lesions (7). The reported sonographic appearance of soft-tissue lipomas is highly variable. While initial reports described them to be sonolucent with few internal echoes (4), subsequent reports described internal echogenicity (5). Classically, lipomas have been described as being homogeneous and hyperechoic, but there are frequent exceptions (6). In one series of 35 lipomas, of which 12 were pathologically proved, 29% were hyperechoic, 22% were isoechoic, 29% were hypoechoic, and 20% were of mixed echogenicity (1). In another study of 25 lipomas occurring in the head and neck region, of which 16 were histologically diagnosed, 76% were hyperechoic, 16% were hypoechoic, and 8% were isoechoic (7). An additional descriptive report of three superficial lipomas found one to be echogenic, while two were completely anechoic (8). Our study further characterizes the sonographic appearance of pathologically proved lipomas. In addition, other soft-tissue masses are included in the blind assessment, with the addition of multiple readers to assess interobserver variability.

In our retrospective review of 25 lipomas, all histologically diagnosed, reader 1 described 60% to be isoechoic, 20% to be hyperechoic, and 20% to be hypoechoic, whereas reader 2 identified 52% as hyperechoic, 28% as isoechoic, and 20% as hypoechoic. There was concordance between the readers for only three hyperechoic lipomas, four isoechoic lipomas, and three hypoechoic lipomas (10 of 25). This demonstrates not only variability between our study and prior reports but also a discrepancy between the two readers within a single study in the description of the echogenicity of soft-tissue lipomas.

It has been postulated that the sonographic appearance of lipomas depends on their internal cellularity, specifically on the amount of fat and water within. On the basis of experimental in vitro results, Behan and Kazam (8) found that a mixture of fat and water is quite echogenic, while pure fat is echo-free. An acoustic impedance mismatch exists at fat-water and water-fat interfaces (8). Since pure fatty tumors have few interfaces and less of an acoustic impedance mismatch, they appear echo-free, whereas lipomas, with a mixed cellularity, have an increased number of interfaces and appear echogenic. It is likely that inherent cellular variability within soft-tissue lipomas causes variability in their echogenicity.

The definition of borders of soft-tissue lipomas in the literature is also highly variable. In the study of 35 lipomas by Fornage and Tassin (1), of which 12 were histologically proved, 60% were well defined and 40% were poorly defined. In the study of 25 lipomas by Ahuja et al (7), of which 16 were pathologically diagnosed, 88% were well defined with an identifiable capsule and 12% were ill defined. In our study, both readers found that the majority of lipomas had ill-defined borders. Reader 1 found 60% of lipomas to be ill defined and 40% to be well defined, and reader 2 found 52% to be ill defined and 48% to be well defined. There was concordance between readers for 10 of the masses in the ill-defined category and for seven masses in the well-defined category (total agreement on border definition in 17 of 25 lipomas).

The results from the ROC analysis show that the Az values for each reader were not significantly different from each other or from chance. The {kappa} value of 0.35 indicates only fair interobserver agreement. In addition, sensitivities of 52%, 40%, 52% and accuracies of 64%, 49%, 64% for the three readers indicate the difficulties of sonography in providing the correct diagnosis of soft-tissue lipoma.

We acknowledge limitations in this study. Because readers 1 and 2 were only able to retrospectively review static images from the original sonograms, there was no real-time assessment of the sonographic characteristics of the soft-tissue masses, which somewhat limited this study. However, the information in the report interpreted by reader 3 was the result of real-time assessment, and there was no statistically significant difference in the diagnosis of lipoma between the readers. Another limitation was that each reader’s confidence rating of the lipoma was somewhat subjective and influenced by prior experience and degree of knowledge related to this topic. The referring physician’s degree of suspicion regarding the initial palpable abnormality may also have determined the choice of imaging modality, which may have produced selection bias. The presence of histologic findings as the standard of reference may be an additional source of selection bias as atypical or unusual masses may be more likely to undergo biopsy or be removed. Last, color or power Doppler evaluation was not assessed. While findings of one study (7) of 25 head and neck lipomas showed no flow on color Doppler sonograms, the significance of increased flow at power Doppler imaging is not known.

In summary, soft-tissue lipomas have a variable sonographic appearance that is subjectively difficult to characterize, which likely accounts for the limitations of sonography in accurately differentiating soft-tissue lipomas from other masses.


   FOOTNOTES
 
Abbreviations: Az = area under the ROC curve, ROC = receiver operating characteristic

Authors stated no financial relationship to disclose.

Author contributions: Guarantors of integrity of entire study, J.A.J., D.P.F.; study concepts and design, all authors; literature research, P.I., J.A.J., D.P.F.; clinical studies, J.A.J., D.P.F., M.T.v.H., P.I., S.V.P., L.O.D.S.; data acquisition and analysis/interpretation, all authors; statistical analysis, P.I., J.A.J., D.P.F., R.C.C.; manuscript preparation, definition of intellectual content, editing, revision/review, and final version approval, all authors


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
 REFERENCES
 

  1. Fornage BD, Tassin GB. Sonographic appearances of superficial soft tissue lipomas. J Clin Ultrasound 1991; 19:215-220.[Medline]
  2. Bloem JL. Imaging of soft tissue tumors. J Belge Radiol 1992; 75:265-273.[Medline]
  3. Sintzoff SA, Jr, Gillard I, Van Gansbeke D, Gevenois PA, Salmon I, Struyven J. Ultrasound evaluation of soft tissue tumors. J Belge Radiol 1992; 75:276-280.[Medline]
  4. Wild JJ, Reid JM. Further pilot echographic studies on histopathologic structure of tumors of the living intact human breast. Am J Pathol 1952; 28:839-861.
  5. Goldberg BB. Ultrasonic evaluation of superficial masses. J Clin Ultrasound 1975; 3:91-94.[Medline]
  6. Lin J, Jacobson JA, Fessell DP, Weadock WJ, Hayes CW. An illustrated tutorial of musculoskeletal sonography. IV. Musculoskeletal masses, sonographically guided interventions, and miscellaneous topics. AJR Am J Roentgenol 2000; 175:1711- 1719.
  7. Ahuja AT, King AD, Kew J, King W, Metrewedi C. Head and neck lipomas: sonographic appearances. AJNR Am J Neuroradiol 1998; 19:505-508.[Abstract]
  8. Behan M, Kazam E. The echographic characteristics of fatty tissues and tumors. Radiology 1978; 129:143-151.[Abstract]
  9. Eng J. ROC analysis: web-based calculator for ROC curves. Available at: www.rad.jhmi.edu/jeng/javarad/roc/JROCFITi.html. Accessed August 2003.
  10. Fleiss JL, Kingman A. Statistical management of data in clinical research. Crit Rev Oral Biol Med 1990; 1:55-66.[Free Full Text]



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