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1 Department of Radiology, Mayo Clinic Scottsdale, 13400 E. Shea Blvd.,
Scottsdale, AZ 85259.
2 Department of Pathology, Mayo Clinic Scottsdale, Scottsdale, AZ 85259.
Received August 28, 2002;
accepted after revision October 14, 2002.
Address correspondence to C. C. Roberts
(roberts.catherine{at}mayo.edu).
Abstract
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MATERIALS AND METHODS. We searched the past 7 years of our institution's radiology database for reports of MR images containing the word "lipoma." Medical records were reviewed to identify patients with palpable masses corresponding to fat. Two radiologists retrospectively reviewed the MR images for the presence of an encapsulated or nonencapsulated fatty mass in the region of the palpable abnormality. Pathologic specimens, when available, were also reviewed.
RESULTS. Between 1995 and 2001, 184 palpable subcutaneous fatty masses were evaluated on MR imaging. Of these, 46% (85/184) were encapsulated lipomas and 54% (99/184) were nonencapsulated fatty masses on MR imaging. Four masses (three encapsulated and one nonencapsulated) were surgically resected and had pathology consistent with lipomas.
CONCLUSION. Many palpable fatty masses do not have definable capsules on MR imaging. We propose that a palpable mass that corresponds to a nonencapsulated prominence of subcutaneous fat on MR imaging should be reported as a nonencapsulated lipoma. More definitive reporting of this relatively common lesion will assure the referring clinician of the benign nature.
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Those masses that image identically to the adjacent subcutaneous tissue on all pulse sequences and are surrounded by a thin, smooth capsule on MR imaging can be easily diagnosed as lipomas. In our practice, however, we have observed numerous patients with palpable subcutaneous masses that are not distinguishable from the surrounding fat on MR imaging. For the past 7 years, we have termed these lesions "nonencapsulated lipomas" because we can communicate the benign nature of the mass to the clinician and potentially save patients from unnecessary additional imaging that could occur if a diagnosis of no mass were reported.
These palpable masses are often more apparent clinically than radiologically and may go undepicted on MR imaging if they do not produce a contour abnormality of the overlying skin. The importance of a marker over the palpable mass to differentiate it from surrounding tissue has been previously described [3, 4]. An existing surface deformity may be position-dependent for visibility or may be eliminated by compression by the MR imaging table or surface coil. When radiologists are not aware of the precise location of the mass in question, they cannot make the diagnosis of a nonencapsulated lipoma.
Defining these palpable but indistinct subcutaneous masses as nonencapsulated lipomas has precedence in the field of pathology. The identification of a capsule is not considered necessary for a pathologic diagnosis of lipoma, as long as the mass is composed of mature adipose tissue. The capsule of a lipoma can be difficult to differentiate histologically from septations in the mass or septations in the surrounding subcutaneous fat. Also, lipomas are not always removed in one piece, so the morphology of the lesion can be distorted when the pathologist evaluates it.
The purpose of our study was to evaluate the MR imaging characteristics of palpable fatty masses and to propose terminology for nonencapsulated lesions.
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To ensure that the region of a palpable mass had been included on the MR imaging examinations, we required one of the following: The mass had been localized with skin markers on the images, the MR imaging report stated that the radiologist had examined the patient and noted the location of the palpable mass, or the precise location of the mass was noted by the referring clinician in the medical record. One hundred eighty-four masses that were found in 184 patients fit these criteria and composed our study population.
A consensus panel of a musculoskeletal radiologist and a musculoskeletal fellow, who were unaware of patient information, reviewed the studies in random order. For imaging performed from January 1995 to June 1998, the MR studies were reviewed on film. For the examinations performed from July 1998 to December 2001, the MR images were reviewed on a Litebox PACS workstation (General Electric Medical Systems, Milwaukee, WI).
The reviewers classified the MR imaging findings for each examination as showing either an encapsulated or a nonencapsulated mass that followed fat signal intensity on all sequences in the region of the marked or described palpable abnormality. We included partially encapsulated lesions in the "encapsulated" category. A fatty mass was designated as being encapsulated if it had a low-signal (on T1-and T2-weighted images) smooth, linear margin that surrounded at least 25% of the mass circumference in at least one plane. This definition was an arbitrary one used to exclude the normal septations seen in fat. A mass was considered nonencapsulated if it had less than 25% of its circumference delineated by a low-signal linear margin in all planes. Lesions that had asymmetrically infrequent distributions of fat septations compared with surrounding subcutaneous fat were included in the nonencapsulated category. For those masses that were not distinguished by a capsule or lack of septations, the presence of a subcutaneous nonencapsulated fatty mass was determined by a combination of a palpable mass and a lack of an underlying mass or cystic lesion in the deep tissues (muscle, bone, fascia, and neurovascular structures).
MR imaging sequence protocols varied slightly during the 7 years of studies being reviewed. Minimal sequences included in the study were T1-weighted (TR range/TE range, 300–600/10–20) and T2-weighted (2500–6000/60–90) fat-suppressed series in two planes. The matrix used was 256 x 256–192 for all images. The field of view and slice thickness parameters were varied to tailor the examination to the area of interest. All examinations from 1996 to 2001 were imaged with a surface phased array coil or quadrature extremity coil. Eight studies were performed in 1995 with a body coil.
Patient records were searched for operative and pathology reports of resected subcutaneous masses. The histology of representative sections of the resected masses was reviewed. Because of the retrospective nature of the study, we could not review the gross specimens, which had previously been destroyed.
The distribution of locations of encapsulated lipomas was compared with the distribution of locations for nonencapsulated fatty masses. The statistical significance was calculated using the Freeman-Halton extension of the Fisher's exact test.
For each location, calculating the percentage of encapsulated lipomas and comparing this value to 50% assessed the relative frequency of encapsulated and nonencapsulated lipomas. Calculating the 95% confidence interval (CI, margin of error) for the percentage of encapsulated lipomas assessed the statistical significance. CIs were calculated using the exact binomial method.
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Of these 184 masses, we classified 85 (46%) as partially or completely encapsulated (Figs. 1A, 1B) on MR imaging and the remaining 99 (54%) as nonencapsulated (Figs. 2 and 3). The lipomas were distributed throughout the body (Table 1). The distribution of anatomic locations for encapsulated lipomas appeared to differ substantially from that of nonencapsulated fatty masses (p = 0.006). In particular, lipomas were more likely to be encapsulated than nonencapsulated in the shoulder (95% CI, 53–92%) and less likely to be encapsulated in the thigh (95% CI, 9–42%). The sample of lipomas at each of the other sites was too small to form a strong conclusion. Overall, encapsulated lipomas occurred in frequency approximately equal to that of nonencapsulated fatty masses (95% CI, 39–54%).
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Four fatty masses in the study group had been surgically removed, all for cosmetic reasons. Three fatty masses were encapsulated and one was nonencapsulated on MR imaging. Representative sections from these lipomas showed a similar microscopic appearance, which was that of mature adipose tissue. Specimens contained uniform, polygonal, mature lipocytes with bland peripheral nuclei and fibrous connective tissue septa (Fig. 4). The presence of a capsule was not mentioned in the pathology reports and could not be retrospectively assessed because the gross specimens were no longer available and a capsule is indistinguishable from septations microscopically.
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According to a noted soft-tissue tumor pathologist, lipomas "so closely resemble normal fat that it is usually not possible to recognize them histologically without the benefit of an accompanying clinical or gross description" [8]. Microscopically, a lipoma can have thin fibrous septations that are virtually indistinguishable from a capsule [9]. On gross pathology, the lipomas are typically soft and well circumscribed. They tend to have lobulated round shapes with yellow, greasy cut surfaces [1, 4]. This architecture can be distorted when lesions are removed piecemeal and sent for pathologic evaluation. We elected to use the term "encapsulated" to denote those masses that are demarcated by a thin rim of tissue on MR imaging even though whether lipomas have a true capsule versus a pseudocapsule (compression of normal adjacent tissue) is controversial among some pathologists.
The age range and mean of our population correlated well with the values for lipomas in the literature [5]. However, our study population was predominantly female. This distribution differs from the reported male predominance or equal distribution between the sexes [1, 8]. This discrepancy could have been caused by an asymmetric distribution of lipomas in our referral population, by a greater likelihood for women to seek medical attention for subcutaneous masses, or by different selection biases in previous studies.
The main limitation of this study is that it did not assess whether the fat in these palpable, nonencapsulated masses differs from normal fat. We cannot be certain that these masses are not islands of nonneoplastic, asymmetric fatty deposition, fatty hypertrophy, previously traumatized fat, or areas of fat surrounded by fibrosis. Neither can we be certain that these masses constitute a distinct fatty mesenchymal neoplasm. A prospective, longitudinal study with clinical follow-up may provide answers to some of these questions, although it will be difficult to justify obtaining surgical and pathologic correlation for most of these benign, mildly deforming lesions.
This study is limited by its retrospective nature. If the word "lipoma" did not appear in the report (either in the indication or in interpretation), the case would not have been identified in the database search. There is also limited pathologic proof because only four masses were removed. Finally, for cases reviewed on film, a thin capsule could have been obscured by the window and level settings.
Even if we do not know for certain the causes of these simple nonencapsulated fatty lesions, we do know that they are common and that they are almost certainly benign.
Cytogenetic analysis of nonencapsulated fatty masses may be helpful for determining the origin of these masses because approximately half of all lipomas have distinctive cytogenetic abnormalities [10]. This analysis requires fresh, viable tissue and could not be performed retrospectively.
Diagnosing palpable subcutaneous fatty protuberances as nonencapsulated lipomas corresponds with the nomenclature that is used by pathologists at our institution, who assign a diagnosis of lipoma to masses that are composed of fat and are surrounded by a capsule (or pseudocapsule) identified grossly. They also designate nonencapsulated superficial fatty lesions as lipomas, as long as there are collaborative clinical findings of a discrete mass.
In conclusion, many palpable subcutaneous masses correspond to nonencapsulated fatty lesions on MR imaging. Because these lesions are often difficult to distinguish from normal fat, the interpreting radiologist should be aware of the exact location of the palpable mass in question. We propose reporting these superficial fatty masses as "nonencapsulated lipomas" to assure the referring physician as to the benign nature of the lesion and to avoid additional workup.
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