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Differentiation of lipoma and atypical lipomatous tumor by a scoring system: implication of increased vascularity on pathogenesis of liposarcoma
© Nagano et al.; licensee BioMed Central. 2015
Received: 2 May 2013
Accepted: 2 February 2015
Published: 22 February 2015
Well-differentiated liposarcoma (WDL)/atypical lipomatous tumor (ALT) is considered a low-grade malignancy that rarely metastasizes but should be carefully followed because recurrence or dedifferentiation may occur. It is recognized that WDL and ALT are essentially synonymous, describing lesions that are identical both morphologically and karyotypically, and that site-specific variations in behavior relate only to surgical resectability. Preoperative differential diagnosis between lipoma and ALT has been well studied because their clinical and image characteristics are very similar. We evaluated the factors that may differentiate ALTs from lipomas, and validated a tentative scoring system for the diagnosis of the 2 tumor types.
Forty-eight lipomas and 12 ALTs were included. The mean age, location and depth of the tumor as well as the compartment were not significantly different between the 2 groups. To evaluate the vascularity of the tumors, the average number of intratumoral vessels on pathological sections was calculated and compared between cases of lipoma and ALT.
The tumor size was significantly larger in ALT cases than in lipoma cases (P < 0.001). Magnetic resonance imaging (MRI) revealed septal structures in 91.6% of ALTs, whereas 20.8% of lipomas showed septa. Contrast enhancement in MRI was found significantly more often in ALTs (81.2%) than in lipomas (18.8%) (P < 0.001). We created a “ALT score” to discriminate between lipoma and ALT (0–6 points). ALT cases gave significantly higher point values (average 5.1 points) than lipoma cases (average 1.7 points) (P < 0.001). We found a significantly increased number of vessels in cases of ALT than in cases of lipoma (P = 0.001).
Our ALT score may help surgeons to differentiate a suspected ALT from a lipoma and could recommend a marginal resection in cases of suspected ALT. Increased intratumoral vascularity in ALT is reflected in the MRI findings and may play a key role in the acquisition of a malignant phenotype in adipocytic tumors.
Adipocytic tumors are the soft tumors most frequently encountered by orthopaedic physicians in clinics. Benign adipocytic tumors, lipomas, can be conservatively observed unless patients experience symptoms due to the presence of the mass. However, tumors that are preoperatively suspected to be lipomas, can sometimes be intermediate (locally aggressive)-type adipocytic tumors or well-differentiated liposarcoma (WDL)/atypical lipomatous tumors (ALTs). WDL is considered a low-grade malignancy that rarely metastasizes but should be carefully followed because recurrence or dedifferentiation may occur . It is recognized that WDL and ALT are essentially synonymous, describing lesions that are identical both morphologically and karyotypically, and that site-specific variations in behavior relate only to surgical resectability . The term WDL is now used for tumors of the retroperitoneum, mediastinum, and deep pelvis, whereas the term ALT includes tumors of the extremities and superficial sites. Preoperative differential diagnosis between lipoma and WDL/ALT has been well studied because their clinical and image characteristics are very similar [3,4]. Magnetic resonance imaging (MRI) is currently the most popular modality for the screening and diagnosis of soft tissue tumors, including adipocytic tumors. MRI findings of lipomas usually show high intensity in both T1- and T2-weighted images, reflecting their uniform structure with fatty tissue. In contrast, high-grade liposarcomas, including myxoid, round cell, pleomorphic, and dedifferentiated liposacrcoma, show low intensity in T1-weighted images. The MRI features of WDL/ALT are similar to those of lipoma, which makes differentiation between them difficult. In general, a larger size, deeper localization, or enhancement with contrast medium in MRI is suggestive of malignant soft tissue tumors. In this study, we evaluated the factors that may differentiate ALTs from lipomas and aimed to establish a feasible scoring system to help in the diagnosis of the 2 tumor types. Furthermore, we examined if increased vascularity in the surgical specimen could be a finding that pathologically differentiates ALTs from lipomas, and affect the clinical behavior of ALT.
We retrospectively reviewed the records of 48 patients with lipomas and 12 patients with ALT. According to the definitions of WDL and ALT, tumors of the extremities and superficial trunk come under the term ALT . In this series, we aimed to study tumors of the extremities and superficial trunk treated in our department of orthopedic surgery. Therefore, no cases of WDL were included in this study. All patients underwent surgical excision of the tumor, and a pathologist established the pathological diagnosis. Age, sex, tumor location (limb or trunk), size (diameter in MRI), and depth (superficial, subcutaneous or deep, or under the fascia), and intracompartmental or extracompartmental location were evaluated in all cases. In the MRI analysis, the presence of septal structures (more than 2 mm thick) was assessed. On fat-suppressed T1-weighted images after the administration of contrast-enhancing medium, enhancement of intratumoral lesions was evaluated in all cases. All tumors were resected by marginal resection, and pathological diagnosis was established by pathologists.
To evaluate the vascularity of the tumors, the number of vessels was counted in 10 randomly taken microscopic pictures of hematoxylin and eosin stained sections. The average number of intratumoral vessels was calculated and compared between cases of lipoma and ALT.
The average value of age and tumor size was analyzed with a Student’s t-test. All other factors were analyzed using the Chi-square test. A P value of less than 0.05 was considered significant.
The ethical committee in Kagoshima University approved the study (reference number, 353).
Summary of patient characteristics
Scoring for the diagnosis of ALT
One of the 12 ALT patients had a recurrence with dedifferentiation 4 years after resection. At the time of dedifferentiation, the entire tumor measured 28 cm and contained 5 cm of a dedifferentiated lesion. The tumor was deep-seated, had septa, and was enhanced by gadolinium in MRI (6 points by the ALT scoring system). This patient underwent wide resection of the ALT and dedifferentiated lesion. Although there was no sign of recurrence or metastasis 3 months after the resection, careful follow-up is required.
Previously, other researchers reported on the significance of septal structures in WDL/ALT [4,11]. Gaskin et al. tried to differentiate WDL/ALTs from lipomas based upon the viewpoint that simple lipomas may contain thin, discrete septa, whereas WDL/ALTs usually contain thick or nodular septa or enhancement . MRI analysis of 126 fatty masses by musculoskeletal radiologists reached the correct diagnosis in all 6 WDL/ALT cases (sensitivity, 100%); however, 10 of the suspected ALT tumor cases turned out to be variants of benign lipomas, such as chondroid lipoma, osteolipoma, or angiolipoma. The differential diagnosis of lipomatous tumors largely depended on the decisions made by the musculoskeletal radiologists. It would be useful for non-oncologist orthopedic surgeons if simplified diagnostic criteria were available. Therefore, we have created a scoring system to discriminate between lipoma and ALT by the combination of 4 values (Table 2). The score can be measured if enhanced MRI is performed (Figures 4 and 5). Based on this score, diagnosis of ALT is possible with 100% sensitivity and 77% specificity. This result is superior to MRI findings of intratumoral septa alone as a diagnostic finding, which showed 91.7% sensitivity and 74.2% specificity. Although the prevalence of hibernoma is very low, its MRI findings are similar to those of WDL/ALT. Vassos recently reported that hibernomas show spotty areas of contrast enhancement as well as prominent fibrovascular septa on MRI . Hibernomas exhibit very high standard uptake values (SUVs) on [18F]fluorodeoxyglucose (FDG)-based positron emission tomography (PET) because they contain abundant mitochondria and are highly metabolically active . One of our cases of hibernoma showed an SUV >40, suggesting that PET might be useful to distinguish hibernomas from WDL/ALTs (manuscript in preparation).
Treatment for WDL/ALT is still controversial because the recurrence rate after surgical resection of WDL/ALT is variable, ranging from 0–69% [15-17]. The recurrence of ALT in our series was seen in only 1 case (8.3%), similar to the findings in the report by Sommerville et al. showing an 8% local recurrence rate after marginal resection of 61 cases of ALT . We agree with Sommerville et al. and Kubo et al. in the idea of “conservative” surgery for ALT to preserve the major vessels or nerves . However, for recurrent ALT cases, we recommend as wide of a resection as possible because tumor margins are not usually clear and there is an increased chance of dedifferentiation. An increased number of intratumoral angiogenic vessels was revealed to be a significant factor that differentiates ALTs from lipomas in this study. Because angiolipomas are characterized by rich vasculature in mature adipose tissue, vascularity alone is not useful for the differentiation of ALTs from lipomas. As Folkmann’s group proposed, angiogenesis could be a switch that turns on the malignant phenotype in adipocytic tumors . In our study, the highest number of intratumoral vessels (21.4 vessels per field) was observed in the case of ALT recurrence, which eventually dedifferentiated. Contrast enhancement MRI definitely reflects the vascular supply in the tumor and also supports the theory.
Our ALT score (0–6 points) can be used to differentiate ALTs from lipomas based on MRI. If the score is equal to or higher than 3, we recommend marginal resection of the tumor to confirm the pathological diagnosis. Cut-off value should be validated by the future study because of the number of the case is not large in this study. Once the diagnosis of ALT is established, careful follow-up is recommended, especially for cases with increased vascularity.
We thank Shinichi Kitajima and Akihide Tanimoto for performing pathological analysis. This work was supported by JSPS KAKENHI Grant Number 24592237.
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