We evaluated and compared cytological findings, MDM2 amplification, and histological diagnosis to identify patient samples either as lipoma or ALT/WDL. The concordance rate for distinguishing between these two conditions based on cytologic specimens and histological findings was 76.7%, indicating that small cytological specimens can predict the whole tissue histological findings with high probability. CTs focused on pleomorphism, nuclear enlargement, and unequal size of nucleus in predicting lipoma or ALT/WDL, but the concordance rates between CTs were low for parameters such as nuclear enlargement and nuclear size. The cytological findings with a high concordance rate among CTs were strongly associated with histology and MDM2 amplification, multinucleated cells, pleomorphism, and large atypical cells. If the sum of the scores for the multinucleated cells, pleomorphism, and large atypical cells was 3 for lipoma and 4–12 for ALT/WDL, and the MDM2 amplification indicated ALT/WDL, positive sensitivity was 90.3%, negative sensitivity was 64.6%, and the total concordance rate was 80.0%. Specimens from ALT/WDL cases, showing MDM2 amplification and all three findings with score 1, showed some multinucleated cells; detailed evaluation of these findings may increase the positive sensitivity. However, few multinucleated cells were observed in lipoma cases without MDM2 amplification. A single CT rescreened a 1-cm2 area of the Papanicolaou-stained specimen in each case and counted the number of large atypical cells. As large atypical cells were not found in the lipoma cases, it was considered a highly specific cellular finding of ALT/WDL. However, the sensitivity of large atypical cells was not high. Comprehensive evaluation of multinucleated cells and pleomorphism is important for predicting the diagnosis of ALT/WDL. MDM2 amplification may be predicted using cytological specimens with even greater probability by considering the above-mentioned three cytological findings, namely multinucleated cells, pleomorphism, and large atypical cells.
In one case, the histological findings indicated ALT/WDL, and amplification of MDM2 was not observed by FISH. This case was diagnosed as ALT/WDL because of the presence of a small number of large nucleated cells. If it is difficult to identify a distinct atypical cell, MDM2 amplification by FISH must be confirmed for final diagnosis. In this case, four of six CTs presumed the sample to be a lipoma based on the cytological specimens. In a few cases of ALT/WDL, atypical cells were identified relatively easily in cytological specimens, even when it was difficult to identify atypical cells in the histological specimens (Fig. 4). Although tissue specimens can be examined over a wide area, cytological specimens can be used to evaluate nuclear atypia in more detail and may help improve diagnostic accuracy in combination with tissue specimens.
The length of the nucleus short diameter in the Papanicolaou-stained cytological specimens was significantly greater in cases showing MDM2 amplification. Additionally, a significant difference in SD values was also observed, suggesting that unequal nuclear size was prominent in the MDM2 amplification group, which may reflect as pleomorphism in cytological findings. In cases without MDM2 amplification, few cells with a nucleus short diameter greater than 7 µm were observed, but none of the cells had a nucleus short diameter greater than 9 µm.
Although most adipocytic tumors are lipoma and ALT/WDL, other adipocytic neoplasms should be included when performing differential diagnosis before surgery. These include adipocytic tumors, such as angiolipomas, myolipomas, chondrolipomas, spindle cell lipomas, pleomorphic lipomas, atypical spindle cell/pleomorphic lipomatous tumors, lipomatous myxoid liposarcomas, dedifferentiated liposarcomas, and pleomorphic liposarcomas. These tumors were not considered in the present study. Angiolipomas are generally more common among young individuals in their late teens and early 20s.8 Myolipoma is a rare tumor that does not show atypical cells, as observed in ALT/WDL.8 A chondroid lipoma is a rare tumor composed of relatively cohesive clusters of mature adipocytes and variably sized lipoblasts in a chondromyxoid matrix.9,10 Approximately 80% of spindle cell lipomas and pleomorphic lipomas arise within the subcutaneous tissue of the posterior neck, back, and shoulders.11,12 Atypical spindle cell/pleomorphic liposomal tumors in the hands and feet include mild to moderately atypical spindle cells, adipocytes, lipoblasts, and pleomorphic cells.8 These benign adipocytic neoplasms can be differentiated from ALT/WDL by examining MDM2 amplification. Preoperative diagnosis of myxoid liposarcoma is important because preoperative radiotherapy and excision methods differ from those for ALT/WDL. Myxoid liposarcoma is more common among individuals in their 30s and 40s and tends to be less pleomorphic than ALT/WDL. Identifying DDIT3 rearrangement using FISH break-apart probes is a sensitive, specific strategy for the diagnosis of myxoid liposarcoma.8 Dedifferentiated liposarcoma often indicates ATL/WDL around the tumor, as revealed by imaging findings.8 Preoperative imaging findings rarely indicate pleomorphic liposarcoma as an adipocytic tumor.13
Currently, while considering treatment strategies for adipocytic tumor, it is important to differentiate among benign lipoma, ALT/WDL with the potential for recurrence and malignancy, and myxoid liposarcoma with the potential for distant metastasis that requires wide resection. FISH examination plays an important role in differentiating among these three conditions. In the present study, the results of MDM2 amplification obtained by FISH of cytological specimens were consistent with the FISH results obtained using FFPE specimens. In two cases with no MDM2 amplification, it was difficult to detect signals by FISH using Papanicolaou-stained specimens. This may be attributed to the strong background staining. In order to avoid background signals, it is necessary to consider methods such as dividing the sample for analysis and conducting Papanicolaou staining and FISH separately. It is necessary to optimize the assay conditions to improve the accuracy of MDM2 FISH in the cytological specimens of adipocytic tumors. If FISH for DDIT3 can be performed on cytological specimens, several adipocytic tumors may be predicted from these samples (Fig. 5).
It is possible to detect genes using small specimens, and minimally invasive FNAC may be used for preoperative diagnosis of various tumors, including soft tissue tumors.5 In the era of genetic analysis of small tissue specimens, preoperative diagnosis by FNAC, a minimally invasive test, may play a major role in determining treatment plans. Familiarity with the cell morphology of cytological specimens derived from tumors is important for making a differential diagnosis. Preoperative diagnosis of lipoma or ALT/WDL can provide important information for deciding the treatment plan. In the future, it will be important to examine and analyze the diagnostic accuracy of cytological morphology in actual aspiration biopsy specimens. Given that the size of the patient dataset was small (n = 20), we will consider a follow-up study with a larger cohort. Our results may facilitate differential diagnoses for patients with lipoma and ALT/WDL and assist clinicians in making treatment decisions.