Well-characterized as "pleomorphic adenomas" in salivary glands, mixed tumors may also occur in the skin and soft palate, and have been recently recognized to be primary in soft tissue [1, 2]. Microscopically, mixed tumors of soft tissue show similar morphologic feature to their salivary gland counterparts. Mixed tumors, myoepitheliomas, and parachordomas originated from the soft tissue share many clinical and histological features, all of which are considered within the same spectrum. The vast majority of these tumors arise in the subcutaneous or deep subfascial soft tissue of extremities, more frequently in the upper than the lower , usually affect adults with a significant number of children [3, 9]. Only 18 cases of mixed tumors of soft tissue were reported , and 33 percent of those were described to meet histological criteria for malignancy resulting in local recurrence or metastasis .
In the present case, diagnostic differential considerations include other benign lesions, such as neurilemoma of degenerative type and organized hematoma that may radiologically demonstrate a granular pattern of low in high signal intensity on T2-weighted MRI images indicating calcification or hemorrhage within the tumor. However, these diagnoses were excluded by the histological finding of tumor cells showing both epithelial and mesenchymal characteristics. Given the morphologic heterogeneity of this tumor, the differential diagnosis is quite broad and depends in large part upon the predominant cell type and stromal component [10, 11]. Other candidate was synovial sarcoma with both epithelial and mesenchymal lesion in the tumor expressing bcl-2. Although the present case showed strong bcl2 expression in the cytoplasm, however diagnosis of synovial sarcoma was deniable as the epithelial cells showed myoepithelial marker p63. Metastasis of myoepithelial carcinoma originated from other organs, such as head and neck might be excluded by the benign histology of the tumor.
This is the first report of Tl-201 scintigraphic features in a benign mixed tumor of soft tissue in our reviewing literatures. It is generally considered that Tl-201 uptake around tumor cells mainly depends on the volume of blood flow and blood pools . In the second step, Tl-201 has similar ion-radius and physical effects similar to potassium , having five times the affinity to cell as potassium, and is transported into the cell instead of potassium as a result . The preferential transportation of Tl-201 mainly depends on Na+/K+-ATPase, which is overexpressed in a variety of malignant tumors [5–8] and some benign tumor cells . In the present case, although vascularity of tumor was not high by morphological assessment, imunohistochemical study revealed expression of Na+/K+-ATPase in most tumor cells. This result may account for a mechanism of the early uptake and continuous accumulation of Tl-201 in a benign mixed tumor of soft tissue.
Conclusive significance of thallium-201 scintigraphy in soft tissue tumors has been still controversial within our knowledge. Positive examination often indicates malignant tumors, but some benign tumors are occasionally estimated as positive [15, 16]. Accumulation of Tl-201 at delayed scan has been reported as a more sensitive finding distinguishing malignancies from benignancies . In our case, we considered the continuous accumulation of Tl-201 at delayed scan as a potential sign indicating malignancy, although it was denied by histological examination including immunohistochemistry.