Cytological features of ossifying fibromyxoid tumor of soft parts.

A case of ossifying fibromyxoid tumor (OFMT) evaluated during an intraoperative pathological consultation is presented. The patient, a 70-year-old woman was being followed because of a tumor in the left buttock. Cytological smears were obtained after scrapping the tumoral cut surface and revealed a myxoid background with fragments composed of a denser, fibrillar metachromatic stroma with accompanying round to oval tumoral nuclei and no vessels. Single cells were predominantly monomorphic with a round to oval morphology and scarce cytoplasm. The most relevant feature of the tumor was its peripheral, plaque-like, calcified consistency. The review of the cytological descriptions of four cases revealed similar findings that can be presumed as those of a low-grade myxoid tumor of round to oval cells. A specific recognition of OFMT based solely on cytological features seems difficult. However, when such features are coupled with characteristic radiological findings (peripheral calcification) this entity must be considered.

Introduction

Ossifying fibromyxoid tumor (OFMT) is a rare soft tissue tumor of uncertain histogenesis first described in 1989.[1] Most cases show a benign clinical behavior but a few others exhibit a more aggressive course.[2] Typically it shows a subcutaneous location, lobular architecture and proliferation of small, bland round cells in a fibromyxoid stroma. The most characteristic feature of the tumor, which may permit preoperative recognition, is a peripheral shell of woven bone.[3-5] Cytological features of four cases have been reported.[6-9] We present a further case evaluated during an intraoperative consultation. It is our experience that cytological findings coupled with characteristic peripheral ossification seen on image studies or macroscopically may lead to suspect this peculiar neoplasm.

Case Report

The patient, a 70-year-old woman, consulted because of a slowly growing mass in the left buttock. The tumor had been present for years and lipoma was considered clinically. Magnetic resonance imaging revealed a 10-cm, well-defined, subcutaneous tumor [Figure 1]. It was vascularized and showed no adipose component so lipoma was excluded. During surgery an intraoperative pathological consultation was requested. We received a 10.5 × 7 × 6.5 cm well-defined tumor that showed a characteristic calcified, laminar consistency on its periphery. Due to these calcifications the tumor was difficult to cut. It was solid with elastic to myxoid cut surface, and gray to brown color. No areas of necrosis were seen. Smears were obtained after scrapping the tumoral surface with a surgical blade. They were air-dried and alcohol-fixed and stained with Diff-Quik and Papanicolaou, respectively. They were moderately cellular with tissue fragments, single cells and myxoid stroma. The fragments were composed of a fibrillar, metachromatic stroma with a moderate number of accompanying round to oval tumoral nuclei [Figure 2]. No hypercellular aggregates were present and capillaries were a rare finding. Single cells were predominantly monomorphic with a round to slightly oval morphology and scarce, if any cytoplasm. Occasionally, larger oval cells with more cytoplasm were present [Figure 2]. Apart from a few, large macrophages with microvacuolized cytoplasm no inflammatory cells were seen. The smears lacked necrosis or cellular atypia. The lesion was interpreted as a low-grade, mesenchymal tumor. Peripheral calcifications were so characteristic that the possibility of OMFT was raised but not mentioned in the intraoperative pathological report. Solitary fibrous tumor of soft parts was also considered. The tumor was removed surgically. Histologically, it showed characteristic lobular growth and an almost complete rim of mature bone tissue [Figure 2]. Foci of lamellar bone were also present within the tumor. Neoplastic cells were uniform with a round to oval morphology and were arranged in bridging cords and nests in a variable myxoid background [Figure 2]. No hypercellular areas, atypia or necrosis were present. Mitotic index was low, with less than 2 mitoses per 50 high-power fields. Immunohistochemistry revealed an intense expression of vimentin and S100 protein. There was no expression of CD34, smooth muscle actin, desmin, b-catenin, bcl2, glial fibrillary acidic protein, epithelial membrane antigen or cytokeratins. The proliferative index, measured with Ki67, was less than 1%.

Figure 1

Magnetic resonance image showing a well-defined, solid and heterogeneous mass located in the buttock

Figure 2

(a) Tissue fragment consisting of metachromatic, fibrillary stromawith well-defined limits, intermixed with oval nuclei (Diff-Quik, ×400). (b) Tumoral cells distributed mostly as single round to oval cells with scarce cytoplasm and occasionally larger spindle ones (Diff-Quik, ×630). (c) The characteristic rim of ossified tissue is clearly visible (H and E, ×200). (d) Tumoral cells are small with an oval morphology and are arranged in ribbons. Note the presence of myxoid stroma (H and E,×400)

Discussion

It is interesting to note that the few reports describing cytological features of OFMT reveal similar findings. Smears show a myxoid background and presence of tissue fragments and single cells. Fragments are composed of dense myxoid substance and tumoral nuclei with scarce vessels. Cells are of small to intermediate size and show round to oval morphology with few spindle ones. Cytoplasm is scarce or absent and nuclei are round or slightly oval. In the present case, samples were obtained by scrapping the tumoral surface, a method that usually renders more material than fine needle aspiration (FNA). Nevertheless, the amount of material obtained by FNA is described as moderately cellular.[6-9]

It is a common practice in the cytodiagnosis of soft tissue tumors to use diagnostic categories. Such categories are mostly based on cellular morphology (spindle, round, pleomorphic, epithelioid) and the presence of stromal elements, mainly myxoid.[10] OFMT will fit into the myxoid category with the relevant fact that most tumor cells are round to oval. The moderate cellularity and lack of pleomorphism seen on smears correlates with a benign or low grade neoplasm. There are at least four previous reports describing cytological features of OFMT [Table 1]. In the case reported by Lax and Langsteger[6] concerning a case of OFMT located in the neck, round cell morphology was so remarkable as to induce confusion with a follicular thyroid neoplasm. This report and further ones[689] also describe a peculiar pseudoacinar arrangement of tumoral cells not present in our case. There is evidence to believe in the existence of a malignant variant of OFMT.[2] These tumors show hypercellularity, nuclear atypia and high mitotic index. Such aggressive features are reflected on cytology as described in the cytological report by Minami et al.[7]

Table 1

Cytological reports describing OFMT

Differential diagnosis should be focused on myxoid sarcomas with scarce atypia. Paucicellular myxoid liposarcoma may show bland cytological features with round to oval cell morphology and myxoid stroma. Distinction from OFMT should be based on the presence of lipoblasts and characteristic network of vessels. In extraskeletal myxoid chondrosarcoma stroma is more abundant and dense and smears are more cellular showing larger cells in strands and evident cytoplasm. Low grade fibromyxoid sarcoma may show similar architectural features but neoplastic cells are spindle rather than oval. Regarding benign lesions, in solitary fibrous tumor the stroma is more collagenous and tumoral cells exhibit spindle morphology. Neurogenic tumors should be also considered. In schwannoma and neurofibroma tissue fragments may be similar, but tumoral nuclei are long and slender, often twisted, as opposed to those seen in OFMT. Finally, ring-like calcifications can also be seen in myositis ossificans, a lesion that cytologically differs from OFMT and shows a mixture of spindle stromal cells, plump osteoblasts and multinucleated osteoclastic cells.

In conclusion, the cytological features of OFMT are those of a low-grade myxoid tumor composed of round to oval cells. A specific recognition of OFMT based solely on cytological features seems difficult. However, when such features are coupled with characteristic radiological findings (peripheral calcification) OFMT must be considered.