A mass protruding from the pancreas featuring extensive myxoid change

Histological correlation and follow-up of the present case

Histologically, the tumor displayed a patternless pattern of spindle tumor cells along with evidence of marked myxomatous change. The tumor displayed a branching vascular pattern along with perivascular hyalinization. Immunohistochemical studies gave results similar to those obtained by ICC, with the addition of positive results for STAT6 [Figure 2]. The histologic diagnosis was myxoid SFT. Furthermore, the tumor was considered to have arisen from the pancreas because of an involvement of pancreatic parenchyma in the tumor periphery within the resected specimen. The patient's postoperative course was uneventful, and no signs of recurrence have been observed during a 3-year follow-up.

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Figure 2

(a) Histologically, the spindle tumor cells had proliferated haphazardly, and thin-walled capillaries were also evident (H and E, ×200); (b) Immunohistochemical study revealed a positive result for STAT6 (×200)

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Brief review of topic

SFTs are uncommon mesenchymal tumors, and although they can occur at any location,[3] more than ten cases with a pancreatic origin have been reported.[456] These previous cases showed tendencies toward: (1) female occurrence, (2) mild clinical symptoms, (3) pancreatic head location, and (4) benign clinical course. The present tumor displayed some unusual features, as follows: (1) located in pancreatic tail, (2) formation of an extrapancreatic protruding, large mass, and (3) histologically extensive myxoid change. The present case is the first case of a myxoid variant of pancreatic SFT. In addition, the present case exhibited histologic features that were also exhibited by the myxoid SFTs at other sites reported by de Saint Aubain Somerhausen et al.[7] and Lau et al.,[8] namely, obscuring of the typical histological characteristics of SFT such as alternating areas of high and low cellularity, a patternless pattern of spindle cells, and blood vessels displaying a typical staghorn formation.

A preoperative cytomorphologic diagnosis by endoscopic ultrasound-guided fine-needle aspiration cytology may be requested in the routine clinical situation, although in the present case it was not. In typical benign SFT cases, the cytology usually exhibits: (1) variable cellularity, (2) a bloody background, (3) oval-to-spindle-shaped cells, with a uniformity of cell shape, (4) bland nuclei without nucleoli, (5) no mitotic figures or necrosis, and (6) intercellular collagen.[9] The cytologic findings in our case matched features 3, 4, and 5 mentioned above. Based on situations in which fine-needle aspiration is employed, Shidham et al. summarized the cytology findings in benign SFT as follows.[10]

1. Greater degree of cohesion

2. Cohesive clusters with haphazard overlap and lack of uniform polarity

3. Some clusters with fascicular pattern with focal suggestion of palisading

4. Bland spindle cells

5. Scant cytoplasm

6. Many naked nuclei

7. Oval-to-elongated nuclei

8. Finely granular, evenly distributed chromatin

9. Inconspicuous nucleoli

10. Insignificant mitotic activity

11. Bloody background

12. Pink, amorphous substance, consistent with collagen

13. No polygonal or epithelioid cells

14. Frequent paucicellular smears.

The cytologic findings in our case matched features 1, 2, 4, 5, 7–10, 13, and 14 mentioned above. In contrast, in malignant SFT findings of few cohesive clusters, larger number of single cells, moderate degree of nuclear pleomorphism, prominent nucleoli, mitotic figures, and high cellular smears have been reported.[10]

For the purposes of differential diagnosis, benign neurogenic neoplasm, benign lipomatous neoplasms, and low-grade GISTs were considered. In neurogenic neoplasms, such as neurilemmoma (schwannoma) or neurofibroma, spindle cell morphology with moderate cellularity and a fibrillary background are common, and nuclei or tumor cells are elongated with pointed ends and have a wavy or twisted appearance.[11] In lipomatous neoplasms, the presence of vacuolated fat cells with nuclei pushed to the periphery and delicate vessels without much fibrous tissue may be a hint for adequate diagnosis.[1112] GIST can show a wide spectrum of cytologic features, such as variable cellularity, cohesive clusters, or dispersed cells. The spindle tumor cells have oval-to-elongated or irregular-shaped nuclei, and chromatin is finely to coarsely granular. There may have fascicles in which tumor cells are orientated or organized in one direction.[13]

In the present case, SFT needs to be considered a candidate in the list for differential diagnosis by cytomorphologic findings because of the nuclear shape and a haphazard arrangement of tumor cells. However, a definitive diagnosis of SFT might be difficult. Indeed, in a cytomorphometric analysis of six SFTs reported by Gupta et al.,[14] no definitive cytologic features for diagnosis could be identified because of morphologic overlap with other tumors. In the present case, the sparse cellularity due to extensive myxoid change increased the difficulty of cytologic diagnosis. However, application of an ICC panel is useful for diagnosing SFT,[10] and a positive result for STAT6 has recently been reported to be effective.[15] Especially when paucicellular specimens are encountered, as in the present case, we consider the application of ICC to be an essential addition to adequate cytologic evaluation to narrow the field of candidates for differential diagnosis.