A rare case of nodular fasciitis presenting as a parotid tumor: Clues to cytodiagnosis

INTRODUCTION

Nodular fasciitis (NF) is a benign, self-limiting, fibroblastic/myofibroblastic tumor, commonly encountered during third and fourth decades of life.^[1] NF is a great masquerader and can be easily misdiagnosed clinically as sarcoma because of its presentation as a rapidly growing mass. Accurate characterization of the lesion is often not possible by radiological modalities such as ultrasonography (USG), computed tomography (CT) scan, and magnetic resonance imaging (MRI).^[2] On fine needle aspiration cytology (FNAC), another very important modality of preoperative diagnosis, offering a confident diagnosis of NF can be quiet challenging. Here, we report an interesting case of NF presenting as a parotid mass in a teenager with emphasis on cytomorphological features.

CASE REPORT

A 14-year-old boy presented to the otorhinolaryngology outpatient department with the complaints of painless, left preauricular swelling of 10 months’ duration. There was no history suggestive of chronic sialadenitis/toothache/trauma to the site. Local examination revealed a 3 × 2-cm, ill-defined, firm, nodular, nonmobile mass in the left preauricular area, just in front of tragus. There was no facial nerve weakness or other intraoral lesions or cervical lymphadenopathy. FNAC yielded paucicellular smears comprising singly scattered histiocyte/myofibroblast-like spindle cells and rare multinucleated giant cells (Figure 1a–c). Occasional collagenous fragments were also noted (Figure 1d). No ductal cells or acinar cells were found. A diagnosis of benign mesenchymal neoplasm was rendered. Subsequently, the patient underwent left superficial parotidectomy by external approach using modified Blair incision (Figure 1e). Intraoperatively, a multilobulated tumor mass was found attached to the superficial lobe of parotid and fibrocartilaginous framework of preauricular cartilage. The lesion was completely excised and measured 2.4 × 1.8 × 1.2 cm in toto (Figure 1f). Histopathology was suggestive of NF (Figure 2a–i). Microscopic examination showed an illcircumscribed cellular neoplasm, abutting and focally infiltrating into an otherwise normal parotid gland. The tumor comprised spindle cells arranged in short bundles and storiformpattern with interspersed multinucleated giant cells. The lesional cells were oval to spindle with bland nucleus, fine chromatin, inconspicuous nucleoli, and ill-defined cyto-plasmic borders. Few mitotic figures were noted. Also noted were foam cells clusters, plenty of extravasated RBCs, and focal areas of keloidal collagenization. On immunohistochemistry, the cells were positive for smooth muscle actin (SMA) and negative for S100, CD34, beta-catenin, and pancytokeratin. The postoperative period was uneventful, and no local recurrence was noted at 18-month’s follow-up.

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Figure 1:

(a) FNAC smear shows singly scattered oval to spindle cells against a haemorrhagic background (May Grunwald–Giemsa stain, ×40); (b) higher magnification showing the histiocyte/myofibroblast-like nature of the lesional cells with wispy amphophilic cytoplasm (May Grunwald– Giemsa stain, ×400); (c) a multinucleated giant cell on cytosmear (May Grunwald–Giemsa stain, ×400); (d) collagenous extracellular material/fragment (May Grunwald–Giemsa stain, ×400); (e) left superficial parotidectomy by external approach using modified Blair incision; (f) excised tumor along with superficial lobe of left parotid gland. FNAC, fine needle aspiration cytology.

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Figure 2:

Histomorphological features: (a) Tumor (right top) seen in juxta position to the parotid gland (left bottom) (H&E, ×40); (b) tumor focally infiltrating into the parotid parenchyma. Blue arrows point to the entrapped glandular epithelium of parotid by tumor (H&E, ×40); (c) the tumor– soft tissue interface. The surgical margins were free of tumor (H&E, ×40); (d) tumor displaying tissue culture–like appearance and storiform arrangement of cells (H&E, ×40); (e, f) oval to spindle tumor cells with bland nucleus, fine chromatin, inconspicuous nucleoli, and ill-defined cytoplasmic borders. Inset shows diffuse smooth muscle actin positivity on immunohistochemistry (H&E, ×400); (g) an interspersed multinucleated giant cell (center) (H&E, ×400); (h) a focus depicting cluster of foam cells (circled area), few cholesterol clefts (asterisk), and plenty of extravasated RBCs (H&E, ×40); (i) a focus showing keloidal collagenization (H&E stain, ×40). H&E, hematoxylin–eosin.

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DISCUSSION

NF usually presents as a painful/tender, subcutaneous lesion with rapid growth pattern.^[1] It can potentially occur any-where in the body but most commonly seen in upper extremities, trunk and head and neck region.^[1,3] NF presenting as a parotid mass is a rarity, with only a handful of case reports and case series on the record.^[4] Gibson et al analyzed 30 cases of parotid gland NF and found that male-to-female ratio was 1:1, mean age at presentation was 35.7 years, and the duration of symptoms ranged from 0.2 to 12 months.^[4] They also noted that right parotid was involved more than left (right:left = 19:6) with superficial lobe being commonly affected. The size of the lesion ranged from 0.7 to 6 cm. The clinical parameters of the current case fairly corroborate with the findings of Gibson et al.

Spontaneous resolution of NF is a commonly noted phenomenon, and therefore, arriving at an accurate preoperative diagnosis is crucial in order to avoid surgical excision.^[5,6] It should also be distinguished from its clinicopathological mimickers that occur in the parotid region. In this context, imaging tools such as USG, CT scan, and MRI lack specificity in picking up NF. NF usually displays infiltrative borders and thus can be misinterpreted as a malignant lesion radiologically^.[2] As a preoperative diagnostic modality, FNAC thus lays an important role in narrowing down the clinical differential diagnoses and ruling out certain salivary gland lesions. Wong and Di demonstrated a diagnostic accuracy of 88% for rendering a diagnosis of NF on FNAC.^[6]

The cytosmears of NF display predominant population of singly scattered or loosely cohesive sheets of spindle cellsadmixed with variable amount of myxoid stroma. Inflammatory cells, polygonal cells, myofibroblasts, ganglion cell-like cells, multinucleated giant cells, and fibrocollagenous stromal fragments can be present.^[2–4] Mild nuclear atypia and few typical mitotic figures may be encountered, and their presence should not raise an alarm.^[2,4,5] Few investigators have observed the presence of ganglion cell-like cells to be specific for the cytodiagnosis of NF.^[3,6] A ganglion cell-like cell is a large polyhedral or triangular cell (at least twice the size of a plump fibroblast) with a round to oval nuclei displaying finely dispersed chromatin, prominent nucleoli, and smooth nuclear membrane.^[6]

On cytology, NF presenting as a parotid mass may be misdiagnosed as the commonly encountered pleomorphic adenoma (PA) due to the presence of polygonal cells and myofibroblasts mimicking plasmacytoid myoepithelial cells and fibrocollagenous stromal fragments mimicking chondromyxoid substance as in the present case.^[5] In the present case, a diagnosis of mesenchymal tumor was favored over PA due to clear-cut myofibroblast-like appearance of the spindle cells with basophilic to amphophilic cytoplasm with unipolar and bipolar cytoplasmic extensions. The occasional stromal fragment encountered displayed some degree of metachromatic hue but was clearly more opaque and coarsely fibrillary in nature as opposed to the chondromyxoid substance seen in PA. However, a confident cytodiagnosis of NF could not be made in this case owing to paucicellularity, and superficial parotidectomy was performed. Table 1 provides a comparison of cytomorphological features of present case with reported literature.

Histopathological examination showed typical features of NF, but immunohistochemistry for SMA, S100, CD34, pancytokeratin, and beta-catenin was performed to rule out desmoid-type fibromatosis and deep benign fibrous histiocytoma (BFH) as the management would differ (Table 2).^[7–12]

Desmoid-type fibromatosis are locally aggressive tumors with high recurrence rate ranging from 19 to 77%.^[13] Hence, a multimodal management approach by combining surgery with chemoradiation and endocrinal therapy is often warranted.^[13] Even deep BFH can recur locally in approximately 20% of cases and rare examples of distant metastasis have been reported.^[11] S100 was performed to rule out the rare possibility of myoepithelial lesions of parotid gland with spindle cell morphology and was negative in this case. Histopathology and an appropriate panel of immunostains usually confirm the diagnosis of NF, but in rare cases with atypical morphology, fluorescence in situ hybridization, reverse transcriptase–polymerase chain reaction, or next generation sequencing to look for ubiquitin-specific protease 6 (USP6) rearrangements can be undertaken. The MYH9 gene, located at 22q12.3-q13, is the commonest somatic fusion partner of the USP6 gene in NF.8

CONCLUSIONS

NF can rarely present as a parotid mass. Every effort should be made to accurately characterize the lesion on FNAC, so that tailored treatment options with conservative approach can be provided to such patients. Clues to cytodiagnosis include spindle cell–rich smears with myofibroblastic appearance, histiocytic multinucleated giant cells, and fibrocollagenous stromal fragments.