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Dendritic cell tumor in a salivary gland lymph node: a rare differential diagnosis of salivary gland neoplasms

Dendritic cell tumors are extremely rare neoplasms arising from antigen-presenting cells of the immune system. We report a case of a 69-year-old man with an unremarkable medical history who presented with a 2-months history of a gradually enlarging painless, firm, mobile, 2 × 2-cm swelling at the caudal pole of the left parotid gland without systemic symptoms. Histologically, the tumor consisted of a spindle cell proliferation in an intraparotideal lymph node. Based on the histopathologic, immunohistochemical and electron microscopic findings, a dendritic cell tumor, not otherwise specified (NOS) in an intraparotideal lymph node was diagnosed. The patient underwent complete tumor resection, and is currently free of disease, 2 years after surgery. These extremely rare tumors must be distinguished from other more common tumors in the salivary glands. Awareness that dendritic cell tumors may occur in this localization, careful histologic evaluation and ancillary immunohistochemical and electron microscopical analyses should allow for recognition of this entity.


Background
Dendritic cell sarcomas (DCS) are exceedingly rare entities, arising from antigen-presenting cells of the immune system. DCS are subclassified into the better characterized follicular (FDCS) [1] and interdigitating (IDCS) [2] dendritic cell sarcomas and other rare and less well classifiable dendritic cell tumors like fibroblastic reticular cell tumors, indeterminate dendritic cell tumors and dendritic cell tumors, not otherwise specified (DCT, NOS) [2]. DCS was first described in 1986 by Monda et al. [3]. Since then, nearly 300 cases, most of them FDCS, have been described in the literature. Although most DCS evolve in cervical, mediastinal, axillary and inguinal lymph nodes, also extranodal manifestations have been described [4]. The clinical behaviour of DCS is similar to that of low-grade soft tissue sarcoma, with an approximately 30% overall risk for developing local recurrences and metastases [5]. Because of the rareness of the disease a standardized treatment is lacking.
We herein report a case of a dendritic cell tumor, NOS of an intraparotideal lymph node, emphasizing the important role of ancillary immunohistochemical and molecular studies in establishing this extraordinarily rare diagnosis.

Case report
A 69-year-old man presented with a 2-months history of a gradually enlarging painless, firm, mobile, 2 × 2 × 2 cm swelling at the caudal pole of the left parotid gland without systemic symptoms. His medical history was unremarkable.
Magnet resonance imaging (MRI) showed a 2 × 2 × 2 cm mass with hyperintense signal on T2-weighted images and hypointense signal on T1-weighted sequences and a contrast enhancement which bordered directly to the lateral part of left sternocleidomastoid muscle and displaced the external jugular vein dorsally. Cranially there was no clear demarcation to the left parotid gland (Figure 1). The patient underwent surgical excision of the swelling by a partial left parotidectomy with preservation of the facial nerve. Because of insecure R0-status a follow-up resection with extended partial parotidectomy and ipsilateral selective neck dissection (levels II and III) was conducted. A primary tumor of the upper aerodigestive tract was excluded by panendoscopy. Subsequent total body positron emission tomography with 18-F-fluorodesoxyglucose and computed tomography scan 6 months after surgery were unremarkable. The patient is currently disease free, 2 years after surgery.

Material and methods
The specimen was fixed in 10% buffered formalin, paraffin-embedded, and histologic sections were obtained. Sections were routinely stained with hematoxylin and eosin. Immunohistochemical staining was performed on formalin-fixed, paraffin-embedded tissue sections on an automated immunostainer (Ventana Medical Systems © , Tucson AZ) following the manufacturer's protocols. The monoclonal antibodies used are listed in Table 1.
For electron microscopical (EM) evaluation, small pieces of tissue were dissected out of the paraffin bloc, rehydrated stepwise and postfixed in 1% OsO 4 in 0.1 M cacodylate buffer (pH 7.4) and then again dehydrated in an ethanol series (50, 70, 96, 100%). The 70% ethanol was saturated with uranyl acetate for contrast enhancement. Dehydration was completed in propylene oxide. The specimens were embedded in Araldite (Serva © , Heidelberg, Germany). Ultrathin sections were produced on a FCR Reichert Ultracut ultramicrotome (Leica © , Bensheim, Germany), mounted on pioloformcoated copper grids, contrasted with lead citrate and analyzed and documented with an EM10A electron microscope (Carl Zeiss © , Oberkochen, Germany). The pictures were digitized and processed with Adobe Photoshop.

Gross and histological findings
The specimen consisted of parotid gland tissue containing a single firm, nodule 2.3 cm in diameter. The cut surface was soft and homogeneous light-grey. Microscopic examination revealed a spindle cell neoplasia in an intraparotideal lymph node. Tumor cells featured moderate nuclear pleomorphism, bulked chromatin and a clearly perceptible nucleolus (Figure 2A, B). Furthermore an elevated mitotic rate (up to 27 mitoses per 10 high power fields) was found.

Immunohistochemical Findings
To further characterize the histogenesis of this tumor, immunohistochemical stainings were performed. The tumor cells were positive for vimentin, fascin and HLA-  DR ( Figure 2B, D). In contrast, the neoplastic cells were negative for all epithelial, melanocytic, lymphoid, histocytic and follicular dendritic cell markers used. The proliferation, as assessed by Mib1 index was 40-50%. The results of the immuohistochemical stainings are shown in table 1.

Electron Microscopic Findings
The electron microscopical aspect of the tissue was characterized by cells which were devoid of characteristic structures as desmosomes or Birbeck granules. There was a certain heterogeneity of nuclei concerning their amount of heterochromatin. However, the cytoplasm of the cells (size, content of mitochondria or endoplasmatic reticulum) was less well differentiated ( Figure 3A). The most prominent finding was the large number of slender long-spacing collagen fibers dividing the tissue in many compartments ( Figure 3A). Fibrous long-spacing collagen the period length of which was in the order of 200 nm in this case ( Figure 3B), is not described in the literature to be specific for any disease, but has been found in a variety of normal and pathological tissues. Therefore, it cannot be used as a pathognomonic marker here.
In summary, the morphological and immunophenotypic features were consistent with a dendritic cell tumor, not otherwise specified (NOS) originating in an intraparotideal lymph node. A differentiation towards a follicular or interdigitating dendritic cell sarcoma could not be demonstrated.

Conclusions
Dendritic cell sarcomas (DCS) are rare neoplasms arising from antigen-presenting cells of the immune system [2]. DCS are being recognized with an increasing frequency. These tumors can occur, where dendritic cells are located. One of the most common manifestation sites are the head and neck region. Under-recognition of this entity may contribute to its rarity. The WHO subclassifies DCS into five groups [2]: Langerhans cell histiocytosis, Langerhans cell sarcoma, interdigitating dendritic cell sarcoma (IDCS), follicular dendritic cell sarcoma (FDCS) and other rare and less well classifiable dendritic cell tumors like fibroblastic reticular cell tumors, indeterminate dendritic cell tumors and dendritic cell tumors, not otherwise specified (DCT, NOS) [2]. FDCS is the most common subtype followed by IDCS.
The clinical course of this neoplasia is similar to that of soft tissue sarcomas [3,6]. Since only 306 cases of DCS have been reported worldwide (table 2), there is no general consensus on the treatment of this rare disease. Surgery has remained the gold standard in localized disease, but a recurrence rate up to 50% has been reported [5]. Alternatively some DCS have also been treated additionally by radiotherapy and or chemotherapy [7]. For our patient we chose the option of surgical therapy and he is well and free of disease for 2 years after surgery now.
For establishing the diagnosis a large number of immunohistochemical stainings and ultrastructural analyses has been performed. Other rare differential diagnosis of salivary glands, like lipomatous pleomorphic adenoma [8] or sebaceous adenoma [9], were excluded. Despite this extensive work-up a definite assignation to an established entity of dendritic cell tumors could not be achieved. Therefore a diagnosis of dendritic cell tumor, NOS was made.
In summary, we report on the clinical, pathologic, and immunohistochemical features of a dendritic cell tumor, NOS in an intraparotideal lymph node. Metastatic disease was ruled out by PET-and CT imaging. Recognition of this extremely rare tumor, and its distinction from more common malignant tumors of lymph nodes and salivary glands largely depends on judicious application of immunohistochemical and ultrastructural techniques. The precise histogenesis of these extraordinarily rare tumors remains to be established.

Consent
Written informed consent was obtained from the patient for publication of this Case Report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.