DIAGNOSTIC DILEMMAS

Malignant Melanoma With aMyxoid Stroma:A Diagnostic Pitfall on Fine-NeedleAspiration BiopsyDanielle Elliott, M.D., and Martha Bishop Pitman, M.D.*

Malignant melanoma (MM), both primary and metastatic, may beassociated with a prominent myxoid stromal reaction causingdiagnostic confusion on fine-needle aspiration biopsy (FNAB),most often with sarcomas that demonstrate a myxoid stroma,particularly malignant peripheral nerve sheath tumor (MPNST).We present a case of a 32-yr-old man with no past medical historywho presented with a unilateral neck mass clinically suspicious forlymphoma. FNAB produced a specimen composed of large sheetsof anaplastic cells encased in a myxoid stroma that was S100 andvimentin-positive but HMB-45-negative. A diagnosis of MPNSTwas made. Excision demonstrated a metastatic MM of unknownprimary, with a prominent myxoid stromal reaction. A repeatHMB-45 was again negative. Electron microscopy demonstratedintracytoplasmic melanasomes and cisternae of rough endoplas-mic reticulum with intracisternal parallel tubules, confirming thediagnosis. Although HMB-45 is typically negative in both tumors,S100 should be strongly positive in myxoid MM and only focal inMPNST. Diagn. Cytopathol. 2001;25:185–190.© 2001 Wiley-Liss, Inc.

Key Words:cytology; myxoid; melanoma; sarcoma; pitfall

We report on our experience with a case of metastaticmalignant melanoma (MM) with a myxoid stroma thatmimicked a myxoid sarcoma on fine-needle aspiration bi-opsy (FNAB). This rare and unusual morphologic feature ofMM has received little attention in the surgical pathology1–5

and cytopathology6–9 literature and is one that presented asignificant diagnostic pitfall, despite the well-known pro-tean nature of MM. Since FNAB is increasingly the firstdiagnostic modality of choice in mass lesions, it is importantthat this entity be familiar to pathologists who interpretFNAB specimens and that it is considered upon examina-tion of any myxoid tumor. The clinical management of apatient with metastatic melanoma is distinctly different

from that of a patient with a primary sarcoma, so accuratediagnosis is imperative for proper patient care.

Case ReportA 32-yr-old man with no significant past medical historypresented with an 8 weeks complaint of a rapidly enlargingleft neck mass. Physical examination revealed a 6.0 cm,relatively mobile mass in the left supraclavicular fossa. ACT scan confirmed the mass as solid and probably mattedcervical nodes. A thorough head and neck examinationidentified no other masses or lesions. The clinical suspicionwas lymphoma, and FNAB was requested to confirm thisdiagnosis.

FNAB of the mass was performed by an experiencedcytopathologist using a 23-gauge needle. One smear wasstained for rapid interpretation using a modified Diff-Quiktstain (using a rapid ethanol-fixed smear in Diff-Quikt stains2 and 3 rather than a methanol-rehydrated air-dried smear).This slide demonstrated small clusters and many single,large, rather polygonal cells with central malignant nuclei,prominent macronucleoli, scattered intranuclear inclusions,and rare intracytopalsmic dark, round pigment granules(Fig. C-1A). The rapid diagnosis made was malignant neo-plasm consistent with MM, and excluded lymphoma.

The clinician was very skeptical of the diagnosis, as theyoung man had no history of melanoma and did not appearto have any suspicious skin lesions. The remaining alcohol-fixed and air-dried smears were processed the followingday, using standard Papanicalaou and Diff-Quikt stains,respectively. These smears were much more cellular thanthe rapidly stained smear and showed a different cytologicalpicture than seen in the rapidly stained smear. The tumorwas present in large cohesive sheets, with very few smallgroups or single cells (Fig. C-1B). The individual tumorcells displayed large pleomorphic, anaplastic nuclei withmultiple nucleoli and intranuclear inclusions and a fibril-lary-appearing mesh of cytoplasmic processes or stromalmatrix material (Fig. C-1C). No additional intracytoplasmic

Department of Pathology, Massachusetts General Hospital, Boston,Massachusetts

*Correspondence to: Martha Bishop Pitman, M.D., Department of Pa-thology, Massachusetts General Hospital, 14 Fruit Street, Boston, MA02114. E-mail: mpitman@partners.org

Received 11 December 2000; Accepted 7 March 2001

© 2001 WILEY-LISS, INC. Diagnostic Cytopathology, Vol 25, No 3 185

Fig. C-1. A: Rapidly stained smears demonstrate large polygonal cells with central nucleus, prominent nucleolus, scattered intranuclear inclusions and rarepigment granules (Diff-Quikt stain, 3400). B: Subsequently stained smears showed anaplastic cells in large cohesive sheets, with rare single cells(Papanicalaou stain,3200).C: Individual tumor cells displayed large pleomorphic, anaplastic nuclei with multiple nucleoli and intranuclear inclusions anda fibrillary-appearing mesh of cytoplasmic processes (Papanicalaou stain,3460). D: Air-dried smears highlighted the prominent myxoid stroma(Diff-Quikt stain,3300).E: Tumor cells displayed strong staining for S100 (S100 immunocytochemistry,3200).F: Histologic section of resected lymphnodes with metastatic melanoma cells in an extensive myxoid stroma (hematoxylin-eosin stain,360).

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melanin pigment was seen. Air-dried smears demonstrated aprominent myxoid stroma (Fig. C-1D). Commercially avail-able immuncytochemical stains showed the tumor cells tobe strongly positive for vimentin (Dako Corp., Carpinteria,CA) and S100 (Dako Corp.) (Fig. C-1E), but negative forkeratin (cocktail of AE1,3, Signet Pathology, Dedham, MA,and Cam 5.2, Becton-Dickenson, San Diego, CA), leuko-cyte-common antigen (Dako Corp.), desmin (Dako Corp.),and HMB-45 (Dako Corp.). After much consultation andreview of the slides, the final diagnosis rendered was ofmalignant neoplasm consistent with peripheral nerve sheathorigin (malignant peripheral nerve sheath tumor; MPNST).

A left modified radical neck dissection was performed.Histology revealed malignant melanoma in 3 of 37 lymphnodes, with an extensive myxoid change in the connectivetissue surrounding the tumor cells (Fig. C-1F). Repeatedimmunohistochemcial stains showed the tumor cells to bestrongly vimentin, S-100, NK1/C3 (Biogenics, San Ramon,CA), a melanoma marker, and focally epithelial membraneantigen (Dako Corp.) and Leu-7 (Becton-Dickenson)-posi-tive. HMB-45, keratin, and glial fibrillary acidic protein(Signet Pathology, Dedham, MA) were negative.

Electron microscopy demonstrated tumor cells that werelarge with round to oval, irregularly shaped nuclei contain-ing pseudoinclusions and prominent nucleoli. The cyto-plasm had numerous organelles, including varying amountsof granules consistent with melanosomes ranging fromstages I–IV (Fig. 1A). The cytoplasm contained cisternae ofrough endoplasmic reticulum with intracisternal paralleltubules (Fig. 1B). This finding, although not specific formelanoma, has been described in a small percentage ofcases.10

Despite thorough intraoperative exploration of the headand neck, no primary tumor was identified. Two monthspostoperatively the patient presented with metastases to theliver, and 1 year later to the hip. He was unresponsive toradiation and chemotherapy, and expired 1 year after theinitial diagnosis.

DiscussionWe present a case of metastatic MM sampled by FNAB inwhich the presence of a myxoid stroma coupled withHMB-45 negativity by immunocytochemistry stain pre-sented a diagnostic pitfall and led to a misdiagnosis ofMPNST.

The diagnosis of malignant melanoma (MM) relies on aclassic constellation of cytologic features, the production ofmelanin, or a characteristic immunophenotype of S100 pos-itivity, keratin negativity, and positivity for HMB-45, ananti-melanoma monoclonal antibody that marks premelano-somes.11–13 Given the nonspecificity of S100,14,15 the addi-tion of HMB-45 to the roster of ancillary tests has greatlyaided in making this diagnosis, particularly for those tumorspresenting with either an unusual clinical picture or with

unusual cytologic features.16 The classic appearance of MMon FNAB is one in which a predominantly dishesive pop-ulation of large, polygonal tumor cells are present withlarge, relatively central nuclei, prominent nucleoli, andlarge intranuclear inclusions. Intracytoplasmic melanin pig-ment is supportive if present, but often it is not present.17,18

In addition to the absence of melanin, establishing thecytologic diagnosis can be complicated by an array ofmorphologic characteristics that closely mimic other tu-mors, including carcinoma, lymphoma, and sarcoma.19–22

Predominant cell cytology has led to various morphologiccategories of MM, each with its own differential diagnosis.8

The myxoid variant of MM is an extremely rare subtypeof MM first described by Bhuta et al.1 in 4 patients withmetastatic disease in 1986. Twenty-eight additional cases ofprimary and metastatic myxoid MM have since been de-scribed, less than a third of which had a myxoid primary,and all of which were diagnostically challenging due to thismorphologic feature.2–9 Rocamora et al.6 were the first todescribe this variant on FNAB cytology in a single case

Fig. 1. A: Electron microscopy of tumor cells illustrated organelles con-sistent with stage I–IV melanosomes (35,000).B: Ultrastructure of cyto-plasm revealed cisternae of rough endoplasmic reticulum with intracister-nal parallel tubules (315,000).

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report. To our knowledge, only 4 other cases have beenreported in the cytology literature.7–9 Cytologically, themyxoid stromal background, believed to be produced byreactive stromal cells rather than tumor melanocytes,1 in-troduces a differential diagnosis of myxoid sarcoma. Al-though HMB-45 is very sensitive and specific for the diag-nosis of MM, not all MM are positive for this marker.11–13,16

In fact, as in our case, myxoid MM is frequently and evenmore commonly negative for HMB-45,3,8,9 adding to thediagnostic challenges of such a case.

Myxoid soft-tissue tumors with either a similar morphol-ogy or a similar immunophentype to myxoid MM to con-sider in the differential diagnosis include myxoid liposar-coma (LPS), extraskeletal myxoid chondrosarcoma(ESMC), myxoid malignant fibrous histiocytoma (MFH),chordoma, and MPNST (Table I).

Myxoid LPS and extraskeletal myxoid chondrosarcomahave a similar immunophentype to myxoid MM; however,their cytomorphology and usual clinical presentations aresufficiently distinctive to rule out these tumors simply onthat basis. Myxoid LPS is the most common subtype ofliposarcoma, occurring in the deep soft tissues of adults,most commonly thighs, buttocks, and retroperitoneum.23

The cytologic hallmark of this tumor is the presence of anetwork of branching capillaries intimately associated withtumor cells (Fig. 2A).24 Smears of FNAB reveal this char-acteristic vasculature among small, round to oval or slightlyspindled cells with a primitive appearance, and a variablenumber of scattered lipoblasts with the classic vacuolatedcytoplasm, and a small nucleus with scalloped edges.22 Bothcell types, along with the capillaries, are embedded in amyxoid stroma. These features, when present, make the

diagnosis of myxoid LPS straightforward. Extraskeletalmyxoid chondrosarcoma (MCS) is a tumor typically deep-seated, located in the extremities and often associated withpain.23 These tumors are composed of round to slightlyspindled cells, widely scattered and arranged in cords orstrands within a thin myxoid background (Fig. 2B). Thenuclei are small, hyperchromatic, and surrounded by a rimof deeply eosinophilic cytoplasm. The cells are regular inshape and size and are infrequently arranged in whorls orsmall aggregates. Mitotic activity is scarce, correspondingto the slow-growing behavior of this lesion. The smears maybe hypercellular, but the cells tend to lack pleomorphism aswould be expected in melanoma.24 In contrast to MM, themyxoid component of chondrosarcomas stains with alcianblue in the presence of hyaluronidase.26

Myxoid MFH (Fig. 2C) consists of a pleomorphic cellpopulation ranging from stellate, lipoblast-like cells to largecells with abundant eosinophilic cytoplasm and bizarre,occasionally multinucleated cells associated with a myxoidstroma. Spindled and bizarre epithelioid cells are inter-spersed among large, hypocellular areas comprised of myx-oid material.26 This finding correlates with the inverse re-lationship between the amount of myxoid stroma and thedegree of cellularity seen cytologically.27 Cytologic prepa-rations also reveal aggregates of large, eosinophilic cellswhich resemble melanocytes and are more commonly en-countered in high-grade MFH, recurrences, and metasta-ses.28 Rarely, curvilinear capillaries may be seen.27 MyxoidMFH tends to occur in the extremities of older adults, andpresents as a slow-growing, painless mass.23,28 Staining forS-100 is typically negative, but may rarely be focal in MFH;

Table I. Tumor Profiles in Differential Diagnosis of Myxoid Malignant Melanoma

Tumor Presentation Cytology ICC

Myxoid LPS Average age 46 yr, deep soft tissues of thigh,buttock, and retroperitoneum

Uniform, small round cells with lipoblasts and “chickenwire” vascular pattern; lacy myxoid background

S1001Vim1Ker2HMB452

Extraskeletal MCS Average age 65 yr, deep soft tissues of extremities Small, round to oval cells, pleomorphic mesenchymalcells in an abundant loose myxoid stroma

S1001Vim1Ker1HMB452

Myxoid MFH Average age 65 yr, thigh, trunk, head, and neck Large, pleomorphic, bizarre cells in a myxoid background S1001Vim1Ker1HMB452

Chordoma Any age for spheno-occipital site Small, round to oval cells, physaliphorous cells in a loosemyxoid stroma

S1001Vim1Ker1EMA1HMB452

MPNST Association with VRNF in 3–35-yr range, M. F Epithelioid variant: large pleomorphic cells,6intranuclearinclusions; may contain melanin

S1001, focalVim1Ker2Des2HMB456

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strong S-100 positivity favors MM, and HMB-45 positivityin MM and negativity in myxoid MFH would certainlydistinguish the two.9,16

Chordoma enters into the differential diagnosis of thisparticular case of myxoid MM by virtue of the patient’syoung age and tumor location in the cervical neck region.This entity commonly occurs in bone of the sacrococcygealand spheno-occiput regions. Cytologic preparations typi-cally contain two populations of cells located singly and inclusters among a myxoid stromal background (Fig. 2D).29

The first cell type is the physaliferous cell: a large cell with

abundant, vacuolated cytoplasm and round to oval, irregularnuclei. The vacuoles tend to be large and occasionallydisplace the nucleus to an eccentric location within the cell.The second cell type is an epithelial-like cell with a mod-erate amount of eosinophilic cytoplasm and a small, roundto oval nucleus. These cells occur in clusters or cords, andthe myxoid stroma can be seen encircling individual cells.30

However, it is the abundance of the latter cell type and theabsence of the former with the myxoid background whichmay compel the cytologist to include myxoid MM in thedifferential diagnosis on cytological grounds. The immuno-

Fig. 2. A: Myxoid liposarcoma with a network of branching capillaries admixed with the tumor cells (Diff-Quikt stain,3200).B: Round to spindled tumorcells arranged in cords within a myxoid background, as seen in MCS (Diff-Quikt stain,3400).C: Pleomorphic cells with abundant eosinophilic cytoplasmand occasional bizarre cells are characteristic of myxoid MFH (Diff-Quikt stain,3400).D: Smears of chordoma with two cell populations: vacuolatedphysalipherous cells and epithelioid cells, both in a myxoid background (Papanicalaou stain,3200).E: MPNST with round and spindled nuclei, myxoidstroma, and rare melanin granules (Papanicalaou stain,3400).

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phentype is distinctive, however, with tumor cells of chor-doma showing keratin positivity (Table I).

MPNST (Fig. 2E) represents the most difficult tumor todistinguish from myxoid MM. The misdiagnosis of myxoidMM as an MPNST has been described as a pitfall in thedifferential diagnosis by histopathology, and even with im-munohistochemical analysis.5,8,31 MPNST may contain adistinctive myxoid background with rare melanin or mela-nosomes, and, being of neurectodermal origin, may expressantigens which react with monoclonal antibodies for HMB-45. The presence of spindled and epithelioid componentswithin the same specimen can be seen in both MM andMPNST. In contrast to MM, staining for S-100 in MPNSTtends to be focal or weak, and electron microscopy revealscytoplasmic processes among other features of Schwann-cell differentiation.32 The deep-seated location of the lesion,typically in the subcutis, a history of neurofibromatosis, andthe absence of an intraepidermal component favor the di-agnosis of MPNST.27 Melanin can be focally identified insome tumors and corresponds to melanosomes on electronmicroscopy. As such, the rare melanin granules seen on thesmear evaluated for rapid diagnosis supported the initialdiagnosis of melanoma, but were not considered contradic-tory to the final diagnosis of MPNST. In retrospect, thediffuse S-100 positivity in this case (Fig. C-1E) supportedthe initial rapid interpretation of MM.

In summary, our case represents a rare but significantdiagnostic pitfall of a myxoid stroma associated with met-astatic malignant melanoma. Although myxoid MM is typ-ically HMB-45-negative, strong rather than focal S100 pos-itivity is a helpful clue in distinguishing this tumor fromMPNST.

AcknowledgmentsThe authors thank Joanne Schiavo and Tricia Crafts for theirsecretarial assistance, and Ms. Michelle Forrestall for pho-tographic and digital image production assistance.

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