CHAPTER 14 Skin and subcutis

Clinical aspects

The place of FNAC in the investigative sequence

Fine needle biopsy (FNB) and cytodiagnosis has found only limited application in primary tumors of the skin and subcutis due to the ease of surgical biopsy and of excision. The main indications for FNB are rapid, non-invasive investigation of suspected metastatic malignancy, and distinction between neoplasia and a reactive process likely to resolve spontaneously or respond to conservative treatment.^1,² In patients with known malignancy, the nature of any nodules or thickenings related to surgical scars or elsewhere in the skin or subcutis the distinction can easily be made between suture granuloma, infection or other reactive process and recurrent or metastatic tumor. The possibility of a second primary or of de-differentiation of the original tumor can also be decided by cytology.

Some knowledge of the cytological features of skin adnexal tumors is needed to avoid mistaking these for metastatic malignancies. With some exceptions, a type-specific diagnosis of primary adnexal tumors by FNB is difficult or impossible but is not often of practical importance. From a clinical point of view, the information sought is mainly if the lesion is benign or malignant, and if malignant whether it is primary or metastatic. Cytology as a supplement to history and clinical findings can be useful in the management of patients with skin tumors.

Incisional biopsy or punch biopsy of pigmented lesions is generally contraindicated in view of the risk of spreading melanoma and of interfering with the histological diagnosis and staging on subsequent excision. Although not proven, FNB could have similar adverse effects and is therefore not recommended for localized pigmented skin lesions. However, since pigmented lesions are occasionally examined by FNB, cytopathologists must be familiar with the cytomorphology of melanocytic skin lesions (see below).

Scrape smears from tumors involving the epidermis or a mucous membrane is an alternative to biopsy in some cases, e.g. to confirm suspected squamous cell carcinoma, basal cell carcinoma or Paget’s disease. Scrape smears can also be used to confirm advanced ulcerated malignant melanoma. Dermatologists seeking an alternative treatment to surgical excision of basal cell carcinoma have shown some interest in cytological confirmation of this diagnosis.

Subcutaneous lipoma is one of the commonest ‘tumors’ seen in a community-based cytology practice. It may seem unnecessary to subject these clinically characteristic lesions to biopsy, but the patient and sometimes the referring doctor are often concerned and demand immediate reassurance. We have had some surprise findings in such cases and feel that FNB is generally justified. One example of several: a small, discrete, soft nodule was incidentally felt in the lower anterior abdominal wall of a 45-year-old woman. The clinical diagnosis was lipoma, but cytology showed a highly cellular, sarcomatous tumor, histologically confirmed as synovial sarcoma.

In general, FNB is not useful in the investigation of inflammatory skin disease except to provide material for microbiology, for example in tuberculosis and leprosy.

Accuracy of diagnosis

Only a few series of FNB of primary skin tumors with histologic correlation have been reported.³^–⁸ In one of the largest series, 89% of primary skin tumors were correctly diagnosed as benign or malignant, and specific typing was possible in 81% of cases.⁴ Some skin adnexal tumors, in particular pilomatrixoma, have been reported as diagnostic pitfalls and a possible cause of false-positive cytological diagnosis.⁹ Clinical correlation is essential.

Complications

The hypothetical risk of tumor cell seeding and of compromising subsequent histological examination as a result of FNB has been mentioned. Other significant complications have not been reported.

Technical considerations

We generally use 25- or 27-gauge needles and the non-aspiration technique for skin tumors, 23–25-gauge for subcutaneous lesions. Local anesthesia is rarely necessary. Two to four passes are usually required to obtain sufficient material and to compensate for possible tumor heterogeneity. Alcohol-fixed H&E- or Pap-stained smears and air-dried smears stained with Diff-Quik or MGG should be used in parallel. FNB of skin lesions often yield cohesive tissue fragments. Alcohol-fixation makes tissue fragments transparent whereas cell detail is poorly seen in fragments in air-dried smears. Squamous differentiation is more easily recognized in Pap-stained smears. On the other hand, air-dried MGG smears provide more information on secretory products and stromal components.

Thin superficial skin lesions, particularly if eroded or ulcerated, are best sampled by scraping cells off the surface. Keratin, crust and inflammatory exudate must be removed to ensure that intact, well-preserved cells are obtained from as deeply into the lesion as possible. Scraping with a sterile scalpel blade held at a blunt angle until the lesion bleeds slightly is recommended.¹⁰ Wooden spatulas or other soft materials are less suitable since they tend to absorb the fluid component. FNB can be successful in some thin lesions using a 25–27-gauge needle inserted tangentially.

Cytological findings

Non-neoplastic lesions

Inflammatory processes

Purulent material aspirated from suppurative inflammation or abscess can be used for microbiological studies. The presence of many mature and keratinized squamous epithelial cells suggests an inflamed epidermoid cyst or suppurative hidradenitis, hair shafts indicate a dermoid cyst or a pilonidal sinus. Demodex may be identified in scrape smears of infectious folliculitis.¹¹ A malignancy, mainly squamous cell carcinoma can be obscured by inflammatory exudate and smears should be examined carefully for single neoplastic cells.

FNB samples from a non-suppurative chronic inflammatory process usually contain fragments of inflammatory/reparative granulation tissue composed of clustered, relatively cohesive reactive fibroblasts and histiocytes, strands of endothelial cells representing capillary vessels, and various inflammatory cells. Granulation tissue can be quite cellular in smears, macrophages and fibroblasts may look atypical and mitotic figures are not uncommon, but the overall findings in the appropriate clinical context are characteristic.

Inclusion bodies may be found in scrape smears of viral skin lesions. The cytological features of herpes virus infection are well known from gynecological cytology. Molluscum bodies of molluscum contagiosum look similar in smears and in histological sections (Fig. 14.1).

Fig. 14.1 Molluscum contagiosum

Three darkly staining molluscum bodies and some inflammatory cells (MGG, HP).

Clusters of epithelioid histiocytes with poorly defined cytoplasm and pale nuclei of a banana-, bean- or footprint-like shape, associated with multinucleated histiocytic giant cells are consistent with granulomatous inflammation. The differential diagnosis includes a number of conditions: foreign body granuloma, tuberculosis, leprosy, sarcoid, fungal infections, etc.¹² Multinucleated histiocytic giant cells are prominent in foreign body granuloma, and the presence of birefringent particles, suture material, etc. supports the diagnosis (Fig. 14.2). Caseous necrosis suggests tuberculosis, to be confirmed by staining for acid-fast bacilli; fibrosis with no evidence of necrosis is more in keeping with sarcoid. The cytological patterns found in leprosy have been described.^13,¹⁴ The etiology of granulomatous inflammation cannot be decided on the basis of cytomorphology alone and must be pursued by staining for microorganisms, bacteriological culture, serological tests, and other investigations.

Fig. 14.2 Sarcoid-like foreign body granuloma

(A) Large multinucleated giant cells, clustered histiocytes and some inflammatory cells. Birefringent foreign material present but not seen in this microphotograph (Pap, IP); (B) Corresponding tissue section. Sarcoid-like cluster of many discrete granulomata of epithelioid histiocytes and some giant cells; no evidence of caseation; foreign bodies seen in polarized light (H&E, LP).

A vaguely granulomatous chronic inflammatory pattern with a background of fat and evidence of fat necrosis seen in FNB smears of a subcutaneous induration on the leg suggests a panniculitic process, such as erythema nodosum.¹⁵ The findings are not diagnostic, only suggestive, and necessitate a surgical biopsy. Cytological findings in fat necrosis are described in Chapter 7 (see p. 167).

Cysts and other non-neoplastic lesions

Aspirates from epidermal or dermoid cysts consist of thick, greasy, foul-smelling material. Smears show mature squamous epithelial cells, a high proportion of which are keratinized cells or ghost cells, and a background of debris and often inflammatory cells. Foreign body giant cells and calcium granules may be present (Fig. 14.3). The presence of hair shafts suggests a dermoid cyst.¹⁶ Reactive epithelial atypia in inflamed cysts can look worrisome.⁴ On the other hand, cells of partly necrotic and degenerate, well-differentiated metastatic squamous cell carcinoma can look deceptively bland in FNB samples. The content of trichilemmal cysts is similar, but clumps of structureless keratin are more prominent, there are no granular or parakeratotic cells and cholesterol crystals are often seen. Cytological findings in proliferating trichilemmal cyst and malignant proliferating trichilemmal tumor have been described.^17,¹⁸

Fig. 14.3 Epidermal cyst

Clumps of keratin and squamous cells, multinucleate histiocytes, inflammatory cells and debris (Pap, IP).

A postoperative lymphocele yields clear, yellowish, mucoid fluid on aspiration which contains a variable number of mature lymphocytes. Smears from a hematoma show altered blood, hemosiderin-containing macrophages, amorphous material with cholesterol crystals, or fragments of reparative granulation tissue with hemosiderin pigment, depending on the age of the process.

The material aspirated from a ganglion is thick, colorless, glassy and jelly-like. It is so characteristic that the diagnosis is already obvious from the macroscopic appearance of the aspirate in the appropriate clinical setting. Smears show a small number of single cells with abundant cytoplasm and small oval nuclei and a background of abundant myxoid material, which may show interesting drying artifacts (Fig. 14.4).¹⁹ FNB of bursal cysts has been described as similar to that of ganglion.²⁰

Fig. 14.4 Ganglion

A single histiocyte-like cell; background of myxoid material showing peculiar drying artifact (MGG, HP).

A few cases of rheumatoid nodules with FNB findings have been reported.^21,²² Samples are scanty of amorphous granular acidophilic material with a variable number of fibroblasts and/or histiocytes. Small multinucleated histiocytic giant cells may be seen. Gouty tophi are sometimes subjected to FNB if the clinical diagnosis is doubtful. Thick, putty-like material is aspirated. Smears show clumps of non-cellular material of birefringent needle-shaped crystals that are better preserved than in formalin-fixed tissue, and a few histiocytes and giant cells (Fig. 14.5).^21,²³

Fig. 14.5 Gouty tophus

Clumps and more thinly spread crystalline material (A, MGG, IP; B, polarised light, IP).

Calcinosis of subcutaneous or soft tissue may present as a mass lesion. It is usually of dystrophic or metabolic etiology but there is also a rare, primary inherited form called tumoral calcinosis.²⁴ FNB yields amorphous calcified material, laminated concretions and variable numbers of histiocytes, lymphocytes and osteoclast-like giant cells.

Endometriosis can present as a poorly defined, tumor-like induration in the subcutaneous tissue or in relation to a scar of the anterior abdominal wall in a premenopausal woman. Cytological findings are described in Chapter 13 (see page 357).²⁵

Amyloid tumor of subcutaneous tissue may also present as a mass lesion. FNB smears display fragments of acellular amorphous matrix, scattered histiocytic cells and occasionally small calcifications. FNB of periumbilical adipose tissue of the anterior abdominal wall can be of value in the diagnosis of secondary systemic amyloidosis although sensitivity is low.^26,²⁷ Rings of amyloid around fat cells and amyloid deposits in vessel walls can be demonstrated by Congo red staining and polarization.

Benign epithelial skin tumors

The cytology of benign primary epithelial skin tumors has mainly been described in single case reports, and criteria are not well established. A wide range of tumors occur in this site and specific entities are difficult or impossible to identify in smears. Clinically, the distinction between benign and malignant tumors, particularly those of metastatic nature, is the most important, and this is possible by FNB with a high level of accuracy. In general, cell-rich smears of more or less cohesive small basaloid cells mixed with a variable number of squamoid cells and sometimes glandular elements suggest a primary cutaneous neoplasm, and close study of the nuclear cytomorphology is likely to predict malignancy. The microarchitectural pattern and certain stromal components can provide clues to a type-specific diagnosis in some cases.

Our limited experience does not permit a systematic description of the cytology of skin adnexal tumors. Some examples selected from our files are briefly described and illustrated and references are provided to relevant case reports and small series published in the literature.

Adenomatous tumors of sweat gland origin

Smears of cutaneous cylindroma (’turban tumor’) are usually cell-rich, of pseudopapillary fragments of cohesive basaloid epithelial cells and hyaline stromal material often seen as globules. The cells are small, crowded with a high nuclear : cytoplasmic ratio and relatively uniform round or ovoid hyperchromatic nuclei. The pattern can closely resemble adenoid cystic carcinoma but the nuclear chromatin is bland (Figs 14.6 and 14.7). Cutaneous cylindroma should be remembered in the differential diagnosis of tumors in the head and neck.²⁸

Fig. 14.6 Cutaneous cylindroma

Pseudopapillary tissue fragments of cohesive bland basaloid cells; hyaline stromal globule (A, MGG; B, Pap, IP); (C) Corresponding tissue section (H&E, IP)

(Courtesy Dr J. Klijanienko, Inst. Curie, Paris).

Fig. 14.7 Cutaneous cylindroma

Clustered basaloid epithelial cells; mild nuclear atypia; hyaline stromal globules; note similarity to adenoid cystic carcinoma (MGG, HP)

(Courtesy Dr K. Lindholm, Malmo, Sweden).

Spiradenoma is closely related to and resembles cylindroma. Smears are highly cellular, of clustered, variably cohesive small basaloid epithelial cells with uniform oval dark nuclei and a homogeneous chromatin. A more or less obvious acinar/tubular arrangement of the cells is discernible, but a dual population of small dark and larger pale epithelial cells as seen in histological sections is difficult to appreciate in smears. Globules of hyaline stromal material are characteristic but were scant in the case illustrated here (Fig. 14.8). The main differential diagnosis is adenoid cystic carcinoma.²⁹

Fig. 14.8 Eccrine spiradenoma

(A) Clustered and dissociated small basaloid cells with relatively uniform dark nuclei; suggestion of acinar/tubular grouping; minimal amount of hyaline stroma (MGG, IP); (B) Corresponding tissue section (H&E, IP).

The cytology of nodular hidradenoma³⁰^–³² and of clear cell hidradenoma^33,³⁴ has been described in single cases. A benign skin adnexal tumor from our files, histologically reported as cystic eccrine hidradenoma, is shown in Figure 14.9. The FNB sample was of mucoid fluid which contained clusters of variably cohesive uniform epithelial cells with a moderate amount of cytoplasm and small dark ovoid nuclei. A dual population was not discernible. The cytology was reported as a benign skin adnexal tumor without further specification.