Round Cell Tumours

Round cell tumours exfoliate readily and typically provide highly cellular, diagnostically rewarding aspirates. They contain discrete, non-cohesive cells, making them particularly amenable to FNA evaluation.

Mast cell tumours (Figure 11) consist of round, discrete cells with abundant cytoplasmic metachromatic granules. Granule density may vary with tumour grade. Eosinophils are commonly present owing to mast cell–derived chemotactic mediators.

Figure 11 – Mast cells with obvious magenta granules and some capillaries. Modified Wright’s stain, ×60 objective.

Cutaneous histiocytomas (Figure 12) form sheets of histiocytic cells with pale to lightly basophilic cytoplasm and round to indented nuclei. Regressing lesions often contain numerous small lymphocytes, reflecting immune-mediated regression.

Figure 12 – Cytological features of a histiocytoma. The smear shows a population of medium‑sized round cells with abundant pale blue cytoplasm and a few reniform to indented nuclei (Red arrow), typical of reactive Langerhans‑cell origin. Modified Wright’s stain, ×40 objective.

Other, less common, round cell tumours in the skin include lymphoma, histiocytic sarcoma, and extramedullary plasma cell tumours.

Epithelial Tumours

Epithelial neoplasms usually exfoliate in cohesive clusters, sheets, or acinar arrangements due to strong intercellular adhesion.

Sebaceous adenomas (and more commonly, nodular areas of sebaceous hyperplasia) and sebaceous epitheliomas yield cohesive groups of epithelial cells with abundant foamy, lipid-rich cytoplasm and small, uniform nuclei. The presence of mature sebocytes with minimal pleomorphism supports a benign process; however, increased numbers of smaller basal cells raise suspicion for epithelioma (Figure 13).

Figure 13 – Cytological features of a sebaceous epithelioma. The smear shows clusters and sheets of basaloid epithelial cells with some having abundant finely vacuolated, foamy cytoplasm consistent with sebaceous differentiation. Nuclei are round to oval with smooth chromatin and small nucleoli. Modified Wright’s stain, ×40 objective.

Cystic and keratinising lesions—including non‑neoplastic cysts—may yield only keratinaceous debris with scant or absent epithelial cells (Figure 14).

Figure 14 – Cytological features of keratinaceous debris from an epidermal cyst. The smear shows abundant anucleate keratin squames admixed with amorphous keratinous material. Modified Wright’s stain, ×40 objective.

Mesenchymal Tumours

Mesenchymal tumours exfoliate poorly and often produce low-cellularity preparations. When present, cells often appear spindle-shaped, stellate, or elongate, with wispy cytoplasm and oval to elongated nuclei. They frequently lie within or adjacent to pale extracellular matrix, supporting a mesenchymal origin (Figure 16).

Lipomas, a very common mesenchymal tumour, but these are not spindle-shaped, and are easily identified due to the presence of free lipid droplets and adipocytes distended with clear cytoplasm and a small peripheral nucleus (Figure 15).

Figure 15 – Cytological features of a lipoma. The smear shows numerous mature adipocytes appearing as large round cells with clear, well‑demarcated cytoplasmic vacuoles causing marked peripheral displacement of small, dense nuclei. Modified Wright’s stain, ×40 objective.

Figure 16 – Cytological features of a well‑differentiated sarcoma. The smear shows a population of spindle‑shaped mesenchymal cells arranged individually and in loose aggregates, with moderate pale cytoplasm and elongate to oval nuclei exhibiting mild anisokaryosis. Scattered extracellular matrix (collagen) may be present (red arrows). Modified Wright’s stain, ×40 objective

While cytology can strongly suggest a spindle cell tumour, differentiating benign from malignant is often unreliable, and histopathology is usually required for definitive classification.

Melanocytic Tumours

Melanocytic tumours show considerable cytologic variability. Melanin pigment may be abundant, scant, or absent (amelanotic forms), the latter making diagnosis more challenging.

Benign lesions (e.g., melanocytomas) typically contain well‑differentiated cells with minimal atypia. Malignant melanomas (Figure 17) often demonstrate marked anisocytosis and anisokaryosis, prominent nucleoli, and occasional bi‑ or multinucleation. Dense pigment granules—when present—may obscure nuclear detail. However, histopathology is required to make the distinction.

Figure 17 – Cytological features of a poorly differentiated melanoma. The smear shows a population of pleomorphic round to spindle‑shaped cells with scant to moderate basophilic cytoplasm and marked anisokaryosis. Nuclei are irregular with coarse chromatin and prominent nucleoli, and occasional binucleated or multinucleated forms may be present. Fine intracytoplasmic melanin pigment may be seen but is often sparse. Modified Wright’s stain, ×40 objective.