What does the presence of spindle cells in a Fine Needle Aspiration Cytology (FNAC) sample indicate?

Spindle Cells in FNAC: Diagnostic Significance and Implications

The presence of spindle cells in Fine Needle Aspiration Cytology (FNAC) samples indicates a heterogeneous group of lesions that can range from benign reactive processes to malignant neoplasms, requiring careful cytomorphological evaluation and often ancillary studies for definitive diagnosis.

Diagnostic Significance of Spindle Cells in FNAC

• Spindle cells in FNAC represent a diverse group of lesions including reactive conditions, benign neoplasms, and malignant tumors that may be primary or metastatic 1
• These cells are characterized by elongated, fusiform, or spindle-shaped nuclei with variable cytoplasm and may be arranged in fascicles, whorls, or as dispersed individual cells 2
• The finding of spindle cells requires careful evaluation as they can be seen in both benign and malignant conditions, making specific diagnosis challenging without additional studies 1, 3

Common Entities Presenting with Spindle Cells in FNAC

Benign Conditions

• Reactive processes including granulomatous inflammation, diabetic mastopathy, and inflammatory pseudotumors may show spindle cell morphology 3
• Benign neoplasms such as fibroadenomas with cellular stroma, solitary fibrous tumors, schwannomas, and granular cell tumors can present with spindle cells 1, 3
• Fibromatosis and nodular fasciitis are benign proliferative lesions that commonly show spindle cell morphology on FNAC 4

Malignant Conditions

• Primary malignancies with spindle cell components include sarcomas (leiomyosarcoma, synovial sarcoma), spindle cell carcinomas, and metaplastic carcinomas 2, 3
• Small cell lung cancer may contain spindle-shaped cells with scant cytoplasm, ill-defined cell borders, and nuclear molding 5
• Metastatic tumors, particularly melanoma, can present with spindle cell morphology in FNAC samples 3
• Thyroid carcinomas, especially papillary variants, may occasionally present with spindle cell features 5, 6

Diagnostic Approach to Spindle Cell Lesions in FNAC

• Initial cytomorphologic evaluation should assess cellularity, arrangement patterns (fascicles, whorls, storiform), nuclear features (shape, chromatin pattern, nucleoli), and cytoplasmic characteristics 2, 7
• Background elements such as necrosis, inflammation, and stromal components provide important diagnostic clues 3
• The presence of epithelial components alongside spindle cells may suggest metaplastic carcinoma or carcinosarcoma 1
• Cystic degeneration in spindle cell lesions can occur, particularly in metastatic papillary thyroid carcinoma or oropharyngeal carcinoma 5

Ancillary Studies for Definitive Diagnosis

• Immunocytochemistry is essential for accurate classification of spindle cell lesions, with markers selected based on differential diagnosis 2, 7
• Common immunomarkers include cytokeratins (epithelial), vimentin (mesenchymal), S100 (neural, melanocytic), CD34 (vascular, solitary fibrous tumor), and smooth muscle markers (SMA, desmin) 7
• Cell block preparation from FNAC material is valuable for additional immunostaining and molecular studies 5
• In cases with indeterminate cytology, molecular testing may provide additional diagnostic information 6

Diagnostic Challenges and Pitfalls

• Spindle cell lesions are often difficult to definitively classify on FNAC alone, with diagnostic accuracy rates ranging from 80-85% 3, 4
• False negative results can occur due to sampling error, particularly in heterogeneous lesions 3
• Reactive spindle cells may be misinterpreted as neoplastic, leading to false positive diagnoses 4
• Certain entities like nodular fasciitis can mimic low-grade sarcomas cytologically 4
• Cystic degeneration in spindle cell lesions can lead to paucity of diagnostic cells, requiring repeat aspiration or image-guided sampling 5

Clinical Approach to Spindle Cell Findings

• When spindle cells are identified in FNAC, correlation with clinical and radiological findings is essential 3
• For thyroid nodules with spindle cell features, ultrasound characteristics should be carefully evaluated for suspicious features 6, 8
• In cases of inadequate or indeterminate FNAC results, repeat FNAC with image guidance or excisional biopsy may be necessary 5
• For cystic lesions with spindle cells, sampling of solid components or the cyst wall is recommended to improve diagnostic yield 5

Conclusion for Clinical Practice

• The finding of spindle cells in FNAC requires careful cytomorphologic evaluation, correlation with clinical and radiological findings, and often ancillary studies for definitive diagnosis 1, 3
• A specific diagnosis can be rendered in approximately 80-85% of spindle cell lesions by FNAC when appropriate ancillary techniques are employed 3
• In cases where a definitive diagnosis cannot be established by FNAC, excisional biopsy should be considered, particularly when malignancy cannot be excluded 4