Fine needle aspiration cytology in cancer diagnosis

Fine needle aspiration cytology (FNAC) entails using a narrow gauge (25-22G) needle to collect a sample of a lesion for microscopic examination. It allows a minimally invasive, rapid diagnosis of tissue but does not preserve its histological architecture. In some cases this limits the ability to make a definitive diagnosis. As with any invasive procedure there are risks, and as with all diagnostic tests involving sampling and interpretation, important diagnoses can be missed. False negative results and occasional complications of the technique have been reported as proof that it is “useless and dangerous.”¹ However, accuracy and complications need to be compared with robust published data about alternative techniques before abandoning fine needle aspiration. Clinicians require clear communication with the cytopathologist to ensure that the procedure is appropriate for the question being addressed and that both understand the answer in the same terms. The rapid diagnosis possible with fine needle aspiration can shorten or avoid hospital admissions, and speed a patient's route to an appropriate specialist. In one study examining the impact of a FNAC service on the management of patients, the savings exceeded the cost of the service by a factor of four.² Used sensibly, aspiration cytology offers a relatively cheap, quick, and accurate tool for the diagnosis and follow up of cancer.³

Published series of FNAC show a wide variation in diagnostic accuracy and complication rates.⁴ This reflects the efficacy of the technique for the particular site being sampled and the expertise of those taking and examining the samples. Specimens taken by individuals who use the technique only occasionally are often of poor quality.⁵ A study of 5226 fine needle aspiration samples taken from the six commonest sites by cytopathologists and clinicians showed inadequate rates of 15-30%, depending on the site, for all aspirators but an overall rate of 12% when the sample was taken by a cytopathologist.⁶

Cytology is not the same as histology. The published literature contains an abundance of case reports describing diagnosis after fine needle aspiration of various rare conditions. Although some unusual diseases have characteristic appearances that an expert cytopathologist may recognise, many reports of more subtle diagnoses represent retrospective assessment of the cytology once a histological diagnosis is known or cases where the final diagnosis was one of a list of possibilities raised by the cytology. Professionals using fine needle aspiration need to have a realistic approach to what is achievable. The cytopathologist should indicate the degree of certainty when offering a diagnosis, making clear when a clinical or radiological correlation is required and when histological confirmation is necessary. The clinician should recognise when fine needle aspiration has narrowed down the possibilities and when the diagnosis has been confirmed. This is particularly important when FNAC is used to make a primary diagnosis of malignancy.

In symptomatic breast disease, FNAC used alongside clinical and radiological assessment allows rapid, inexpensive, and accurate diagnosis.³ However, review of published series has shown that core biopsy with histology is more sensitive and specific than fine needle aspiration in diagnosing most impalpable radiological lesions.⁷ Histology facilitates the diagnosis of benign lesions, allows assessment of whether carcinoma is invasive or in situ, and gives some indication of grade and subtype of carcinomas.

Thyroid nodules are common; most are part of the spectrum of nodular goitre. FNAC offers direct sampling and identification of the minority that are definite or probable malignancies and those that are follicular neoplasms, which require surgery with full histological assessment to exclude malignancy. Fine needle aspiration is most sensitive at detecting anaplastic (almost 100%) and papillary (around 90%) carcinomas.^8-10 Although FNAC has greater accuracy in identifying tumours than alternative imaging or biochemical methods, it misses 5-10% of cancers. Even so, its incorporation into the diagnosis of thyroid nodules reduces the requirement for excision by at least 25% and doubles the yield of cancer in those that are excised. Core biopsy is more traumatic and has not been shown to increase accuracy of diagnosis.

Diagnosing metastatic or recurrent malignancy by FNAC generally has a high specificity and sensitivity.³,¹¹ Many lymph nodes and other metastatic sites are readily accessible, and the ability to match the new lesion with the previous primary reduces the opportunity for error. However, in diagnosing metastases with no known primary, such as liver metastases as a first presentation, consideration should be given to the need for histological material. Cytology is ideal for confirming metastasis from a clinically or radiologically suspected primary site and distinguishing between limited alternatives such as small cell or non-small cell lung cancer. However, if there is no indication of a possible primary site, core biopsy facilitates more thorough assessment of a tumour's architecture and immunohistochemical profile, allowing better prediction of origin and prognosis. The trauma and risks of biopsy are greater than for fine needle aspiration,¹¹ and we need to consider for each patient which technique is most suitable and how the result of any invasive test would alter management.

Used appropriately FNAC remains a powerful tool in the diagnosis and management of patients with malignancy. A realistic approach to what is achievable, with clear communication between clinician and cytopathologist, is vital.