Needle tract seeding of thyroid cancer: incidence, risk factors, and technical considerations

Fine-needle aspiration cytology (FNAC) is widely used for the diagnostic evaluation of thyroid nodules and has a well-established safety profile. Needle tract seeding (NTS) is an exceptionally rare complication reported mainly in retrospective institutional series and case reports. Reported incidence ranges from 0.02% to 0.19% across large retrospective cohorts. Papillary thyroid carcinoma accounts for most reported cases, whereas follicular and anaplastic thyroid carcinomas may be relatively overrepresented on a per-case basis, although this observation is limited by small case numbers. The latency between initial aspiration and clinical detection of NTS ranges from several months to over a decade. NTS typically presents as a superficial neck nodule along a previous needle trajectory, and histopathologic confirmation is generally required to distinguish it from benign subcutaneous lesions. Surgical excision has achieved favorable local control in most reported cases, while overall prognosis appears to reflect the biological behavior of the primary tumor. Technical measures such as small-gauge needles, limited passes, non-aspiration or hybrid sampling, and careful withdrawal may reduce theoretical risk, but direct preventive evidence remains lacking. FNAC remains a safe and essential diagnostic procedure despite this rare complication.