Rarely do medical procedures have non-zero risks of side effects; medical (and pharmaceutical) professionals these days are disclosing every possibly side effect making it more difficult to filter down to those that we should be concerned about. It does not help when someone posts these rare side effects associated with the terms "spreads like wildfire". I doubt the medical professionals that related this side effect to you used these terms to describe the likely risks of "seeding cancerous cells" during a fine needle aspiration. Perhaps in the future you should provide better context for risks associated with procedures, pharmaceuticals, etc; of course that would not be as sensational or provocative.
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So for context; here is a literature review indicating a non-zero risk:
"Needle track seeding following biopsy of liver lesions in the diagnosis of hepatocellular cancer: a systematic review and meta-analysis"
http://gut.bmj.com/content/57/11/1592.short
Results: Eight studies identified by systematic review on biopsy of HCC were included in a meta-analysis. The pooled estimate of a patient with seeding per 100 patients with HCC was 0.027 (95% confidence interval (CI) 0.018 to 0.040). There was no difference whether a fixed or random effects model was used. Q was 4.802 with 7 degrees of freedom, p?=?0.684; thus the observed heterogeneity was compatible with variation by chance alone. The pooled estimate of a patient with seeding per 100 patients per year was 0.009 (95% CI 0.006 to 0.013), p?=?0.686.
There are additional studies on other cancerous tumors with different risks; all the ones I found during a cursory review of scientific literature were non-zero and very low like in this report.