In the lung, the terminology is
✴️ Typical carcinoid tumor
✴️ Atypical carcinoid tumor
✴️ Small cell carcinoma
✴️ Large cell neuroendocrine carcinoma
We don’t terms such as “well differentiated NET” because...this is #pulmpath and we are special 😜
Carcinoid tumors are very similar on H&E to low grade NE tumors in the GI tract or pancreas. Nesting, odd patterns, bland cytology, granular chromatin (“salt and pepper”)
✴️ Most carcinoid tumors in the lung are typical carcinoids
✳️ you can’t tell typical from atypical carcinoids on cytology or in small biopsies
✴️ In these specimens, calling them “carcinoid tumor” is fine
✳️ In resections, you can tell typical from atypical
Criteria for typical vs. atypical carcinoid tumor:
✴️ Mitoses (2/10 HPF is the cutoff)
✳️ Necrosis (none in typical, “punctate” in atypical)
✴️ Either one of these is sufficient for the label of atypical carcinoid
Do NOT differentiate these based on atypia or mets
Grossly, carcinoid tumors often look a bit yellow and tend to be located within a bronchus or adjacent to a bronchus. Post-obstructive mucus plugs are common.
However, carcinoid tumors can also occur in the periphery of the lung.
Atypical carcinoid tumors are more aggressive clinically than typical carcinoids. Both can metastasize to hilar/mediastinal nodes (even rarely to distant sites) but this is more common with atypical carcinoids. So this is why we count mitoses...
What do typical and atypical carcinoids have in common?
✳️ Low grade cytology
✴️ Absence of extensive necrosis
✳️ Almost always positive for synaptophysin and chromogranin
✴️ Less than 10 mitoses per high power fields (usually far less!)
✳️ Treated surgically
Another tumor that many worry about but shouldn’t: paraganglioma
✅ Yes, they can look like carcinoid tumors
✅ But primary paraganglioma of the lung is vanishingly rare. Carcinoid is common. When your hear hoofbeats, think 🐎not 🦓
✅ if in doubt, get a pankeratin