Hi pathology friends! Lung pathology is on everyone’s minds these days… but lets talk about a different type of #pulmpath issue. Lung mass in a 60-yo patients. Core bx. What is your favored Dx on H&E? Would you just call it or get any stains? #natpathpuzzler @PulmPathSoc

Hi everyone. This is not the answer yet - pl stay tuned for that. Just wanted to say I'm LOVING the spirited discussion. Thank you @kis_lorand for moderating! Please keep your thoughts coming. They are all perfectly on the spot with DDx.

Thanks for all posts and lively discussion! If I saw this case just few years ago – for sure would have called squamous cell carcinoma without stains. Keratinization Intercellular bridges Or are they?? Here is IHC. This is a solid adenocarcinoma mimicking SqCC! More 1/

Now that we use IHC more routinely to subtype NSCLC, this look of lung adeno is much better recognized (as many of you commented). Lets look at this more closely. 2/

The cells with dense pink cytoplasm and dark nuclei really evoke the look of keratinizing SqCC… but these are just pyknotic cells. I know – hard to believe, right? 3/

So how do we know there are NOT truly keratinizing cells? Indeed, p40 is negative in such cells (you should NOT use IHC for such cases!) But here is a great example of a similar case. Keratinizing cells should stain w/ high molecular weight keratins - these do not! 4/

Issue 2: Are there intercellular bridges? The answer is no. For trainees - here is the perfect look of IC bridges (i.e. desomsomes). Actually, in lung these are usually present in overtly keratinzing SqCC, so on their own are rarely diagnostically helpful. 5/

So what about our case? Cells do have sharp borders but NOT perfect IC bridges. Very important to know of this look of solid eosinophilic adeno in the lung – it is not uncommon! These tumors have molecular alterations of adeno (not SqCC) so important to be aware of these! 6/

So lets summarize. Do not over-react to eosinophilic large cell look with sharp cell borders as evidence of squamous differentiation in NSCLC! When pyknotic cells are present – these can fully mimic SqCC. 7/

PS: I do NOT advocate IHC for all cases of NSCLC. If there is CLEAR squamous and adeno differentiation – TTF-1/p40 IHC is NOT needed. However, important to be aware of mimics of this type to know when IHC is useful... as most of you know based on your comments. 8/

PS, PS: @kis_lorand already handled really well the question of what to order upfront (thanks Lorand!). Agree 100%. Here is my take – if a tumor looks like likely NSCLC – start with ONLY TTF1 & p40 as vast majority of case will be solved by this. If negative – then expand. 9/

Here is nice reference from our group on lung adenos that mimic SqCC - https://www.ncbi.nlm.nih.gov/pubmed/25871623 . 10/

Thanks very much for your attention and engagement. This is a difficult time for many. Please stay safe and sane! The end. 11/