What shape is the lesion?

This lesion was found in the ascending colon.
HOW WOULD YOU DESCRIBE IT?

■ Flat elevated lesion (IIa)

Only lesions 10mm or smaller are referred to as 'IIa lesions'

■ Flat elevated lesion with a central depression (IIa+IIc)

But that depression dissappear when lumen is inflated!

■ Laterally spreading tumour of the granular type (LST-G)

Absolutely !

■ Laterally spreading tumour of the non-granular type (LST-NG)

But the surface is 'cobbled' not smooth!

■ Laterally spreading tumour with a central depression (LST-D)

LST-D's are always TA's (and therefore have a smooth surface)

explanation

Lets just state something from the start! A flat elevated polyp is of course a IIa type of lesion BUT when larger than 10mm in diameter is no longer referred to as a IIa lesion.  Now, it's a 'Laterally Spreading Tumour'!  
There are 4 types as follows:

• LST-G (granular type) – usually found in the rectum or caecum and are almost always TVA’s harbouring LGD only
• LST-NG (non-granular) – can be found anywhere in the colon and are always TA’s and usually harbour HGD (which should be your guess if the crypt openings are very small) or even cancer
• LST-M (mixed type) – these are usually LST-G’s which have developed a dominant nodule. Resect the nodule first because if there is cancer, it will be found at the site of the dominant nodule
• LST-D (depressed type) – this is the least common of all the LST’s are almost always TA’s in which early cancer has developed. 

I must admit to working from the crypt pattern and ‘backwards’ when deciding on the LST subtype.  Of course this is because the LST-G are always TVA’s with a gyrate crypt pattern whilst all the others are TA’s with slit-like crypts or small round crypts, usually found in LST-D's which usually harbour at least HGD.  This is because as the degree of dysplasia increase, the crypts wither, become smaller and more angular.   

In this case the crypts are gyrate, (type IV crypts), making it a TVA. Laterally Spreading Tumours with a gyrate crypt pattern are all TVA's and almost never harbouring more than LGD.  OK but it looks depressed in the centre doesn't it?  No, it isn't depressed in the centre!  It's simply they way the large lesion has folded itself. This is something of a 'trick question' as I deliberately didn't inflate the colonic lumen fully in the first image.   When the lumen is inflated, you can see the true shape of the lesion; a LST-G ! 

A meta-analysis in 2013 by Voorham et al. [Voorham QJM. AM J Gastro 2013;108(7):1042-] concluded that pedunculated polyps were more likely to harbour KRAS mutation and APC mutations than flat lesions, and that flat lesions were more likely to harbour BRAF mutations.  Depressed lesions and LST-NG’s were particularly unlikely to contain KRAS.  

Below are some examples of LST's