This case raises one of the difficulties regarding TNM staging in colorectal carcinoma; in particular, the interpretation of pericolic tumor as it relates to the T (tumor) and N (node) categories.

The specimen photograph demonstrates colorectal carcinoma with extensive pericolic fat involvement. There is direct tumor invasion into pericolic fat, consistent with at least T3 (in this case the histology showed extension to the free serosal surface so it was classified as T4). For us (I speak as a Pathologists) this is the easy bit. 

The difficult part can arise with the classification of separate tumor nodules in the perinephric fat. Tumor nodules in pericolic fat can be seen in several settings;

• tangential cutting of direct tumor extension - this can occur when irregular 'tongues' of the main tumor are sliced tangentially, giving the impression of separate nodules
• extramural vascular invasion
• metastatic lymph node involvement 

Nodules in the setting of direct tumor extension should be identified, and therefore avoided, at macroscopic 'cut-up'. Identification of lymph node metastases is easy when there is residual lymph node present. When there is no residual lymph node, by convention the nodule is regarded as completely replaced lymph node metastasis if the nodule is rounded and circumscribed (N category), and as extramural vascular invasion (under T category) if the nodule is irregular. While this can be additionally designated with a V prefix (V1 if seen microscopically and V2 if seen macroscopically) in practice it rarely is, instead being describe descriptively as extramural venous/vascular invasion.  

The take-home message is that the exact cause of pericolic tumor involvement is sometimes difficult, often requiring histological assessment for accurate classification.