How skin carcinoma is staged in Britain: UICC TNM, and why it is not the American system

**Which system, and why.** The Royal College of Pathologists assessed both and chose UICC, noting that UICC TNM 'is the version favoured by NCRAS in the UK' and that the decision between the two rested on the staging of non-melanoma skin cancer, because for melanoma and Merkel cell carcinoma the two are now identical (G123). The difference is one of coverage. The AJCC eighth edition has a chapter on cutaneous squamous cell carcinoma of the head and neck, which also takes in basal cell and adnexal carcinoma but not Merkel cell carcinoma, and it has no staging system for squamous cell carcinoma anywhere else on the body. The UICC has both a head-and-neck chapter and a separate carcinoma-of-the-skin chapter for the rest, essentially trunk and limbs but excluding the eyelid, vulva, penis and perianal skin. The boundary between the two regions is the acromioclavicular joint in front and the upper margin of the shoulder blade behind. A further small discrepancy has never been resolved: UICC divides the size categories at 20 mm or less, over 20 to 40 mm and over 40 mm, while AJCC divides at under 20 mm, 20 to under 40 mm and 40 mm or more, and the RCPath judged the UICC breakpoints more likely to be correct because they match the ones both bodies use for Merkel cell carcinoma and lip and oral cavity tumours.

**The categories, in the ninth edition now in force.** pTis is carcinoma in situ. pT1 is 20 mm or less in maximum dimension, pT2 over 20 up to 40 mm, pT3 over 40 mm; pT3 is also reached by minor bone erosion, by perineural invasion or by deep invasion, whatever the size. Deep invasion means invasion beyond the subcutaneous fat, or more than 6 mm measured from the granular layer of adjacent normal epidermis to the base of the tumour. Perineural invasion means tumour within the nerve sheath of a nerve lying deeper than the dermis, or measuring 0.1 mm or more in calibre, or involvement of five or more nerves in a section, short of foramen or skull base involvement. pT4a is gross cortical or marrow invasion and pT4b is axial skeleton invasion including the vertebral foramen and epidural space. Nodal categories differ between the two body regions: for trunk and limbs they run on ipsilateral node size alone, 30 mm and 60 mm, and a contralateral node counts as distant metastasis; for head and neck they are subdivided by extranodal extension as well. The stage groups are simple: stage 0 is Tis, stage I is T1N0, stage II is T2N0, stage III is T3N0 or any of T1 to T3 with N1, stage IV is anything more.

**Why almost nobody is told a stage.** Cancer Research UK puts it plainly: most basal cell carcinomas do not need staging because it is very rare for them to spread, and staging is more likely with a squamous cell carcinoma because those can spread, although that is still rare. So the odd position in Britain is that basal cell carcinoma is formally stageable, with its own UICC appendix in the national reporting dataset, and is almost never staged; while in the United States there is no system for it outside the head and neck at all. Neither country routinely gives a person with a basal cell carcinoma a stage number, and a reader who cannot find one on their letter has not been overlooked.