The first thing to say is that it is very important that you do not mix up the term "level" with the term "stage".
The level of your melanoma refers to the thickness of the melanoma within your skin.
Stage is an assessment of the risk of your melanoma spreading and an assessment of whether spread has already occurred and how far that spread has occurred.
For staging the information used is the information from the pathology report, the examination findings observed by your doctor about whether she/he can feel any enlarged lymph glands and where appropriate scans of your body looking for spread in the internal organs ( PET, MRI and CT scans may all be used for this purpose). Scans are not always appropriate in early melanoma.
The International system used is a TNM system:
This TNM system is the basis on which the Stage is decided.
We are currently at a significant point of change in regards melanoma staging.
The 8th edition of the AJCC ( American Joint Committee on Cancer)staging document has been decided and released so practitioners can get used to the changes which will come into effect on January 1, 2018.
For 2017 we remain using the 7th Edition of the AJCC document.
This gets quite complex and is probably best discussed directly with your doctor once all the relevant information is available.