A tumor designated as Carcinoma in Situ is labeled as which T category?

• A. T1
• B. Tis
• C. T2
• D. T3

Answer: (B)

In this concept, the “T” in TNM staging describes how far the primary tumor has grown and invaded surrounding tissues. Carcinoma in situ means the malignant cells are confined to the surface layer (the epithelium) and have not breached the basement membrane to invade underlying tissues. That lack of invasion is why the T category is designated as Tis, which stands for tumor in situ.

Because there is no invasion, the tumor isn’t given a T1, T2, or T3 designation, which are reserved for cancers that have penetrated deeper or grown larger. If invasion were present, the T category would reflect the depth or extent of that invasion (and, in many cancers, also size). So, carcinoma in situ is best labeled Tis, indicating a pre-invasive state.