Which of the following stages indicates that a tumor is in situ?

The designation "Tis" specifically refers to a tumor that is classified as "carcinoma in situ." This terminology indicates that the tumor is at an early stage of cancer development where it has not invaded surrounding tissues or metastasized to other parts of the body. In this stage, the cancer cells remain localized within the layer of tissue where they originated, which means it has not yet progressed to any degree of invasiveness. Recognizing this stage is critical for treatment decision-making, as in situ tumors generally have better prognoses when detected early.

The other options represent different stages of tumor development, with "T0" indicating no evidence of primary tumor and "T1" describing a small tumor that has invaded local tissue but is still considered localized. "T4," on the other hand, signifies a more advanced tumor that has invaded other structures or has a more extensive disease presentation. Thus, "Tis" is uniquely defined and recognized as the marker for in situ tumors within the staging system.