melanoma in situ
Meaning of melanoma in situ: The earliest recognized stage of melanoma, still trapped in the outer skin layer where no blood or lymph vessels reach.
Definition of melanoma in situ
Melanoma in situ is melanoma whose abnormal pigment cells remain entirely within the epidermis, the thin outer layer of skin. They have not crossed the basement membrane into the deeper tissue below, where blood and lymph vessels run. Because the cells have no route out, melanoma in situ cannot spread to other parts of the body. It is also called stage 0 melanoma.
What does in situ mean here?
The phrase comes from Latin and means in its original place. Skin is layered: the epidermis on top, then a boundary called the basement membrane, then the dermis below it. Only the dermis carries blood and lymph vessels. Pigment cells that have turned abnormal but stay above that boundary have no transport system to reach the rest of the body, which is the whole significance of the term in situ.
| Melanoma in situ | Invasive melanoma | |
|---|---|---|
| Depth | Epidermis only | Into the dermis or deeper |
| Access to vessels | None | Blood and lymph vessels reached |
| Ability to spread | No | Yes |
| Stage | Stage 0 | Stage 1 and above |
How is it found?
It appears as a flat patch rather than a lump, since nothing has grown downward yet. Uneven color, an irregular edge, and slow enlargement over months are the usual features, and a spot that looks different from the others on the same skin draws attention. On long sun-exposed skin, particularly the face of an older person, a form called lentigo maligna spreads sideways for years as a mottled brown stain. Diagnosis rests on removing the area and examining it, because depth cannot be judged by eye. Reading the edges under the microscope is harder than for a lump, as the abnormal cells fade gradually into normal ones.
Why is it still taken seriously?
Two reasons. First, what is seen on the surface may not be the whole picture: an area reported as in situ on a small sample can contain an invasive focus elsewhere in the same lesion, which is why the full specimen is examined. Second, if left alone, a proportion progress. Abnormal cells that keep dividing eventually break through the basement membrane, and once through, the depth of that invasion becomes the strongest predictor of outcome for melanoma. Treatment at the in situ stage is aimed at removing the whole area with a clear margin, and the outlook once that is achieved is excellent.
Also known as
Used in a sentence
Pathology reported melanoma in situ with margins clear of abnormal cells.
Good to know
Because the epidermis contains no blood or lymph vessels, melanoma in situ has no route to spread, and the reported depth of invasion is what separates it from higher stages.
Word origin
Where “melanoma in situ” comes from
In situ is Latin for in its original position; melanoma joins the Greek melas, meaning black, with -oma, a swelling or tumor.