American Melanoma Institute · 4 min read
A line-by-line guide to the terms on a melanoma pathology report, what each one means, and which ones change treatment.
A melanoma pathology report is written by one doctor for other doctors, so it reads like a checklist. Most labs use a standard template, and the same items appear in nearly the same order everywhere. Going through it one line at a time makes it far less intimidating.
The first line names the diagnosis and the body site. The words that matter most here are in situ and invasive. Melanoma in situ means the abnormal cells are confined to the epidermis, the thin top layer of skin. Invasive melanoma means cells have entered the dermis, the thicker layer underneath, where blood and lymph vessels run.
A subtype often follows, such as superficial spreading, nodular, lentigo maligna, or acral lentiginous. The subtype describes the growth pattern the pathologist sees. It seldom changes treatment by itself, because the measurements below carry more weight.
Breslow thickness is the depth of the melanoma in millimeters, recorded to the nearest 0.1 mm. This single number drives more decisions than any other line. Ulceration, reported as present or absent, means the epidermis over the tumor has broken down when viewed under the microscope.
Mitotic rate counts dividing cells per square millimeter. A higher count points to a faster-growing tumor. The current staging system doesn't use it to assign the T category, yet pathologists still report it because it carries information about outlook.
Some reports also list the Clark level, a Roman numeral from I to V that names the anatomic layer the tumor has reached. Breslow thickness and ulceration replaced it in staging years ago. You may also see growth phase (radial or vertical), an older way of describing whether the tumor has begun to grow downward.
Margins are the cut edges of the specimen. The lab inks them so the pathologist can tell whether melanoma cells reach an edge. Peripheral margins are the sides. The deep margin is the bottom.
A positive or involved margin on a first biopsy is common and expected, since a biopsy is meant to diagnose and the wider surgery comes later. The deep margin deserves a closer look. If melanoma reaches the bottom of the sample, the report will give the thickness as at least a certain number, because the true depth could be greater.
Microsatellites are tiny deposits of melanoma in the skin or fat that sit apart from the main tumor, with normal tissue in between. Under the AJCC 8th edition, finding even 1 places the melanoma in stage III. Pathologists see them more often in the wide excision specimen than in the first biopsy.
Lymphovascular invasion means melanoma cells are visible inside a lymphatic channel or a blood vessel. Neurotropism, also called perineural invasion, means cells are tracking along small nerves. Both are linked with a higher chance of the melanoma coming back, and both can affect how the surgical team plans treatment of the area.
Regression describes a zone where melanoma cells have disappeared and scar-like tissue and inflammation remain. Doctors think the immune system destroyed part of the tumor there. Its meaning for outlook is still debated, and some pathologists note that a regressed tumor may once have been thicker than it now measures.
Tumor-infiltrating lymphocytes, shortened to TILs, are immune cells mixed in among the melanoma cells. Reports grade them as absent, non-brisk, or brisk. A brisk response has been linked with better outcomes in several studies, though experts disagree on how much weight to give it.
Near the end you'll find a code such as pT1a or pT2b. The p stands for pathologic. The number reflects thickness: T1 is 1.0 mm or less, T2 is over 1.0 to 2.0 mm, T3 is over 2.0 to 4.0 mm, and T4 is over 4.0 mm. The letter a means no ulceration and b means ulceration, with one special rule for T1 that also uses a 0.8 mm cutoff.
pTis means melanoma in situ. pTX means thickness couldn't be assessed, which can happen when a sample is fragmented or wasn't cut at the right angle. The N (lymph node) and M (distant spread) parts of the stage usually can't be assigned from a skin biopsy alone.
Regression, TILs, subtype, Clark level, and growth phase mostly add detail about outlook. They rarely change the plan by themselves.
Many reports end with a free-text comment. This is where the pathologist explains anything unusual, lists special stains or molecular tests that were used, and names any colleague who also reviewed the slides. A comment that mentions a second reviewer means more than one specialist looked at your case before the report was signed.
