Clark’s Level vs. Breslow Thickness: Why Level III and IV Still Trip People Up
Category: Fundamentals Friday
Clark’s Level vs. Breslow Thickness: Why Level III and IV Still Trip People Up
Ask a resident to define Breslow thickness and you’ll usually get a confident, correct answer: a linear measurement, top of granular layer to deepest invasive cell, reported to the nearest 0.1 mm. Ask the same resident to explain the difference between Clark’s Level III and Level IV, and the confidence often drops — which is exactly the gap this post is meant to close.
Clark’s level and Breslow thickness measure two different things. Breslow thickness is a linear, quantitative measurement — millimeters of vertical extent. Clark’s level is a qualitative, anatomic assessment — which named dermal compartment the tumor has actually reached. They usually track together, but not always, and the disagreement between the two is precisely where the diagnostic subtlety lives.
The Five Levels
Level I is melanoma in situ — intraepidermal only, basement membrane intact, no dermal invasion at all.
Level II means the tumor has broken through into the papillary dermis, but the papillary dermis still looks like the papillary dermis — its normal architecture is preserved, just occupied by scattered tumor cells.
Level III is where most of the confusion starts. At this level, the tumor fills and expands the papillary dermis — the papillary dermis is now completely occupied and visibly widened by tumor, pushing down against the superficial vascular plexus, but critically, the tumor has not yet crossed into the reticular dermis.
Level IV is the tumor actually invading the reticular dermis — recognizable by the coarse, thick collagen bundles of the reticular dermis being infiltrated by tumor cells, not just abutted by an expanded papillary dermis from above.
Level V is invasion into the subcutis (adipose tissue).
Why III and IV Get Confused
The Level III/IV distinction is subtle because both show tumor pushing deeply — the difference is entirely about which collagen compartment the tumor has actually entered, not how deep the lesion looks at a glance. A markedly expanded papillary dermis in Level III can look deep and worrisome at low power, especially when it’s bulging down against the vascular plexus, but if the tumor hasn’t actually infiltrated the thick, coarse collagen bundles characteristic of the reticular dermis, it’s still Level III. The practical habit worth building: at the papillary–reticular junction, specifically ask whether tumor cells are present within the coarse reticular collagen, not just adjacent to or pressing on it. This is also exactly why Clark level is acknowledged to be less reproducible between pathologists than Breslow thickness — the reticular dermis boundary is a judgment call in a way that a ruler measurement isn’t.
Why It’s Still Worth Knowing — Even Though It’s Not Used for pT Staging
Here’s the nuance that’s easy to lose: Clark level was removed as a T1 staging criterion in the AJCC 8th edition — T category is now determined by Breslow thickness and ulceration status alone, not anatomic level. Mitotic rate as a dichotomous T1 criterion was also dropped, and T1 is now subcategorized at a 0.8 mm threshold (T1a: non-ulcerated, less than 0.8 mm; T1b: 0.8–1.0 mm, or ulcerated and less than 0.8 mm).
But “not used for pT staging” does not mean “not useful.” The AJCC still recommends anatomic (Clark) level be reported as a primary tumor characteristic, because it remains an independent predictor of outcome — it’s simply been reclassified from a staging criterion to an optional, but recommended, data item. It also does real conceptual work that a millimeter measurement alone doesn’t: it gives you and the clinician a mental picture of exactly where the tumor sits relative to skin architecture, which matters when you’re explaining a case, teaching a trainee, or thinking through a lesion that behaves unexpectedly for its thickness.
The Takeaway
Breslow thickness is what goes in the pT category. Clark level is what goes in your head — a fast, visual anchor for where a melanoma actually lives in the dermis, and a data point still worth recording even when it isn’t driving the stage.
References
- Donica WR, Stephens KR, Philips P, McMasters KM, Egger ME. Predictors of Sentinel Lymph Node Biopsy in Thin Melanoma: Understanding the Impact of the American Joint Committee on Cancer 8th Edition Staging System. J Am Coll Surg. 2025;240(4):518–527.
- Elmore JG, Elder DE, Barnhill RL, et al. Concordance and Reproducibility of Melanoma Staging According to the 7th vs 8th Edition of the AJCC Cancer Staging Manual. JAMA Netw Open. 2018;1(1):e180083.
- Amin MB, Edge SB, Greene FL, et al., eds. AJCC Cancer Staging Manual, 8th ed. New York: Springer; 2017.
