Classic two-step algorithm

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 Author(s): Ralph P. Braun, Ashfaq A. Marghoob

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In 2001, the Board of the Consensus Net Meeting agreed on a two-step algorithm for the classification of pigmented lesions of the skin . In the first level of decision making, the observer must decide whether a lesion is of melanocytic or nonmelanocytic origin. Once the lesion is identified to be of melanocytic origin, one can move on to level two. This second step the decision must be made if a melanocytic lesion is benign, suspect, or malignant. For this purpose, the algorithms found in chapter 9 are most useful.

The revised two-step algorithm

After the initial description of the two-step algorithm, the different types of blood vessels have been described . Since the initial algorithm did not take into consideration the vascular architecture, there was a need to update this algorithm .

Step 1 decision: melanocytic vs. nonmelanocytic lesion

The first step of this two-step dermoscopy decision-making algorithm has different levels . However, before we use this algorithm it is important to be aware that this algorithm is designed to evaluate lesions on glabrous and nonglabrous skin only. It is not intended for special locations because in this case standard dermoscopy criteria do not apply. Each level requires that the observer evaluate the lesion for the presence of specific dermoscopy criteria in the following sequence

Level 1: Criteria for Melanocytic Lesions

Search for pigment network (N.B: one exception; see below), atypical network, streaks (i.e., radial streaming and pseudopods), negative network, aggregated globules, homogenous blue pigmentation, criteria for melanocytic lesions in special locations such as pseudonetwork (face), or parallel pattern (palms, soles, and mucosa). If any of these structures or features are present, the lesion is considered of melanocytic origin and hence, one can proceed directly to the second step of the two-step algorithm. One major exception to the above is when a lesion manifests a delicate peripheral reticulation with a central scar-like area as seen in a dermatofibroma. This particular pattern generally trumps the presence of a network, hence preventing the misdiagnosis of a dermatofibroma as a melanocytic lesion. A further clarification of homogenous blue pigmentation as seen in blue nevi is also warranted here. The quintessential characteristic that differentiates a blue nevus from other lesions possessing a blue-white veil is that in a blue nevus the blue pigmentation occupies the entire surface area of the lesion in a homogenous manner. If the criteria of Level 1 are not met, then one needs to proceed to Level 2.

Level 2: Criteria for Basal Cell Carcinoma

Search for the presence of specific morphologic criteria for basal cell carcinoma, which include arborizing blood vessels (telangiectasias), leaf-like areas, large blue-gray ovoid nests, multiple blue-gray non-aggregated globules, spoke-wheel-like structures, shiny white areas, or ulceration. In the absence of a pigment network, these criteria are highly suggestive of basal cell carcinoma. If, on the other hand, these structures are not seen, then proceed to Level 3.

Level 3: Criteria for Seborrheic Keratoses

Look for multiple milia-like cysts, comedo-like openings, crypts, moth-eaten borders, network-like structures, “fissures and ridges” (also known as gyri and sulci) that sometimes give a brain-like or cerebriform appearance to the lesion, fat fingerlike structures, or light brown fingerprint-like structures. Although milia cysts and comedo openings are very easy to identify by standard nonpolarized dermoscopy, they may not be as apparent when viewed using polarized light dermoscopy. With that being said, if some of these structures are present, then the lesion is probably a seborrheic keratosis. The presence of milia like cysts does not automatically equate a diagnosis of seborrheic keratosis because milia cysts can sometimes be seen in basal cell carcinoma and in melanocytic nevi, especially of the congenital type. Thus, milia cysts should be considered a diagnostic criterion only after insuring that the lesion is not a melanocytic lesion or a basal cell carcinoma. If none of the seborrheic keratosis criteria are seen then one needs to proceed to Level 4.

Level 4: Criteria for Vascular Lesions

The presence of red, maroon, or red-blue to black lacunae (also known as lagoon-like structures), indicates that the lesion is a hemangioma or angiokeratoma.

Level 5: Specific Blood Vessels in Nonmelanocytic Lesions

If none of the morphologic criteria described in the previous levels can be identified, then the lesion does not manifest any obvious features of a melanocytic lesion nor does it manifest any of the features seen in the four common nonmelanocytic tumors. These lesions are usually amelanotic or hypomelanotic. Since blood vessels are often the only “diagnostic criterion” it is important to appreciate both their morphology and distribution. Hairpin vessels surrounded by a whitish halo are characteristic of keratinizing tumors, such as keratoacanthomas and seborrheic keratosis.

Glomerular vessels

Are usually aggregated focally at the periphery of the lesion. They identify the lesion as squamous cell carcinoma. Besides the morphology and distribution of vessels, the arrangement of the blood vessels and the color surrounding the vessels can also assist in the diagnosis.

Pearls on a string

The presence of blood vessels arranged like “pearls on a string” or in a “serpiginous pattern” is a hallmark of clear cell acanthoma

Crown vessels

The presence of crown vessels identifies the lesion as a sebaceous hyperplasia or molluscum contagiosum.

Level 6: Specific Blood Vessels in Melanocytic Lesions

The presence of predominantly comma shaped blood vessels is a hallmark of intradermal nevi . The blood vessel morphology encountered in melanoma includes dotted, linear irregular, atypical hairpin (serpentine) vessel in a pink background, and cork screw or tortuous vessels. If more than one type of vessel morphology is seen within the same lesion, the vascular pattern is termed “polymorphous.” The polymorphous pattern that is most commonly associated with melanoma consists of centrally placed dotted and linear/serpentine vessels . In addition to the blood vessel morphology described above, the presence of multiple shades of pink, also known as milky red areas, can also be seen in melanoma. The milky red color probably represents an increased vascular volume and may be a reflection of neoangiogenesis . Lesions that do not display any of the structures mentioned in levels 1–6 are considered “structureless” and for such lesions one needs to proceed to Level 7.

Level 7: “Structureless” Lesions

Lesions that fail characterization in levels 1–6 are tumors that are often completely featureless or structureless. However, it is important to highlight that these lesions are not always necessarily structureless. Level 7 is simply the default category that includes all lesions that fail to reveal any specific diagnostic structures to help classify them as melanocytic or as one of the non-melanocytic lesions mentioned in levels 1 to 6 of the twostep algorithm. For example, the presence of fine dots, peppering, blue white veil, crystalline structure, and blotches may be present in these lesions. While these structures cannot be used to differentiate melanocytic from non-melanocytic lesions, they can be clues that aid in correctly identifying melanomas and some basal cell carcinomas. With that said, for all lesions in level 7 (the so called “structureless” lesions), it is imperative that melanoma be ruled out. This “worst case scenario” avoids missing melanomas that are devoid of any specific discernable structures. Thus, these lesions either should be biopsied or should be subjected to short-term mole monitoring in an attempt to ascertain their biologic nature.

Step 2 decision: benign nevus vs. suspect melanocytic lesion vs. melanoma

Lesions that are identified as being of melanocytic origin in step one of the two-step algorithm include those described in levels 1, 6, and 7 above. Melanocytic lesions–manifesting structures and features described in these levels can be subjected to the second step of the two-step algorithm. In the second step, the presumed melanocytic neoplasm is evaluated to determine whether the lesion is benign, suspect, or malignant. To accomplish this, many different approaches have been proposed.

Limitations of the two-step algorithm

The main purpose of the two-step algorithm is to avoid missing the diagnosis of melanoma. Although some exceptions to the two-step algorithm exist, they do not affect the sensitivity for melanoma detection. In other words, the two-step process is designed to maximize the sensitivity for melanoma detection . With that said, the main shortcoming of the classic two-step algorithm was that it did not adequately address the evaluation process for amelanotic and hypomelanotic neoplasms. These lesions often appear to be structureless or featureless on initial inspection. However, upon scrutiny many of these neoplasms will in fact reveal blood vessels and are not truly structureless. These new insights prompted us to modify the two-step algorithm by adding two new decision levels to the classic two-step algorithm, which are intended to assist clinicians in correctly classifying some of these so-called featureless neoplasms as either melanocytic or nonmelanocytic tumors.

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