Accuracy of the first step of the dermatoscopic 2-step algorithm for pigmented skin lesions

Objectives: To evaluate the frequency of misclassifications of equivocal pigmented lesions according to the first step of the dermatoscopic 2-step algorithm. Patients and Methods: 707 consecutive cases from 553 patients of central Europe and Australia were included in the study. Dermatoscopic images were evaluated in a blinded fashion for the presence of features described in the 2-step algorithm to determine their melanocytic or non-melanocytic origin. Mucosal, genital and non-pigmented lesions were excluded. Results: The sensitivity of the first step was 97.1% for patients from Australia and 96.8% for patients from central Europe. The specificity was 33.6% for Australian patients and 67.9% for European patients. The most common reasons for misclassification were the presence of a pigmented network in a non-melanocytic lesion (n=68, 25.2%) and the absence of dermatoscopic features of melanocytic and non-melanocytic lesions in 69 (25.6%) non-melanocytic lesions. Conclusion: The first step of the dermatoscopic 2-step algorithm, if applied consistently, has high sensitivity but low specificity. Many non-melanocytic lesions, especially solar lentigines and seborrheic keratoses, are wrongly classified as melanocytic. The worse performance of the first step algorithm in Australian patients is probably due to a higher rate of solar lentigines in patients with severely sun-damaged skin.


Introduction
step especially runs the major risk of misclassification and can therefore lead to wrong diagnoses eventually, regardless of how good the algorithm of the second step is.
A few publications have reported single dermatoscopic features of the first step to be prone to misclassification [11,12], but the overall rate and reasons for wrong classifications have not been reported yet and this is the aim of this study.

Patients and methods
The cases originated from a tertiary referral center at a uni- Dermatoscopic images were evaluated in a blinded fashion by two of the authors (P.T., H.K.) for the presence of every melanocytic and non-melanocytic feature described in the 2-step algorithm [9]. A lesion was regarded of melanocytic origin if either at least one melanocytic feature was present or no dermatoscopic feature was present at all ("melanocytic by default"). If no melanocytic but at least one non-melanocytic feature was present a lesion was classified non-melanocytic. Lesions histologically proven to be a collision lesion of both origins were histologically classified as melanocytic.
Devices used for taking dermatoscopic images were a DermLite Foto® (polarized imaging) and a DermLite Fluid®

Statistical analysis
Sensitivity was calculated by dividing the number of cor-
Two hundred and seventy (39%) of the cases were nonmelanocytic, 432 (61%) melanocytic, and the frequencies of histologic diagnoses are shown in Table 1. The lesions were located on head or neck in 18.4%, on the trunk in 45.7%, on the upper extremities in 11.8%, on the lower extremities in 19.4% and on acral sites in 2.7% ( Figure 1).

Accuracy of the first step
The sensitivity of the first step was 97.1% for patients from

Misclassifications
The most common reasons for misclassification were a pig-  Table 2. Seven percent (n=13) of misdiagnosed lesions were melanocytic but misclassified as non-melanocytic and 161 (92.5%) non-melanocytic were misclassified as melanocytic. Seborrheic keratoses and solar lentigines were most commonly misclassified. The frequencies of misclassification by feature are given in Table   3. Table 4 shows the positive predictive value by feature.

Discussion
In this study we show that the accuracy of the first step of dermatoscopy is only moderate. It is very sensitive for melanocytic lesions but has low specificity. In other words, if the first step for dermatoscopy is used in the way it has been sug-      [13,14]. Solar lentigines with a pigment network contributed largely to the low positive predictive value of the first step in the Australian group.
The introduction of the 2-step algorithm was partly motivated by the relative importance of melanoma in comparison to non-melanoma skin cancer. However, we hold to the opinion that the differentiation between benign and malignant lesions is a better first step than deciding whether a lesion is melanocytic or non-melanocytic [3]. We prefer a system that differentiates between chaotic and symmetric lesions first. This system is not more accurate but conceptually simpler (Figure 4). Our study has a significant limitation.
The authors are very critical with regard to the use of the first step and advocate another method instead [3]. We have tried to minimize any form of bias by blind assessment of the lesions and by selecting consecutive lesions form different parts of the world. We have used the criteria and the algorithm precisely in the way they are advocated. We acknowl-edge that many seborrheic keratosis or solar lentigines with a pigment network can be diagnosed correctly based on other criteria. We are convinced that experienced dermatoscopists are tacitly aware of the limitations of the first step, i.e., its low specificity in solar lentigines and seborrheic keratosis and that they use other criteria to diagnose these lesions with specificity. However, this is not what the first step tells us to do. According to the first-step algorithm, the presence of a "pigment network" trumps all other criteria and thus would lead to a wrong diagnosis if used in a pedantic fashion.

Conclusion
The first step of the dermatoscopic 2-step algorithm, if applied consistently, has high sensitivity but low specificity especially in patients with severely sun-damaged skin.