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Original Investigation
August 14, 2019

Association Between Surgical Skin Markings in Dermoscopic Images and Diagnostic Performance of a Deep Learning Convolutional Neural Network for Melanoma Recognition

Author Affiliations
  • 1Department of Dermatology, University of Heidelberg, Heidelberg, Germany
  • 2Department of Dermatology and Venerology, Medical University of Graz, Graz, Austria
  • 3Department of Dermatology, Lyon Sud University Hospital, Hospices Civils de Lyon, Pierre Bénite, France
  • 4First Department of Dermatology, Aristotle University of Thessaloniki, Thessaloniki, Greece
  • 5Public, Private, and Teaching Practice, Konstanz, Germany
  • 6Department of Dermatology, Allergology and Environmental Medicine II, Klinik Thalkirchnerstraße, Munich, Germany
JAMA Dermatol. 2019;155(10):1135-1141. doi:10.1001/jamadermatol.2019.1735
Key Points

Question  Are surgical skin markings in dermoscopic images associated with the diagnostic performance of a trained and validated deep learning convolutional neural network?

Findings  In this cross-sectional study of 130 skin lesions, skin markings by standard surgical ink markers were associated with a significant reduction in the specificity of a convolutional neural network by increasing the melanoma probability scores, consequently increasing the false-positive rate of benign nevi by approximately 40%.

Meaning  This study suggests that the use of surgical skin markers should be avoided in dermoscopic images intended for analysis by a convolutional neural network.


Importance  Deep learning convolutional neural networks (CNNs) have shown a performance at the level of dermatologists in the diagnosis of melanoma. Accordingly, further exploring the potential limitations of CNN technology before broadly applying it is of special interest.

Objective  To investigate the association between gentian violet surgical skin markings in dermoscopic images and the diagnostic performance of a CNN approved for use as a medical device in the European market.

Design and Setting  A cross-sectional analysis was conducted from August 1, 2018, to November 30, 2018, using a CNN architecture trained with more than 120 000 dermoscopic images of skin neoplasms and corresponding diagnoses. The association of gentian violet skin markings in dermoscopic images with the performance of the CNN was investigated in 3 image sets of 130 melanocytic lesions each (107 benign nevi, 23 melanomas).

Exposures  The same lesions were sequentially imaged with and without the application of a gentian violet surgical skin marker and then evaluated by the CNN for their probability of being a melanoma. In addition, the markings were removed by manually cropping the dermoscopic images to focus on the melanocytic lesion.

Main Outcomes and Measures  Sensitivity, specificity, and area under the curve (AUC) of the receiver operating characteristic (ROC) curve for the CNN’s diagnostic classification in unmarked, marked, and cropped images.

Results  In all, 130 melanocytic lesions (107 benign nevi and 23 melanomas) were imaged. In unmarked lesions, the CNN achieved a sensitivity of 95.7% (95% CI, 79%-99.2%) and a specificity of 84.1% (95% CI, 76.0%-89.8%). The ROC AUC was 0.969. In marked lesions, an increase in melanoma probability scores was observed that resulted in a sensitivity of 100% (95% CI, 85.7%-100%) and a significantly reduced specificity of 45.8% (95% CI, 36.7%-55.2%, P < .001). The ROC AUC was 0.922. Cropping images led to the highest sensitivity of 100% (95% CI, 85.7%-100%), specificity of 97.2% (95% CI, 92.1%-99.0%), and ROC AUC of 0.993. Heat maps created by vanilla gradient descent backpropagation indicated that the blue markings were associated with the increased false-positive rate.

Conclusions and Relevance  This study’s findings suggest that skin markings significantly interfered with the CNN’s correct diagnosis of nevi by increasing the melanoma probability scores and consequently the false-positive rate. A predominance of skin markings in melanoma training images may have induced the CNN’s association of markings with a melanoma diagnosis. Accordingly, these findings suggest that skin markings should be avoided in dermoscopic images intended for analysis by a CNN.

Trial Registration  German Clinical Trial Register (DRKS) Identifier: DRKS00013570