In vivo assessment of optical properties of melanocytic skin lesions and differentiation of melanoma from non-malignant lesions by high-definition optical coherence tomography

Abstract

One of the most challenging problems in clinical dermatology is the early detection of melanoma. Reflectance confocal microscopy (RCM) is an added tool to dermoscopy improving considerably diagnostic accuracy. However, diagnosis strongly depends on the experience of physicians. High-definition optical coherence tomography (HD-OCT) appears to offer additional structural and cellular information on melanocytic lesions complementary to that of RCM. However, the diagnostic potential of HD-OCT seems to be not high enough for ruling out the diagnosis of melanoma if based on morphology analysis. The aim of this paper is first to quantify in vivo optical properties such as light attenuation in melanocytic lesions by HD-OCT. The second objective is to determine the best critical value of these optical properties for melanoma diagnosis. The technique of semi-log plot whereby an exponential function becomes a straight line has been implemented on HD-OCT signals coming from four successive skin layers (epidermis, upper papillary dermis, deeper papillary dermis and superficial reticular dermis). This permitted the HD-OCT in vivo measurement of skin entrance signal (SES), relative attenuation factor normalized for the skin entrance signal (µ raf1) and half value layer (z 1/2). The diagnostic accuracy of HD-OCT for melanoma detection based on the optical properties, µ raf1 , SES and z 1/2 was high (95.6, 82.2 and 88.9 %, respectively). High negative predictive values could be found for these optical properties (96.7, 89.3 and 96.3 %, respectively) compared to morphologic assessment alone (89.9 %), reducing the risk of mistreating a malignant lesion to a more acceptable level (3.3 % instead of 11.1 %). HD-OCT seems to enable the combination of in vivo morphological analysis of cellular and 3-D micro-architectural structures with in vivo analysis of optical properties of tissue scatterers in melanocytic lesions. In vivo HD-OCT analysis of optical properties permits melanoma diagnosis with higher accuracy than in vivo HD-OCT analysis of morphology alone.

Keywords: Absorption; Benign nevi; Dysplastic nevi; High-definition optical coherence tomography; In vivo optical properties; Light attenuation; Melanocytic lesions; Melanoma; Reflectance; Scattering.

Fig. 1

Benign nevus. a, b A 3-D HD-OCT DICOM (digital imaging and communication in medicine) image of each melanocytic lesion is selected. To exclude artefacts such as skin furrows or air bubbles, a square region of interest is chosen in the en face image based on the presence of relevant morphologic features of melanocytic lesions (green square). A plot z-axis profile of the scanned volume is performed. c, d The obtained graph displays the reflectance of the ballistic photons [OCT signal: measured on the y-axis with arbitral units (AU)] versus imaging depth which is indicated on the x-axis (slice numbers from 0 to 200 axial position of depth). The slice numbers are multiplied by factor 3 (slice thickness equals 3 µm) to correspond to the real depth in micron (from 0 to 600 µm: see x-axis in (e). e Semi-log plot: an exponential function becomes a straight line given by y = ax + b. Four successive layers (epidermis, upper papillary dermis, deeper papillary dermis and superficial reticular dermis) with clear exponential decay are identified and plotted. A straight line is fitted in each of the four layers (i = 1–4) resulting in equation of the type y _i = ax _i + b whereby a is proportional to the attenuation coefficient for each of the four layers

Fig. 2

Dysplastic nevus. a Cross-sectional image, b en face image, c section of the cross-sectional image, d graph displaying reflectance versus depth of focus and e semi-log plot

Fig. 3

Malignant melanoma. a Cross-sectional image, b en face image, c section of the cross-sectional image, d graph displaying reflectance versus depth of focus and e semi-log plot

Fig. 4

Comparison of trend lines of relative attenuation factor at different skin layers in benign nevus (upper part), dysplastic nevus (middle part) and melanoma (lower part)