5.2.2.1　Basal cell carcinoma（BCC）

The BCC is one of the prime examples of indications for OCT.Promising results have already been demonstrated in delineating these tumors，and also the diagnostic criteria for BCC have been established.For normal skin，OCT can reliably identify the distinct layers of the skin usually down to the deep reticular dermis（depending on the imaged skin region and the OCT system used）and the dermal-epidermal junction（DEJ）appears as an intact narrow hyporeflective line（Figure 5-4（a））.In BCC lesions，loss of normal skin architecture is an overall finding（Figure 5-4（b））.The specific OCT characteristics of BCC include alteration of the DEJ and dark ovoid areas in the dermis（basal cell nests）surrounded by a white halo（e.g.，stroma），sometimes referred to as a honeycomb pattern.Cellular palisading/peritumoral clefting at the margins of basal cell nests is often seen as a low-intensity OCT signal at the periphery of the cell nests.

Secondary features include absence of normal hair follicles and glands and altered dermal capillaries directed toward the basaloid cell islands.Several studies have shown the correlation of OCT morphology with histology for several types of skin tumors，although earlier studies found it difficult to differentiate between BCC subtypes.HD-OCT has an increased lateral resolution（at the cost of penetration depth）and may permit differentiation of BCC subtypes.The features described in HD-OCT can be visualized only superficially in the lesion and include the combination of distinct lobular organization，a dominant vascular pattern in the papillary plexus，and the presence/absence of a stretching effect on the stroma.(https://www.daowen.com)

As mentioned above，OCT provides relatively high accuracy in distinguishing lesions from normal skin，which is of great importance in identifying tumor borders.In differentiating normal skin from non-melanoma skin cancers（NMSCs）lesions，sensitivity of 79％-94％and specificity of 85％-96％were found for OCT.Looking specifically at the diagnostic accuracy of OCT in identifying BCC，recent extensive studies have investigated this aspect.By using a specific scoring system（e.g.，Berlin score）based on five predetermined diagnostic OCT criteria，Wahrlich et al.found that the sensitivity and specificity for multibeam OCT amounted to 96.6％and 75.2％，respectively，when evaluated by a dermatology specialist familiar with OCT.Specifically，88％of all BCC diagnoses based on OCT were correctly classified，confirmed by histopathology.In a multicenter study by Ulrich and Mayer et al.，the diagnostic value of OCT for BCC in a typical clinical setting was investigated.235 non-pigmented pink lesions of 155 patients suspicious for BCC underwent clinical assessment，dermatoscope，OCT，and biopsy/histological examination，with the diagnosis recorded at each stage.The results showed that sensitivities are high for all three techniques，increasing from 90.0％by clinical examination only to 95.7％with the addition of OCT.However，there was a marked and statistically significant increase in specificity from 28.6％by clinical assessment to 75.3％with the addition of OCT.The positive predictive value and negative predictive value were the greatest for OCT，and the overall accuracy of diagnosis for BCC increased from 65.8％（clinical examination alone）to 87.4％with the addition of OCT.The authors of this study emphasized that OCT should not be used as a replacement for clinical examination or dermatoscope，but that OCT is best utilized as an adjunct non-invasive tool especially in difficult cases and in the management of patients with field cancerization or a large number of suspicious skin tumors.