Exploring Patient Pain Experiences during and after Conventional Red Light and Simulated Daylight Photodynamic Therapy for Actinic Keratosis: A Qualitative Interview Study.

Simulated daylight photodynamic therapy is a relatively new and potentially less painful alternative to conventional red light photodynamic therapy for actinic keratosis. Qualitative research exploring patient experiences of pain and skin reactions during these treatments is scarce. To address this, semi-structured interviews were conducted of 10 patients aged 60-81 years with symmetrically distributed actinic keratoses 4 weeks after split-face treatment with conventional red light photodynamic therapy and simulated daylight photodynamic therapy. The participants were recruited from an ongoing clinical randomized trial. Interviews (median length 35 min) were conducted between June 2022 and January 2023, audio-recorded, transcribed verbatim, and analysed qualitatively using content analysis, as described by Graneheim and Lundman. Participants reported that conventional red light photodynamic therapy was very painful during illumination and transiently painful in the post-treatment period, while simulated daylight photodynamic therapy was almost painless during illumination and led to minor post-treatment pain. Also, skin reactions were more intense and longer-lasting with conventional red light photodynamic therapy than with simulated daylight photodynamic therapy. Most participants expressed a treatment preference for simulated daylight photodynamic therapy but had reservations about its unestablished long-term effectiveness. This study underscores the considerable pain associated with conventional red light photodynamic therapy, and the pivotal importance of shared decision-making when selecting the most appropriate treatment.

Pharmacokinetics, Safety, and Tolerability of a Single 5-Day Treatment of Tirbanibulin Ointment 1% in 100 cm2 : A Phase 1 Maximal-Use Trial in Patients with Actinic Keratosis.

Tirbanibulin ointment 1% is approved in the United States and Europe for the treatment of actinic keratosis with demonstrated efficacy, safety, and tolerability when applied over a field up to 25 cm2 . This Phase 1 maximal-use trial determines the plasma pharmacokinetics, safety, and tolerability of tirbanibulin ointment 1% applied to 100 cm2 of the face or balding scalp in adults with actinic keratosis. Twenty-eight patients self-applied tirbanibulin once daily for a single 5-day treatment course. On Day 5, the mean maximum plasma concentration was 1.06 ng/mL and area under the plasma concentration-time curve during a dosing interval was 16.2 ng â€¢ h/mL. Systemic exposure was approximately 4-fold higher than in a previous pharmacokinetic study with a 25 cm2 field, consistent with the increase in the treated area. Tirbanibulin applied to a 100-cm2 treatment field showed favorable safety and tolerability. The most common treatment-emergent adverse events were application site reactions (in 35.7% of patients). All treatment-emergent adverse events and most of the tolerability signs were mild/moderate and resolved or returned to baseline by Day 29. In summary, under maximal-use conditions, tirbanibulin ointment 1% was safe and well tolerated supporting its potential use over a field up to 100 cm2 .

Shedding new light on actinic keratoses and squamous cell carcinoma in situ.

ABSTRACT: Cutaneous squamous cell carcinoma can arise from various premalignant lesions such as actinic keratosis, Bowen disease, and premalignant genital squamous cell lesions. Identification and treatment can prevent malignant transformation and death. This article describes the causes, epidemiology, and characteristics of suspicious premalignant squamous cell lesions so that clinicians can identify these lesions and refer patients for specialist treatment as appropriate.

Dermoscopic Evaluation of Actinic Changes in the Lips of Indigenous Children Living at High Altitude in Ecuador: A Descriptive Study.

BACKGROUND High altitude increases sunlight exposure, resulting in actinic keratosis, which predisposes people to skin cancer. The dermoscopy procedure evaluates keratotic and pigmented skin changes. This study aimed to describe the clinical and dermoscopic actinic changes in the lips of 25 indigenous children living at high altitude in Ecuador. MATERIAL AND METHODS An observational study was conducted in a public school in the Andes region of Ecuador (August-November 2019). Twenty-five children, males and females, age 5-15 years were assessed by complete physical examination, digital dermoscopic photographs, and punch biopsies. Descriptive statistics and Fisher's exact test were used to summarize and analyze the data. RESULTS We included 17 (68%) boys and 8 (32%) girls with a mean age of 9.8±2.0 years. Clinical lips findings reported desquamation [52% Upper Lip (UL); 40% Lower Lip (LL)], fissuring (8% UL; 8% LL), scabs (8% UL; 8% LL), and discoloration (40% UL; 20% LL). Dermoscopic features included a white-yellow lip color (24% UL; p=0.02). The main morphologic pattern of blood vessels was monomorphic (88% UL; p<0.001), polymorphous (60% LL; p<0.001), dotted pattern (64% UL; 28% LL; p=0.02), and linear-irregular (32% UL; 72% LL; p=0.01). Girls had radiating white structures on UL (p=0.025), while boys presented white structureless areas (UL 63.6%; LL 77.8%; p=0.032). No differences in dermoscopic findings were observed according to Fitzpatrick scale score (FSS). Punch biopsies showed no indications of actinic cheilitis. CONCLUSIONS Dermoscopic features in indigenous children living in high altitudes were related to actinic damage, but histopathological findings were negative.

Needs and challenges among general practitioners in the management of actinic keratosis: a qualitative study.

BACKGROUND: Because of the increasing incidence of actinic keratosis (AK), optimal use of limited healthcare resources is essential. Although most patients can be managed in primary care, dermatology referrals are common. More profound knowledge of general practitioners' (GPs) considerations might assist in enhancing AK care. METHODS: The aim of the current study was to gain insight into AK management in primary care by exploring the needs and challenges among GPs in the Netherlands. A qualitative study was conducted based on semi-structured in-depth interviews with 15 conveniently sampled Dutch GPs, focusing on the needs and challenges in AK management. A literature-informed, predefined topic list guided the interviews, which were recorded, transcribed ad verbatim, and thematically analysed using the Framework Method. RESULTS: All GPs reported AK to be a clinical diagnosis and most GPs indicated that most AK patients could be managed in primary care. Cryotherapy was preferred and experience with 5-FU therapy was limited. Most GPs applied cryotherapy without discussing other treatment options with patients. Reasons for dermatology referrals included an incomplete treatment response, extensive lesions, difficult-to-treat areas, and serious doubts about the diagnosis. GPs reported a need for more education, especially on 5-FU therapy. Their main challenges were dealing with diagnostic uncertainty, treating extensive lesions, managing treatment-related skin reactions, and reconciling patient misconceptions. CONCLUSIONS: This study shows various AK management approaches among Dutch GPs with suboptimal guideline compliance due to diverse underlying barriers. It suggests that more education might contribute to a more standardised and uniform AK management and supports further transition of AK care from hospital to primary care.

Evaluating the Efficacy and Safety of 4% 5-Fluorouracil Cream in Patients with Actinic Keratosis: An Expert Opinion.

Actinic keratosis is a lesion that develops in sun-exposed areas of the skin and is considered to be a precancerous condition or an early in situ squamous cell carcinoma. Treatment of actinic keratosis is important for reducing skin cancer risk, with treatment choice based on patient-, lesion- and treatment-related considerations. Of the topical treatments used for field-directed therapy, those containing 5-fluorouracil are among the most effective and widely prescribed. The most recently developed topical 5-fluorouracil preparation (Tolak®; Pierre Fabre, France) contains 4% 5-fluorouracil in an aqueous cream. This narrative review discusses data on 4% 5-fluorouracil cream to treat actinic keratosis, and provides the authors' expert opinion on issues associated with it use. The effect of the cream has been evaluated in phase 2 and 3 trials of adult patients with actinic keratosis on the face, ears or scalp. These trials included patients with severe baseline disease, defined by high lesion counts and large-size treatment fields, which possibly affected the proportion of patients who were able to achieve complete clearance. Other efficacy parameters (e.g. percentage change in lesion count, ≥ 75% clearance of lesions or clinically significant changes in validated severity scales) should also be assessed to fully evaluate 4% 5-fluorouracil treatment efficacy in these patients. Nevertheless, 4% 5-fluorouracil is associated with high efficacy, a low level of recurrence and a satisfactory safety profile.

Folliculotropism in Actinic Keratoses in Patients not Responding to Treatments: A Pilot Study.

Dear Editor, Actinic keratoses (AK) have a high prevalence in the general population, with greater rates in Caucasian patients after the fourth and fifth decades of life (37.5-60.0%) (1,2). Standard histopathologic reporting of AKs does not provide information on the presence of atypical keratinocytes extending to the hair follicle, also defined as folliculotropism (FLC). Commonly, atypical cells in AKs do not present FLC, but this feature can be observed in bowenoid AKs with full-thickness epidermal atypia (3,4). FLC has been considered a possible element enhancing the chances of a progression toward invasive SCC (iSCC). Fernandez-Figueras et al. (3) reported that the depth of FLC in AKs was correlated with the invasiveness of associated iSCC. Pandey et al. (5) reported a positive association between AKs with FLC and history of invasive cutaneous cancer or melanoma, more often in men at an older age. The role of FLC in cutaneous melanoma is still debated, but it is considered a parameter that may correlate with treatment response in lentigo maligna and disease progression or recurrences in invasive tumors (6,7). These studies draw particular attention to the potential role of hair bulge compartment stem cells in favoring tumor progression through the expression of adhesion molecules, cytokines, and growth factor receptors (8). Aks are known to have a high recurrence rate after topical treatment (1). The risk of evolution to an iSCC is not completely clear, but it has been estimated to be around 0.6% at 12 months and up to 2.5% at 48 months (1,3,7). Considering the possible progression and the heavy burden of AK treatments, including the economic burden, it is imperative to focus on histopathologic features associated with treatment failure. The aim of this preliminary study was to assess the histopathologic features, specifically FLC, of AK samples from patients considered "non-responders" to specific topical treatments. A secondary endpoint was to assess the clinical/dermoscopic features. Patients were considered "non-responders" if the lesions persisted after two alternated completed cycles of treatments with ingenol mebutate, imiquimod, diclofenac 3%, or 5-fluoruracil. Patients with a positive history of immunosuppression or genetic diseases were excluded. The study was approved by the local Ethics Committee. Slides of AKs diagnosed at the Laboratory of Dermatopathology, University of Bologna, Italy from January 2016 to October 2018 were reviewed by two dermatopathologists (CM, PAF). 155 "non-responder" AKs of five main histopathologic subtypes were included, classified from grade I to III according to the Roewert-Huber classification (9) (Table 1). The proliferative and atrophic histopathologic subtypes of AKs were detected in 33.6% and 30.4% samples, respectively. FLC was observed in 75.3% of the cases, subdivided into two categories, periadnexal (48.9%) and intraadnexal (26.4%). Periadnexal FLC was detected in 31.0% of atrophic and in 50.3% of proliferative AKs, while intraadnexal FLC was found in 48.7% and 29.2%, respectively (Figure 1, a, b). At dermoscopy, most lesions had been classified as grade I or II (38.8% and 45.8%), and only 15.4% as grade III, showing an unexpected non-response to treatment according to the dermoscopic criteria. In contrast, almost half of the AKs were classified as grade III at histology, revealing a discrepancy between the dermoscopic grading and histological findings in a majority of cases (77.4%) (Figure 2, c, d). Furthermore, atrophic and proliferative AKs accounted for 64.0% of total cases, and these are the variants associated with a higher probability of evolution toward an iSCC (10). The clinical/histological discrepancy has already been reported in the literature (9) and may represent a misleading factor for treatment choice and outcomes. We believe that a comparative analysis with dermoscopy and histology should be performed in non-responding AKs, in order to choose the best therapeutic option. In fact, some superficial treatments (such as cryotherapy) may not provide a good response in deep hair follicles (4). We also suggest encouraging greater focus on FLC and its description in pathology reports. This is a preliminary observational study, but it reinforces the need to further larger clinical studies investigating the role of specific histopathologic parameters in AKs, including FLC, that may correlate with treatment outcomes.

S3 guideline "actinic keratosis and cutaneous squamous cell carcinoma" - update 2023, part 2: epidemiology and etiology, diagnostics, surgical and systemic treatment of cutaneous squamous cell carcinoma (cSCC), surveillance and prevention.

Actinic keratosis (AK) are common lesions in light-skinned individuals that can potentially progress to cutaneous squamous cell carcinoma (cSCC). Both conditions may be associated with significant morbidity and constitute a major disease burden, especially among the elderly. To establish an evidence-based framework for clinical decision making, the guideline "actinic keratosis and cutaneous squamous cell carcinoma" was updated and expanded by the topics cutaneous squamous cell carcinoma in situ (Bowen's disease) and actinic cheilitis. The guideline is aimed at dermatologists, general practitioners, ear nose and throat specialists, surgeons, oncologists, radiologists and radiation oncologists in hospitals and office-based settings, as well as other medical specialties, policy makers and insurance funds involved in the diagnosis and treatment of patients with AK and cSCC. A separate guideline exists for patients and their relatives. In this part, we will address aspects relating to epidemiology and etiology, diagnostics, surgical and systemic treatment of cutaneous squamous cell carcinoma (cSCC), surveillance and prevention.

Oral Nicotinamide for Actinic Keratosis Prevention in Kidney Transplant Recipients: A Pilot Double-Blind, Randomized, Placebo-Controlled Trial.

BACKGROUND: Oral nicotinamide (NAM) has shown promise in preventing actinic keratoses (AKs) in trials based outside of the United States. We assessed the efficacy of oral NAM supplementation in kidney transplant recipients with a history of keratinocyte carcinoma. MATERIAL AND METHODS: Patients enrolled in a 2-week run-in phase, during which NAM 1000 mg was taken twice daily. After a washout period, patients who tolerated the run-in phase were randomized to NAM 500 mg twice daily or placebo. At baseline, 4, 8, and 12 months, dermatologists conducted full-body skin exams to document area-specific AKs. Routine lab work was collected to ensure the stability of renal allograft function. RESULTS: The dosage was reduced from 1000 to 500 mg due to gastrointestinal symptoms in the run-in phase. Patients were randomized to NAM (n = 10) or placebo (n = 11). At 12 months, mean AK count was 30.8 (95% CI -11.7-73.4) for NAM and 26.6 (95% CI 10.8-42.5) for placebo. The difference in percent AK count change at 12 months compared with baseline was 259.8% (95% CI -385.9 to 905.5) for NAM and 72.4% (95% CI -118.6 to 263.5) for placebo. The between-group difference in percent AK change was not significant (P = .38). There was no attrition in the placebo group and 40% attrition in the NAM arm. DISCUSSION: Nicotinamide did not decrease AK development among kidney transplant recipients. Limitations include drug tolerability, small sample size, and single-center trial nature.

Non-invasive diagnosis of early cutaneous squamous cell carcinoma.

Early cutaneous squamous cell carcinoma (cSCC) can be challenging to diagnose using clinical criteria as it could present similar to actinic keratosis (AK) or Bowen's disease (BD), precursors of cSCC. Currently, histopathological assessment of an invasive biopsy is the gold standard for diagnosis. A non-invasive diagnostic approach would reduce patient and health system burden. Therefore, this study used non-invasive sampling by tape-stripping coupled with data-independent acquisition mass spectrometry (DIA-MS) proteomics to profile the proteome of histopathologically diagnosed AK, BD and cSCC, as well as matched normal samples. Proteomic data were analysed to identify proteins and biological functions that are significantly different between lesions. Additionally, a support vector machine (SVM) machine learning algorithm was used to assess the usefulness of proteomic data for the early diagnosis of cSCC. A total of 696 proteins were identified across the samples studied. A machine learning model constructed using the proteomic data classified premalignant (AK + BD) and malignant (cSCC) lesions at 77.5% accuracy. Differential abundance analysis identified 144 and 21 protein groups that were significantly changed in the cSCC, and BD samples compared to the normal skin, respectively (adj. p < 0.05). Changes in pivotal carcinogenic pathways such as LXR/RXR activation, production of reactive oxygen species, and Hippo signalling were observed that may explain the progression of cSCC from premalignant lesions. In summary, this study demonstrates that DIA-MS analysis of tape-stripped samples can identify non-invasive protein biomarkers with the potential to be developed into a complementary diagnostic tool for early cSCC.

Dermoscopy of atypical pigmented lesions of the face: Variation according to facial areas.

Atypical pigmented facial lesions (aPFLs)-including lentigo maligna (LM) and lentigo maligna melanoma (LMM), solar lentigo (SL), pigmented actinic keratosis (PAK), atypical nevi (AN), seborrheic keratosis (SK) and lichen planus-like keratosis (LPLK)-can exhibit clinical and dermoscopic overlapping features. We aimed to investigate if and how 14 dermoscopic features suggestive for the aforementioned aPFLs vary according to six facial sites among 1197 aPFLs cases (excised to rule out malignancy) along with lesion and patients' metadata. According to distribution and association analysis, aPFLs on the forehead of a male patient aged > 69 years displaying the obliterated follicular openings pattern, appear to be more at risk of malignancy. Of converse, aPFLs of the orbital/cheek/nose area with evident and regular follicular openings with diameter < 10 mm in a female aged below 68 are probably benign. The obliterated follicular openings, keratin plugs, evident and regular follicular openings and target-like pattern features differed significantly among six facial areas in all aPFLs cases. Lesion of the nose may show both features suggestive of malignancy and benignity (e.g. many SL and PAK may display target-like pattern and some LM/LMM cases display keratin plugs and evident and follicular openings), making these features less specific.

Ablative Fractional Laser Enhances Artificial or Natural Daylight Photodynamic Therapy of Actinic Field Cancerization: A Randomized and Investigator-initiated Half-side Comparative Study.

Artificial daylight photodynamic therapy is a near-painless treatment for actinic keratoses, which can be performed indoors using a controlled light dose. Daylight photodynamic therapy is approved only for treatment of grade I-II actinic keratoses. The aim of this study was to evaluate whether fractional laser pre-treatment improves the outcomes of daylight photodynamic therapy for actinic keratoses of all grades. In addition, the study compared the outcomes of artificial and natural daylight photodynamic therapy. This randomized single-blinded split-side comparative study included 60 patients with ≥ 2 actinic keratoses of the head. Fractional laser pre-treatment was assigned randomly for actinic keratoses on 1 side of the head and, subsequently, the entire treatment area was treated with artificial or natural daylight photodynamic therapy. Fractional laser-mediated daylight photodynamic therapy achieved significantly higher complete clearance (50.0% vs 30.3%, p = 0.04), partial clearance (78.6% vs 50.0%, p < 0.01) and lesion-specific clearance (86.2% vs 70.2%, p < 0.01) than daylight photodynamic therapy alone at the 6-month follow-up. No significant differences were found in the outcomes of artificial vs natural daylight photodynamic therapy or grade I lesions vs grade II-III lesions. Thus, fractional laser pre-treatment appears to significantly increase the efficacy of artificial and natural daylight photodynamic therapy, and to be suitable for treatment of actinic keratoses of all grades.

Systemic Photoprotection in Melanoma and Non-Melanoma Skin Cancer.

Non-melanoma skin cancers (NMSCs), which include basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and actinic keratosis (AK), are the most common cancer diseases in the Caucasian race. If diagnosed late and improperly treated, BCC and SCC can become locally advanced and metastasize. Malignant melanoma (MM) is less frequent but more lethal than NMSC. Given the individual and social burdens of skin cancers, performing an adequate prevention is needed. Ultraviolet (UV) ray exposure is one of the main risk factors for skin cancer. Thus, the first-choice prevention strategy is represented by photoprotection that can be both topical and systemic. The latter consists of the oral administration of molecules which protect human skin against the damaging effects of UV rays, acting through antioxidant, anti-inflammatory, or immunomodulator mechanisms. Although several compounds are commonly used for photoprotection, only a few molecules have demonstrated their effectiveness in clinical trials and have been included in international guidelines for NMSC prevention (i.e., nicotinamide and retinoids). Moreover, none of them have been demonstrated as able to prevent MM. Clinical and preclinical data regarding the most common compounds used for systemic photoprotection are reported in this review, with a focus on the main mechanisms involved in their photoprotective properties.

A risk-scoring model for the differential diagnosis of lentigo maligna and other atypical pigmented facial lesions of the face: The facial iDScore.

BACKGROUND: Due to progressive ageing of the population, the incidence of facial lentigo maligna (LM) of the face is increasing. Many benign simulators of LM and LMM, known as atypical pigmented facial lesions (aPFLs-pigmented actinic keratosis, solar lentigo, seborrheic keratosis, seborrheic-lichenoid keratosis, atypical nevus) may be found on photodamaged skin. This generates many diagnostic issues and increases the number of biopsies, with a subsequent impact on aesthetic outcome and health insurance costs. OBJECTIVES: Our aim was to develop a risk-scoring classifier-based algorithm to estimate the probability of an aPFL being malignant. A second aim was to compare its diagnostic accuracy with that of dermoscopists so as to define the advantages of using the model in patient management. MATERIALS AND METHODS: A total of 154 dermatologists analysed 1111 aPFLs and their management in a teledermatology setting: They performed pattern analysis, gave an intuitive clinical diagnosis and proposed lesion management options (follow-up/reflectance confocal microscopy/biopsy). Each case was composed of a dermoscopic and/or clinical picture plus metadata (histology, age, sex, location, diameter). The risk-scoring classifier was developed and tested on this dataset and then validated on 86 additional aPFLs. RESULTS: The facial Integrated Dermoscopic Score (iDScore) model consisted of seven dermoscopic variables and three objective parameters (diameter ≥ 8 mm, age ≥ 70 years, male sex); the score ranged from 0 to 16. In the testing set, the facial iDScore-aided diagnosis was more accurate (AUC = 0.79 [IC 95% 0.757-0.843]) than the intuitive diagnosis proposed by dermatologists (average of 43.5%). In the management study, the score model reduced the number of benign lesions sent for biopsies by 41.5% and increased the number of LM/LMM cases sent for reflectance confocal microscopy or biopsy instead of follow-up by 66%. CONCLUSIONS: The facial iDScore can be proposed as a feasible tool for managing patients with aPFLs.

Non-Invasive Diagnosis of Sun Damaged Skin: Actinic Keratosis Vs Squamous Cell Carcinoma.

IMPORTANCE: Actinic keratosis (AK) is a premalignant lesion that has a1% to 10% potential of progression to squamous cell carcinoma (SCC), but it is not possible to determine which lesions are at higher risk. OBJECTIVE: This study examined the epidermal genetic profiles of actinic keratosis and SCC through non-invasive techniques seeking to develop a biopsy-free method for AK monitoring and aid in the early diagnosis of developing SCC. DESIGN: Ribonucleic acid (RNA) was collected from adhesive tape strips and gene expression levels were measured. A threshold fold change >2 and adjusted P-value <0.05 were used to determine differentially expressed genes. SETTING: Single center dermatology clinic. PARTICIPANTS: Patients who presented to the clinic with lesions suspicious of non-melanoma skin cancer that had never been previously biopsied. MAIN OUTCOME AND MEASURE: RNA was extracted via non-invasive biopsy and sequenced. Low quality samples were filtered out and the remaining samples underwent differential gene expression analysis by DESeq2 in R package. A threshold of fold change >2 and adjusted P-value <0.05 was used for determination of differentially expressed genes. The differentially expressed genes that overlapped between the corrected and uncorrected groups were the most significant for analysis. RESULTS: From 47 lesions, 6 significant differentially expressed genes were found between AK and SCC, and 25 significant differentially expressed genes between in-situ SCC and invasive SCC. Individual samples showed similarities based on diagnosis, suggesting mutations were specific to the disease and not the individual. CONCLUSIONS AND RELEVANCE: These findings highlight which genes may play a role in AK progression to SCC. The genomic differences between in-situ and invasive squamous cell carcinoma open an opportunity for early diagnosis of squamous cell carcinoma and risk prediction of actinic keratosis. J Drugs Dermatol. 2023;22(5): doi:10.36849/JDD.7097.

PRAME immunohistochemistry can distinguish melanocytic pseudonests of lichenoid reactions from melanoma in situ.

BACKGROUND: Distinguishing melanocytic pseudonests encountered in lichenoid dermatoses or lichenoid keratoses from melanoma in situ (MIS) with brisk lichenoid inflammation can prove challenging. METHODS: We designed a case-control study to evaluate the accuracy metrics of PRAME immunohistochemistry to distinguish melanocytic pseudonests in lichenoid dermatoses or keratoses from inflamed MIS. Immunostaining for PRAME was performed on paraffin-embedded formalin-fixed diagnostic tissue using a rabbit monoclonal antibody to PRAME (Abcam), with a 1:3200 dilution on a Leica Bond detection system. RESULTS: Our search identified 21 cases of melanocytic pseudonests (n = 21, 46%) encountered in lichenoid dermatoses and 24 cases of inflamed MIS (n = 24, 53%). Each method of evaluating PRAME immunohistochemistry (PRAME+ clusters, PRAME % of melanocytes by four categories and PRAME+ melanocyte counts per linear mm of epidermal basal layer) showed statistically significant differences between the MIS and the pseudonest cohorts (respectively, p < 0.001; p < 0.001; and p < 0.001). Receiver operating characteristics analysis for PRAME+ melanocyte counts per linear mm of epidermal basal layer revealed an area under the curve of 0.9 ± 0.05 (95% confidence interval 0.9-1.0). When determining an optimal cut-off point for the best Youden index [sensitivity (%) + specificity (%) - 100], the cut-off of 1.0 PRAME+ melanocytes per linear mm showed a sensitivity of 79.2% and specificity of 85.7% (Youden index 0.65) to distinguish MIS from pseudonests. CONCLUSION: PRAME immunohistochemistry may constitute an additional tool for this challenging differential diagnosis.