Keratoacanthoma: a tumor in search of a classification

Radiotherapy for keratoacanthoma of facial skin: A case report and review of literature

Background

Keratoacanthoma (KA) is a benign tumor that arises from the infundibulum of hair follicles. However, some researchers believe that KA is a subtype of squamous cell carcinoma (SCC) or a borderline tumor. There are two types of KA: single-type and multiple-type. Surgical resection is the first-line treatment for KA. The treatment options for patients with large lesions who are not surgical candidates are limited. We present a case of single-type KA patients with basic diseases and large lesions that were untreatable surgically, but the lesions essentially disappeared after radiotherapy. No recurrences were discovered during the two-year follow-up.

Case Description

A 62-year-old male patient was admitted to the dermatology department of our hospital in June 2020 due to the discovery of a red papule on the right face two months prior, with occasional itching, which increased gradually. Pathological examination confirmed the diagnosis of KA. Due to the large lesions and underlying diseases, he was transferred to our radiotherapy department for radiotherapy after consultation. Since the surface of the lesion is uneven and close to the corner of the eye, we adopted intensity modulated radiation therapy (IMRT) at the beginning of radiotherapy. Following the reduction of the lesion, superficial electron beam and added a bolus with thickness of 5mm on the surface of the lesion was continued. The target dose: 42Gy/21 fractions (6MV X-ray, 22Gy; 2Gy/fraction; a total of 11 fractions, 6MeV electron beam, 20Gy; 2Gy/fraction; a total of 10 fractions). By the end of radiotherapy, the patient's facial tumor was dry and subsided. The facial tumor subsided significantly two years after radiotherapy, and the damaged skin on the face recovered to a flat shape.

Conclusions

The treatment experience of this case shows that IMRT combined with superficial electron beam radiotherapy may be an effective treatment for single-type KA patients with basic diseases and large lesions that are not suitable for surgery, and it is worth further study.

Differences in the mitochondrial microsatellite instability of Keratoacanthoma and cutaneous squamous cell carcinoma

Keratoacanthoma (KA) is a common cutaneous neoplasm which often resembles typical squamous cell carcinoma (SCC) in both its clinical and historical presentation. Several studies have attempted to identify methods for distinguishing between KA and SCC, however, none of these have proven to play any obvious roles in these tumors. Given this we went on to evaluate mitochondrial microsatellite instability (mtMSI) in KA and SCC in an effort to understand these tumors better. DNA was isolated from paired normal and tumoral tissues donated by 57 KA patients and 43 SCC patients. MtMSI was then analyzed using eight microsatellite markers and was observed in 2 (3.5%) of the 57 KA patients and 8 (18.6%) of the 43 SCC patients, respectively. MtMSI was also shown to affect different locations depending on tumor type. In KA patients, mtMSI was detected at mitochondrial D514 D-loop and presented with (CA) n repeats, in contrast, all of the SCC patient experienced mtMSI at the D310 with (C)n repeats of the D-loop region. These differences in location were found to be significant, which may support the hypothesis that KA and SCC have different pathogenetic pathways. Our results also suggest that mtMSI may be a candidate for developing novel differential diagnostic methods for KA and SCC.

The first report of diagnosing of keratoacanthoma in Chinese Han patients using dermoscopy and reflectance confocal microscopy

BACKGROUND

Keratoacanthoma (KA) is a special kind of tumor, which is regarded as a variant of squamous cell carcinoma (SCC) in some academic disciplines. But differentiating KA from SCC remains a diagnostic challenge. The noninvasive techniques dermoscopy and reflectance confocal microscopy (RCM) can provide new insights for diagnosis.

OBJECTIVE

To observe the characteristics of KA under dermoscopy and reflectance confocal microscopy (RCM), in order to gain experience and reference for clinicians to facilitate earlier diagnosis.

METHODS

We collected two cases of KA, which were confirmed by clinical and histopathological examination. The two cases were examined by dermoscopy and RCM, respectively. Then, we collected the microscopic characteristics of KA lesions.

RESULTS

The dermoscopy features of KA are concentric circles of central crater, keratin mass, keratin scale, and polymorphic vascular pattern. The RCM features are described as refractile crust, atypical honeycomb pattern, dark center cells, large round nucleated cells, dendritic cells, and linear or round vessels traversing dermal papillae in the dermis.

CONCLUSION

KA has some specific dermoscopy and RCM features, which can provide a basis for doctors to diagnose and intervene earlier.

UV biomarker genes for classification and risk stratification of cutaneous actinic keratoses and squamous cell carcinoma subtypes

Currently, there is no sensitive molecular test for identifying transformation-prone actinic keratoses (AKs) and aggressive squamous cell carcinoma (SCC) subtypes. Biomarker-based molecular testing represents a promising tool for risk stratifying these lesions. We evaluated the utility of a panel of ultraviolet (UV) radiation-biomarker genes in distinguishing between benign and transformation-prone AKs and SCCs. The expression of the UV-biomarker genes in 31 SCC and normal skin (NS) pairs and 10 AK/NS pairs was quantified using the NanoString nCounter system. Biomarker testing models were built using logistic regression models with leave-one-out cross validation in the training set. The best model to classify AKs versus SCCs (area under curve (AUC) 0.814, precision score 0.833, recall 0.714) was constructed using a top-ranked set of 13 UV-biomarker genes. Another model based on a 15-gene panel was developed to differentiate histologically concerning from less concerning SCCs (AUC 1, precision score 1, recall 0.714). Finally, 12 of the UV-biomarker genes were differentially expressed between AKs and SCCs, while 10 genes were uniquely expressed in the more concerning SCCs. UV-biomarker gene subsets demonstrate dynamic utility as molecular tools to classify and risk stratify AK and SCC lesions, which will complement histopathologic diagnosis to guide treatment of high-risk patients.

Resolution of Keratoacanthoma Type Squamous Cell Carcinoma Following Intralesional Therapy With Methotrexate

Keratoacanthoma is considered a variant of squamous cell carcinoma prone to spontaneous involution, but it may also rapidly grow and invade surrounding tissues. Herein, we report a case of keratoacanthoma-type squamous cell carcinoma that resolved after intralesional therapy with methotrexate.

Multicentric cutaneous keratoacanthomas in a free-living marmoset (Callithrix sp.)

Cutaneous neoplasia is common in non-human primates. We describe the gross and microscopic features of multicentric cutaneous keratoacanthomas in a free-living marmoset (Callithrix sp.). Immunohistochemistry for human papillomavirus and herpes simplex virus type I and simplex virus type II was negative. Keratoacanthomas should be included in the differential diagnosis for cutaneous masses in non-human primates.

Epidemiology of skin cancer in the mature patient

Epidermal cancers include keratinocyte cancer, melanocyte cancer, and Merkel cell carcinoma. These cancers account for the vast majority of new cancers diagnosed in Australia, North America, and Europe. Keratinocyte cancer is the most common epidermal cancer and accounts for 7 out of 8 new cancers diagnosed in Australia. Melanoma and Merkel cell carcinoma are less common than keratinocyte carcinoma but are more important causes of mortality in Australia. Keratinocyte cancer has also been demonstrated to be a marker of cancer-prone phenotype. Risk factors for epidermal cancer include intrinsic and environmental factors, in particular exposure to ultraviolet radiation and advanced age. Actinic keratosis has an approximate prevalence of 79% of men and 68% of women between 60 and 69 years of age, and has a low risk of malignant transformation into squamous cell carcinoma. Basal cell carcinoma is the most common malignancy in Caucasians worldwide, with the incidence increasing by 2% per year in Australia. Squamous cell carcinoma is the second most common epidermal cancer, with an incidence of approximately 1035 or 472 per 100,000 person-years in men and women, respectively. Primary risk factors for both basal cell carcinoma and squamous cell carcinoma include light skin color, UV radiation exposure, and chronic immunosuppression. Although the rate of melanoma is increasing, the mortality in Australia is reducing and is currently 9%. The overall incidence of melanoma in Australia is approximately 50 cases per 100,000 persons (62 for men and 40 for women). Keratinocyte carcinoma and melanoma are risk factors for developing further skin cancer and primary malignancy. This contribution reviews the incidence, prevalence, and risk factors associated with the development of epidermal cancer and premalignant epidermal neoplasia.

Imiquimod in dermatology: an overview

Imiquimod is an immune response modifier commercially available as a 3.75 and 5% cream. Topical imiquimod stimulates the innate and adaptive immune responses and induces cytokine production. This allows its use for the treatment of a wide variety of benign and malignant skin conditions due to its potential antiviral, antitumor, and immunoregulatory effects. Currently, topical imiquimod is US Food and Drug Administration (FDA)-approved for the treatment of anogenital warts, actinic keratosis, and superficial basal cell carcinomas. However, it has also shown a beneficial effect in the treatment of many other skin disorders. In this review, we describe existing evidence on the mechanism of action of topical imiquimod, its FDA-approved indications, off-label uses, and side effects.

Non-melanoma Skin Cancer in Canada Chapter 5: Management of Squamous Cell Carcinoma

Background

Squamous cell carcinoma (SCC) is the second-most common form of non-melanoma skin cancer (NMSC).

Objective

To provide guidance to Canadian health care practitioners regarding management of SCCs.

Methods

Literature searches and development of graded recommendations were carried out as discussed in the accompanying introduction (chapter 1 of the NMSC guidelines).

Results

SCCs are sometimes confined to the epidermis, but they can also invade nearby tissues and, in some cases, metastasize to neighbouring lymph nodes or other organs. This chapter discusses the natural history, staging, prognosis, and management of SCC—a tumour type that is less common but typically more aggressive than BCC. For this reason, margin control is strongly preferred in treating SCCs.

Conclusions

Although approaches such as cryosurgery and radiation therapy may be considered for some patients, surgical excision—sometimes coupled with radiation—remains the cornerstone of SCC management. Patients with high-risk SCC may also be considered for referral to an appropriate multidisciplinary clinic.

The Role of p16, p21, p27, p53 and Ki-67 Expression in the Differential Diagnosis of Cutaneous Squamous Cell Carcinomas and Keratoacanthomas: An Immunohistochemical Study

BACKGROUND

Distinguishing squamous cell carcinoma (SCC) from keratoacanthoma (KA) by histopathological features may not be sufficient for a differential diagnosis, as KAs may, in some cases, imitate well-differentiated SCCs.

AIMS

In this study, we investigated whether the expression of the p16, p21, p27, p53 genes and a Ki-67 proliferation index are useful in distinguishing between these two tumors.

STUDY DESIGN

Cross-sectional study.

METHODS

Immunohistochemistry was used to investigate the expression of the p16, p21, p27, p53 genes and the Ki-67 proliferation index was investigated in well-differentiated SCC with KA-like features (n=40) and KA (n=30).

RESULTS

The results of all of the examined markers, except for p27 (p16, p21, p53, and Ki-67) were found to be significantly different between the SCC and KA samples (p<0.05).

CONCLUSION

In well-differentiated SCC with KA-like features and KA cases where the differential diagnosis is difficult from a histopathological perspective, the use of p16, p21, p53 expression and a Ki-67 proliferation index can be useful for the differential diagnosis of SCCs and KAs.

HIPK2 expression in progression of cutaneous epithelial neoplasm

BACKGROUND

Homeodomain-interacting protein kinase 2 (HIPK2) is responsible for a DNA damage response, centrally regulating p53. The aberrant HIPK2 expression is known to be involved in carcinogenesis in several malignancies. However, the correlation of HIPK2 expression along with progression of cutaneous epithelial neoplasm has not been investigated.

METHODS

Using immunohistochemistry and real-time reverse transcription-polymerase chain reaction, we examined the correlation between HIPK2 and HIPK2-related protein expressions and the progression of some cutaneous epithelial neoplasms (i.e., actinic keratosis, Bowen's disease, keratoacanthoma, squamous cell carcinoma, and basal cell carcinoma).

RESULTS

HIPK2 expression was distinct between preinvasive and invasive lesions: the expression decreased in keratoacanthoma (none of eight) and squamous cell carcinoma (five of 35) compared to actinic keratosis (12 of 19) and Bowen's disease (10 of 23) (P < 0.001). HIPK2 expression was also negatively correlated with aggressiveness of basal cell carcinoma; high-risk subtypes showed lower HIPK2 expression than did low-risk subtypes (P < 0.001). HIPK2 mRNA expression of each tumor group was significantly higher than that of normal skin. HIPK2 mRNA expression of each tumor group was not correlated with the relevant HIPK2 protein expression, which was consistent with previous studies.

CONCLUSIONS

HIPK2 expression tends to be decreased along tumor progression and may be involved with the invasive potential, suggesting a possible tumor suppressor role for HIPK2.

Non-melanoma Skin Cancer in Canada Chapter 1: Introduction to the Guidelines

BACKGROUND

Non-melanoma skin cancer (NMSC), including basal and squamous cell carcinoma, represents the most common malignancy.

OBJECTIVE

The aim of this document is to provide guidance to Canadian health care practitioners on NMSC management.

METHODS

After conducting a literature review, the group developed recommendations for prevention, management, and treatment of basal cell carcinomas, squamous cell carcinomas, and actinic keratoses. These tumour types are considered separately in the accompanying articles. The Grading of Recommendations Assessment, Development and Evaluation system was used to assign strength to each recommendation.

RESULTS

This introduction describes the scope and structure of the guidelines and the methods used to develop them. The epidemiology of NMSC is reviewed, as are the pathophysiologic changes occurring with damage to the skin, which lead to the formation of actinic keratoses and invasive squamous or basal cell carcinomas.

CONCLUSIONS

This introduction describes the need for primary prevention and offers an overview of treatment options that are discussed in later chapters of the guidelines.

Cutaneous keratoacanthomas/squamous cell carcinomas associated with neutralization of transforming growth factor β by the monoclonal antibody fresolimumab (GC1008)

Fresolimumab is an antibody capable of neutralizing all human isoforms of transforming growth factor beta (TGFβ) and has demonstrated anticancer activity in investigational studies. Inhibition of TGFβ by fresolimumab can potentially result in the development of cutaneous lesions. The aim of this study was to investigate the clinical, histological, and immunohistochemical characteristics of cutaneous neoplasms associated with fresolimumab. Skin biopsies (n = 24) were collected and analyzed from patients (n = 5) with treatment-emergent, cutaneous lesions arising during a phase 1 study of multiple doses of fresolimumab in patients (n = 29) with melanoma or renal cell carcinoma. Blinded, independent histological review and measurements of Ki-67, p53, and HPV integration were performed. Based on central review, four patients developed lesions with histological characteristics of keratoacanthomas, and of these patients, a single case of well-differentiated squamous cell carcinoma was also found. Expression of Ki-67, no evidence of p53 overexpression, and only focal positivity for human papillomavirus RNA by in situ hybridization in 4/18 cases were consistent with these findings. Following completion of fresolimumab, lesions spontaneously resolved. Therefore, benign, reversible keratoacanthomas were the most common cutaneous neoplasms observed, a finding of importance for adverse event monitoring, patient care, and optimization of therapies targeting TGFβ.

Keratoacanthoma Pathobiology in Mouse Models

Recently we described skin tumors driven by skin-specific expression of Zmiz1 and here we define keratoacanthoma pathobiology in this mouse model. Similar to human keratoacanthoma development, we were able to segregate murine keratoacanthomas into three developmental phases: growth, maturation, and regression. These tumors had areas with cellular atypia, high mitotic rate, and minor local invasion in the growth phase, but with development they transitioned to maturation and regression phases with evidence of resolution. The early aggressive appearance could easily be misdiagnosed as a malignant change if the natural pathobiology was not well-defined in the model. To corroborate these findings in the Zmiz1 model, we examined squamous skin tumors from another tumor study in aging mice, and these tumors followed a similar biological progression. Lastly, we were able to evaluate the utility of the model to assess immune cell infiltration (F4/80, B220 Granzyme B, CD3 cells, arginase-1) in the regression phase; however, because inflammation was present at all phases of development, a more comprehensive approach will be needed in future investigations. Our study of keratoacanthomas in selected murine models suggests that these squamous tumors can appear histologically aggressive during early development, but with time will enter a regression phase indicating a benign biology. Importantly, studies of squamous skin tumor models should be cautious in tumor diagnosis as the early growth distinction between malignant versus benign based solely on histopathology may not be easily discerned without longitudinal studies to confirm the tumor pathobiology.

Generalized eruptive keratoacanthoma: proposed diagnostic criteria and therapeutic evaluation

Generalized eruptive keratoacanthoma (GEKA) of Grzybowski is a sporadically occurring, extremely rare variant of keratoacanthoma characterized clinically by severely pruritic, generalized eruption of numerous small follicular papules, often with a central keratotic plug, and histologically by typical features of solitary keratoacanthoma. Despite the continuous addition of new case reports, no definite diagnostic criteria have been established, and an optimum treatment is not yet determined. Herein, we review the different aspects of this rare entity, including pathogenesis, clinical and histopathological features, differential diagnosis, course and prognosis. Different therapeutic approaches and their impact on the course and prognosis of the disease are also evaluated and presented. We propose two sets of diagnostic criteria to define the disease more precisely and to avoid overlapping and confusion with other types of multiple keratoacanthoma. The first set comprises constant clinical and histopathological features that almost always present in every case and the second set includes variable features that were reported in some patients, and to which any emerging finding could be added to avoid missing cases. Although different therapeutic options have been used, either as single agents or in combinations, there is no standard therapy for GEKA and the disease still represents a therapeutic challenge.

Phase I study of GC1008 (fresolimumab): a human anti-transforming growth factor-beta (TGFβ) monoclonal antibody in patients with advanced malignant melanoma or renal cell carcinoma

Background

In advanced cancers, transforming growth factor-beta (TGFβ) promotes tumor growth and metastases and suppresses host antitumor immunity. GC1008 is a human anti-TGFβ monoclonal antibody that neutralizes all isoforms of TGFβ. Here, the safety and activity of GC1008 was evaluated in patients with advanced malignant melanoma and renal cell carcinoma.

Methods

In this multi-center phase I trial, cohorts of patients with previously treated malignant melanoma or renal cell carcinoma received intravenous GC1008 at 0.1, 0.3, 1, 3, 10, or 15 mg/kg on days 0, 28, 42, and 56. Patients achieving at least stable disease were eligible to receive Extended Treatment consisting of 4 doses of GC1008 every 2 weeks for up to 2 additional courses. Pharmacokinetic and exploratory biomarker assessments were performed.

Results

Twenty-nine patients, 28 with malignant melanoma and 1 with renal cell carcinoma, were enrolled and treated, 22 in the dose-escalation part and 7 in a safety cohort expansion. No dose-limiting toxicity was observed, and the maximum dose, 15 mg/kg, was determined to be safe. The development of reversible cutaneous keratoacanthomas/squamous-cell carcinomas (4 patients) and hyperkeratosis was the major adverse event observed. One malignant melanoma patient achieved a partial response, and six had stable disease with a median progression-free survival of 24 weeks for these 7 patients (range, 16.4–44.4 weeks).

Conclusions

GC1008 had no dose-limiting toxicity up to 15 mg/kg. In patients with advanced malignant melanoma and renal cell carcinoma, multiple doses of GC1008 demonstrated acceptable safety and preliminary evidence of antitumor activity, warranting further studies of single agent and combination treatments.

Trial Registration

Clinicaltrials.gov NCT00356460

Conjunctival keratoacanthoma

An 83-year-old man presented with a 1-month history of a rapidly enlarging conjunctival mass. On examination, slit lamp biomicroscopy revealed a leukoplakic tumour at the temporal limbus. The lesion was excised with cryotherapy application to the limbus and conjunctival margins. Histopathology revealed a keratoacanthoma (KA). KA typically occurs on sun-exposed areas of the skin. Conjunctival KA is very rare, and differentiation between conventional squamous cell carcinoma (SCCA) and KA can be challenging. The present case highlights the indication for excisional surgery in patients with conjunctival KA using the no touch technique, cryotherapy, amniotic membrane and the histopathological differentiation between KA and SCCA.

Ectopic expression of Zmiz1 induces cutaneous squamous cell malignancies in a mouse model of cancer

Cutaneous squamous cell carcinoma (SCC) is the second most common form of cancer in the human population, yet the underlying genetic mechanisms contributing to the disease are not well understood. We recently identified Zmiz1 as a candidate oncogene in non-melanoma skin cancer through a transposon mutagenesis screen. Here we show that transposon-induced mutations in Zmiz1 drive expression of a truncated transcript that is similar to an alternative endogenous ZMIZ1 transcript found to be overexpressed in human SCCs relative to normal skin. We also describe an original mouse model of invasive keratoacanthoma driven by skin-specific expression of the truncated Zmiz1 transcript. Unlike most mouse models, Zmiz1-induced skin tumors develop rapidly and in the absence of promoting agents such as phorbol esters. Additionally, we found that the alternative Zmiz1 isoform has greater protein stability than its full-length counterpart. Finally, we provide evidence that ZMIZ1 is overexpressed in a significant percentage of human breast, ovarian, and colon cancers in addition to human SCCs, suggesting ZMIZ1 may play a broader role in epithelial cancers.

Keratoacanthoma centrifugum marginatum with atypical scar

Keratoacanthoma centrifugum marginatum (KCM) is a rare variant of keratoacanthoma (KA). It is characterized by a progressive peripheral expansion and central healing leaving atrophic scar. It is sometimes confused with squamous cell carcinoma (SCC) both clinically and histopathologically. We here report a case of KCM over the extensor aspect of the right forearm in a 57-year-old man with an abnormal looking scar.

Strategies to use immune modulators in therapeutic vaccines against cancer

Cancers so much resemble self that they prove difficult for the immune system to eliminate, and those that have already escaped natural immunosurveillance have gotten past the natural immune barriers to malignancy. A successful therapeutic cancer vaccine must overcome these escape mechanisms. Our laboratory has focused on a multistep "push-pull" approach in which we combine strategies to overcome each of the mechanisms of escape. If tumor epitopes are insufficiently immunogenic, we increase their immunogenicity by epitope enhancement, improving their binding affinity to major histocompatibility complex (MHC) molecules. If the anti-tumor response is too weak or of the wrong phenotype, we use cytokines, costimulatory molecules, Toll-like receptor ligands, and other molecular adjuvants to increase not only the quantity of the response but also its quality, to push the response in the right direction. Finally, the tumor invokes multiple immunosuppressive mechanisms to defend itself, so we need to overcome those as well, including blocking or depleting regulatory cells or inhibiting regulatory molecules, to pull the response by removing the brakes. Some of these strategies individually have now been translated into human clinical trials in cancer patients. Combinations of these in a push-pull approach are promising for the successful immunotherapy of cancer.

The nuclear factor kappa B p50 subunit and cortactin as markers to distinguish between keratoacanthoma and well-differentiated squamous cell carcinoma

BACKGROUND

Distinguishing keratoacanthoma (KA) from well-differentiated squamous cell carcinoma (SCC) is sometimes difficult. Recent evidence indicates that the nuclear factor kappa B p50 subunit (p50) and cortactin might be useful to distinguish between these two conditions.

AIM

To verify whether p50 and cortactin are useful differentiation markers to distinguish between subungual KA and well-differentiated SCC.

METHODS

Immunohistochemistry using p50, cortactin and Ki-67 was performed on 20 patients with KA and 20 patients with facial well-differentiated SC. Ki-67 staining was also evaluated and scored.

RESULTS

Both p50 and cortactin had higher levels of expression in KA than in SCC. Both were localized to the basal-cell layer of KA, whereas they were scattered without polarity throughout the SCC lesions. Although the Ki-67 index was not significantly different between KA and SCC, the staining pattern also showed loss of polarity in SCC.

CONCLUSION

p50 and cortactin might be useful makers to distinguish between KA and well-differentiated SCC.

Treatment of keratoacanthoma with 5% imiquimod cream and review of the previous report

Keratoacanthoma (KA) is a benign epidermal tumor, characterized by rapid and abundant growth, a tendency toward spontaneous regression and histopathologic similarity to squamous cell carcinoma (SCC). Because KA can be easily misdiagnosed as SCC, surgery is considered the treatment of choice. Recently, regression of KAs following application of 5% imiquimod cream (Aldara®) has been reported. We present 4 cases of KA treated with topical imiquimod, applied 3 to 4 times a week. Obvious improvement was observed after 4 to 6 weeks of application and the lesions were almost cleared leaving scars after 9 to 11 weeks. These results show that topical imiquimod can be an effective option for the conservative management of KA as previously reported. We also suggest that lesions treated with imiquimod cream should be considered for biopsy to judge histopathological remission after 5 to 8 weeks of application to shorten the duration of the treatment.

Keratoacanthoma: hyperplasia, benign neoplasm, or a type of squamous cell carcinoma?

Keratoacanthomas are common self limited squamous proliferations. They have been considered a benign neoplasm with involution and complete resolution within few months. Although considered the prototypical example of cutaneous pseudomalignancy, some believe that these tumors are squamous cell carcinomas and through the years there have been sporadic reports of "metastasizing keratoacanthomas". The question has been raised as to whether keratoacanthoma is an unreliable histological diagnosis or these tumors have a latent, albeit rare, malignant potential. To date, just a handful of "metastasizing keratoacanthomas" have been reported. Since a benign lesion is incapable of metastasis, some other explanation must be considered; the most likely one being a misdiagnosis. While it is clear that in some cases, the histological and cytological features of squamous cell carcinoma and keratoacanthoma are difficult to distinguish by current techniques, these occasional limitations in diagnosis do not make keratoacanthomas a carcinoma. We believe the evidence supports that keratoacanthomas are benign squamous proliferations. The diagnosis can be made with confidence in appropriate biopsies and using well established clinicopathological criteria.