Cases with a spontaneous regression of an infiltrating non‐crateriform keratoacanthoma and squamous cell carcinoma with a keratoacanthoma‐like component

Abstract

We herein report the natural course of an early/proliferative stage keratoacanthoma (KA) with infiltrating islands of cytological malignancy (case 1) and a squamous cell carcinoma (SCC) with a KA-like component (case 2), which were observed until their complete regression. The presented case 1 suggests that one of the histopathological forms of KA includes this unusual, infiltrating, non-crateriform architecture, and also indicates the possibility of complete remission in the KA associated with infiltrating islands of cytological malignancy. In the presented case 2, the peripherally-associated KA-like focus was histopathologically considered to be either a remnant of KA focus or verrucous keratosis (hyperplasia). Therefore, the complete spontaneous regression of case 2 suggests that SCC arising in KA still has the potential of spontaneous regression, or that an extremely rare event, namely, the spontaneous regression of (traditional) SCC occurred in the present case.

Natural course of keratoacanthoma and related lesions after partial biopsy: Clinical analysis of 66 lesions

“…The histopathological diagnostic criteria for each lesion were defined based on the classification of Misago et al . and according to many other studies …”

“…It is noteworthy that regression is observed in a certain percentage of KA with an SCC component. We recently reported a case of SCC with a KA‐like component in which the tumor showed complete regression . Although the present study has a small sample size and other limitations, KA with an SCC component still has the potential for regression.…”

“…For an accurate histopathological diagnosis of early/proliferative stage KA, it is important to confirm that there is virtually no nuclear atypia in the proliferating cells of the invaginated infundibular structures. However, nuclear atypia and/or mitosis are detectable in the infiltrating border of the tumors …”

“…Nevertheless, it appears that wait‐and‐see management can be used in cases of histopathologically typical KA without an SCC component, where complete resection is difficult because of anatomical location or poor general condition. It should be noted that it is not always easy to histopathologically distinguish early/proliferative KA from KA with malignant transformation . In addition, infundibular SCC and crateriform SCC arising from solar keratosis usually show no regression and are associated with clinically malignant behavior …”

“…Early/proliferative stage KA has a predominantly invaginated infundibular structure; large pale pink cells are located only in deep lobules, whereas lobules of pale pink cells predominate in well‐developed KA . In early/proliferative stage KA, the invaginated infundibular structures anastomose or merge, but rarely form a crater or a central keratotic plug . In contrast, well‐developed stage KA is characterized by formation of a relatively distinct crater and a central keratotic plug .…”