More than 200 patients with prurigo pigmentosa, a disease described first by Nagashima in 1971, have been reported on in Japan, but only 28 non-Japanese patients have come to notice as of today. In order to establish reliable, repeatable criteria for diagnosis of the disease, we studied 25 patients with prurigo pigmentosa and reviewed the literature pertaining to it as recorded in another 182 patients.Clinically, prurigo pigmentosa presents itself as pruritic urticarial papules, papulovesicles, and vesicles arranged in reticular pattern and distributed symmetrically on the back, neck, and chest. Lesions involute in a matter of days, leaving behind netlike pigmentation. Exacerbations and recurrences are the rule. Histopathologically, prurigo pigmentosa begins with a superficial perivascular infiltrate of neutrophils. Shortly thereafter, neutrophils are scattered in dermal papillae and then sweep rapidly through an epidermis in which spongiosis, ballooning, and necrotic keratocytes are accompaniments. En route, abscesses may form in the surface epithelium. Very soon, eosinophils and lymphocytes come to predominate over neutrophils in a dermal infiltrate that assumes a patchy lichenoid pattern. Intraepidermal vesiculation follows on spongiosis and ballooning and, sometimes, subepidermal vesiculation on vacuolar alteration at the dermo-epidermal junction. As the epidermis becomes hyperplastic, parakeratotic, and slightly hyperpigmented, melanophages begin to appear in the dermis. Studies by immunofluorescence are negative invariably. Dapsone or minocyclin are effective treatments; both of those agents inhibit migration and/or function of neutrophils. The cause and pathogenesis have yet to be determined. Prurigo pigmentosa is unique among inflammatory diseases of the skin and the singularity of it is manifest both clinically and histopathologically.

The classification of benign sebaceous neoplasms has been challenged both by the assertion that sebaceous adenomas are really carcinomas and by difficulties in drawing the boundaries between sebaceomas and other lesions. We performed a clinicopathologic study of 30 cases of basaloid neoplasms with sebaceous differentiation, excluding cases of definite sebaceous carcinoma with severe nuclear atypia invading deep within the subcutaneous tissue and those of ocular sebaceous carcinoma. We tried to classify sebaceous neoplasms in six categories with defined histopathologic criteria. All the neoplasms were characterized by aggregations of basaloid cells admixed with sebocytes and sebaceous duct-like structures located in the dermis with or without connection to the epidermis. The categories were 1) sebaceoma (14 cases); 2) trichoblastoma with sebaceous differentiation (3 cases); 3) apocrine poroma with sebaceous differentiation (2 cases); 4) low-grade sebaceous carcinoma (6 cases); 5) sebaceous carcinoma (4 cases); and 6) basal cell carcinoma with sebaceous differentiation (1 case). The sebaceoma was further subclassified as classic type (12 cases) or verruca/seborrheic keratosis type (2 cases). Although most sebaceomas can be distinguished from other lesions, there are problematic cases. We discuss the histopathologic diagnostic problems associated with sebaceoma and also argue in favor of the concept of sebaceous adenoma.

The terminology and classification of keratoacanthoma (KA) and other types of squamous cell carcinoma (SCC) with crateriform architecture have not been clarified. The study evaluated the clinicopathological features of 41 nodular (exo-endophytic) SCC lesions with a central keratin-filled crater, including KA (well-developed stage). The lesions were histopathologically classified into six categories: (i) KA (well-developed stage) (27 lesions); (ii) KA-like SCC (three lesions); (iii) KA with malignant transformation (three lesions); (iv) infundibular SCC (crateriform) (four lesions); (v) crateriform SCC arisen from actinic keratosis (three lesions); and (vi) crateriform Bowen's disease (one lesion). The true characteristics of KA-like SCC remain unresolved, but there are three possibilities, namely, that it is one step in the evolution of KA, it is a borderline lesion between KA and invasive SCC, or it is one form of "KA with malignant transformation". KA, KA-like SCC, KA with malignant transformation and infundibular SCC (crater form) are considered to be hair follicle-related neoplasms. In contrast, crateriform SCC arisen from actinic keratosis and crateriform Bowen's disease are SCC, which are not related either to the hair follicles or KA. From an etiological standpoint, the presented lesions in these six categories are considered to be mixed up due to the similarity of crateriform architecture between the various types of lesions. However, the information provided in this report is intended to help physicians to make an accurate differential diagnosis of these conditions in clinical practice. The present study provides an opportunity to standardize the terminology for KA and related neoplasms.

Two types of squamous cell carcinoma (SCC), which are considered to show infundibular differentiation, have been described so far; namely, follicular SCC and infundibulocystic SCC. The latter includes (1) a well-differentiated form, (2) a less-differentiated form, and (3) an infiltrative variant. This study examined the clinicopathological features of 8 cases of SCC with infundibular differentiation, which included follicular SCCs and infundibulocystic SCCs (a less-differentiated form and an infiltrative variant). The present study confirmed that these SCCs with follicular differentiation are clinicopathologically distinct from keratoacanthoma. However, one example of infundibulocystic SCC (less-differentiated form) proved to be difficult to distinguish from keratoacanthoma. The relationship between the follicular SCC and the less-differentiated form of infundibulocystic SCC was investigated. At the periphery of the latter lesions, a focus corresponding to the follicular SCC or advanced follicular SCC lesions was seen. Therefore, these 2 types of SCCs are considered to be similar and thus represent the same neoplastic disease. The less-differentiated form of infundibulocystic SCC is considered to be a more aggressive condition. A unified term, infundibular (follicular) SCC, was used to describe these 2 conditions in this study. The clinicopathological features of the infiltrative variant of infundibulocystic SCCs were unique and distinct from the other 2 types of SCCs. This variant of infundibulocystic SCC is therefore considered to be a distinct entity and therefore has been simply called infundibulocystic SCC in this study. Infundibulocystic SCC may therefore be related to either a microcystic adnexal carcinoma or a malignant counterpart of the trichoadenoma of Nikolowski.

KA is a follicular neoplasm with infundibular/isthmic (upper segmental region of hair follicles) differentiation. It is considered that early or proliferative stage tumors show keratin-filled invaginations with infundibular differentiation and gradual isthmic differentiation. Well-developed examples of KA generally show isthmic differentiation in the whole lesions. The regressed stage KAs lose the features of this type of follicular differentiation and show epidermal characteristics. No expression of CK15 (clone C8/144B) was observed in KAs, although this finding is insufficient to completely rule out the correlation between the regression of KA and the hair follicle cycle.

There is some confusion regarding the classification of keratoacanthoma (KA) and related lesions that have crateriform architecture. We examined the clinical courses of 66 KA lesions and related lesions after a partial biopsy to clarify the nosological concept of KA. We histopathologically classified these lesions into five types: (i) KA at various stages (53 lesions); (ii) KA-like squamous cell carcinoma (SCC) (3 lesions); (iii) KA with malignant transformation (3 lesions); (iv) infundibular SCC (5 lesions); and (v) crateriform SCC arising from solar keratosis (2 lesions). We analyzed the clinical course in each group. The regression rate of KA was 98.1% and that of KA-like SCC/KA with malignant transformation was 33.3%. No regression was observed in either infundibular SCC or crateriform SCC arising from solar keratosis. Thus, KA is a distinct entity that should be distinguished from other types of SCC with crateriform architecture based on the high frequency of regression. The regression rate of 33.3% in KA-like SCC/KA with malignant transformation indicated that KA lesions with an SCC component still have the potential for regression. However, this result also indicated that KA is biologically unstable, and some KA tend to evolve into conventional SCC with a gradual loss of the capacity for the spontaneous regression. Infundibular SCC and crateriform SCC arising from solar keratosis are fundamentally different from KA, not only according to the histopathological findings but also based on the biological properties.

There is a histopathological similarity between cutaneous apocrine carcinoma (CAC) and breast carcinoma. Cutaneous histiocytoid or signet-ring cell (SRC) carcinoma is a rare neoplasm, which usually occurs on the eyelid, and less commonly on the axilla. The precise histogenesis of this carcinoma remains controversial. We report the case of a man with a cutaneous histiocytoid SRC carcinoma of the axilla having histopathological and immunohistochemical features that were quite similar to histiocytoid lobular carcinoma (histiocytoid LC) of the breast, which is a subtype of classic invasive lobular carcinoma of the breast with apocrine differentiation. We consider this case to be a type of CAC equivalent to histiocytoid LC of the breast, based on the features and the occurrence on the axilla. The patient was treated with adjuvant chemotherapy according to the general guidelines for the treatment of breast carcinoma.

Both FF/TD and FP are hamartomas composed of perifollicular or interfollicular connective tissue and a hair follicular epithelial component, which may be caused by an abnormal functioning of the hair follicle bulge cells.