Letters to the Editor
To the Editor:
Misago et al1 have recently published 6 cases of infundibular (follicular) and 2 examples of infundibulocystic squamous cell carcinoma and have redefined the original criteria for both follicular squamous cell carcinoma and infundibulocystic squamous cell carcinoma.2,3 They propose that the term infundibular squamous cell carcinoma was synonymous with follicular squamous cell carcinoma, and infundibulocystic carcinoma was a distinctive tumor sharing features with microcystic adnexal carcinoma.
In the initial description of 16 examples of follicular squamous cell carcinoma by Diaz-Cascajo et al,2 these squamous cell carcinomas arose in follicles and formed nodular tumors that replaced the follicles but did not demonstrate follicular differentiation even when the tumor infiltrated the surrounding tissue. From a clinical perspective, follicular squamous cell carcinomas in the original study were described as dermal nodules that did not resemble keratoacanthoma and were diagnosed as basal cell carcinoma. In contrast, the 6 examples of infundibular (follicular) squamous cell carcinomas described by Misago et al1 were characterized by either a nodule with a central keratotic or ulcerative crater and clinically shared features with keratoacanthoma or squamous cell carcinoma but not basal cell carcinoma. The presence of infundibular and also cystic changes in their histopathologic description of these tumors indicates that the process does not merely reflect the replacement of hair follicles by squamous cell carcinoma as the prime criteria for follicular squamous cell carcinoma but represent a truly follicular tumor demonstrating infundibular differentiation. Understandably, as the structure of the epidermis is identical to the infundibulum, the development of squamous cell carcinoma is not surprising, but it is the infundibular proliferation often associated with a cystic or central keratotic crater that defines infundibular and infundibulocystic squamous cell carcinomas.
The original description of infundibulocystic squamous cell carcinoma3 indicated that the tumors were allied to keratoacanthomas but included less differentiated examples and examples in which the architecture did not fulfill the usual pattern of keratoacanthoma including a rare form presenting as a broad plaque with a deeply infiltrative infundibulocystic component.
Infundibular and infundibulocystic squamous cell carcinomas are part of the same spectrum of carcinomas, but Misago et al1 propose that the term infundibulocystic carcinoma should be restricted to the infiltrative plaque form. Infundibulocystic features occur in nodular tumors and were also reported by the authors in the histopathologic description of the group termed infundibular (follicular) squamous cell carcinoma. The cystic nature is reflected by the central keratotic component and the craters noted clinically. These less well-differentiated tumors may not fully resemble keratoacanthomas. The deep infiltrative variant does represent a distinct tumor that is not nodular or crateriform and may be viewed as a follicular variant of microcystic adnexal carcinoma. At the same time, these tumors are characterized by large irregular cysts in the deeper dermis and commence in the setting of a broad-based infundibulocystic hyperplasia linking these tumors to the wider spectrum of infundibulocystic squamous cell carcinomas.
The term well-differentiated infundibulocystic squamous cell carcinoma could be applied to keratoacanthoma, but the aim of the initial description3 of this recognizable histopathologic characteristic was to expand the spectrum of the infundibular pathway to include lesions, which may not fulfill all the clinical and histopathological criteria for keratoacanthoma. The problem of totally isolating keratoacanthoma from the group of infundibulocystic squamous cell carcinomas is that less well-differentiated tumors may arise directly from clearly defined keratoacanthomas in 25% of tumors in at least one large study.4
Misago et al1 have illustrated excellent examples of infundibular and infundibulocystic carcinomas, but until more is known in respect to the oncogenes and pathway governing this group of tumors, it may be preferable to maintain the link with keratoacanthoma. There is a prospect that systemic agents may harness the pathway of involution seen in keratoacanthoma to manage eruptive, infiltrative, and less differentiated tumors linked by their histopathological features.
1. Misago N, Inoue T, Toda S, et al.. Infundibular (follicular) and infundibulocystic squamous cell carcinoma: a clinicopathological and immunohistochemical study. Am J Dermatopathol. 2011;33:687–694.
2. Diaz-Cascajo C, Borghi S, Weyers W, et al.. Follicular squamous cell carcinoma of the skin: a poorly recognised neoplasm arising from the wall of hair follicles. J Cutan Pathol. 2004;31:19–25.
3. Kossard S, Tan K-B, Choy C. Keratoacanthoma and infundibulocystic squamous cell carcinoma. Am J Dermatopathol. 2008;30:127–134.
4. Sanchez-Yus E, Simon P, Requena L, et al.. Solitary keratoacanthoma: a self healing proliferation that frequently becomes malignant. Am J Dermatopathol. 2000;22:305–310.