The association of malignant neoplasms arising contiguous with or adjacent to seborrheic keratoses has been previously documented. In this study a retrospective analysis was performed to further characterize these combined neoplasms. A total of 54 examples of malignant neoplasms in conjunction with seborrheic keratoses were found, of which 43 were basal cell carcinomas, six were Bowen's disease, three were keratoacanthomas, and two were malignant melanomas. The average age of the patients was over 65 years. Men were affected more frequently then women. The posterior thorax was the most common site for malignant neoplasms associated with seborrheic keratoses. The superficial type of basal cell carcinoma was the most common type of basal cell carcinoma found in this association. All subtypes of seborrheic keratosis may be found together with malignant neoplasms. We believe the appellation collision tumor is not valid because it is unknown whether the association of seborrheic keratoses with malignant neoplasms is a random event or whether there is, in fact, a pathogenic relationship in the development of two distinctive neoplasms together. Therefore, the term compound tumor is proposed to designate the finding of two distinctive neoplasms either directly contiguous with each other or immediately adjacent to each other.

Inflammation of the subcutaneous tissue represents a dynamic process that shows different histopathologic findings at different stages of development; therefore, the stage of evolution of a lesion at the time of biopsy influences the microscopic appearance significantly. Furthermore, location and type of inflammation may vary among different examples of the same panniculitis independent of the stage of evolution. For these reasons, the histopathologic diagnosis of panniculitides is often difficult. Currently, the most common approach to diagnosis is differentiation between predominantly septal and predominantly lobular panniculitis, followed by the distinction between lesions with and without vasculitis. Although these criteria are important for diagnosis, they are often insufficiently specific. To determine an alternative method of diagnosis, 329 cases of panniculitis were histopathologically analyzed using the following parameters: location and type of inflammatory infiltrate within and around the subcutaneous tissue, presence or absence of fat necrosis, type of necrosis, presence or absence of vascular changes, and presence or absence of associated findings (e.g., hemorrhage, sclerosis). On the basis of the results of this study and of an extensive review of the literature, tables of histopathologic findings for the diagnosis of panniculitides are presented.

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