The lack of well-established diagnostic criteria for pigmented solar (actinic) keratosis (PSK) along with its poorly understood etiopathogenesis has contributed to underrecognition.

The clinical, dermatoscopic, and histopathologic features of PSK and the cause of the pigmentation are elucidated.

In all, 167 histologic specimens, 22 clinical images, and 17 dermatoscopic images of PSK were reviewed. In 38 cases, Melan-A–stained sections were available for analysis.

The majority of the lesions were located on the head and neck (84%). A separate pigmented lesion was adjacent to or admixed within PSK in 138 (83%) of the cases indicating that PSK represents a collision between a nonpigmented solar keratosis and a pigmented lesion. Solar lentigo (72%) was the most commonly associated pigmented lesion followed by seborrheic keratosis and melanoma. PSK was suspected clinically in 17% of the cases. There were no significant differences in the quality and quantity of the melanocytes between pigmented and nonpigmented solar keratosis.

This was a single-center retrospective study. The sample sizes were small for the clinical and dermatoscopic images and Melan-A stains.

In the majority of the cases, a collision between a nonpigmented solar keratosis and a separate coexistent pigmented lesion, primarily a solar lentigo, accounts for the pigmentation in PSK rather than from any fundamental changes in the quantity or quality of the melanocytes. The collision phenomenon accounts for the spectrum of the clinical and dermatoscopic features observed in PSK and its underrecognition.