Collision Tumors and Exceptions to Rules

Collision tumors can display a combination of features or sometimes, reveal a lack of specific criteria for rendering a diagnosis. The different entities comprising collision tumors can often be correctly identified via dermoscopy; however, a focus of a malignancy arising in association with a benign lesion such as a seborrheic keratosis may be overlooked due to anchoring bias and search satisfaction. On the other hand, malignancies mimicking benign lesions may lead to delays in diagnosis. Benign lesions mimicking malignancy will be included in the differential diagnosis of melanoma and other pigmented tumors (melanocytic or nonmelanocytic). During routine examination, dermatologists must have in mind that collision tumors are frequent and can be diagnosed easily if suspected. Collision tumors are frequently found on sun-damaged skin, SK almost always being involved. Rarely, melanomas (primary or metastatic) can clinically and dermoscopically resemble vascular lesions, such as hemangiomas and angiokeratomas.