If I have hundreds of moles on my back, how would a doctor know if they are cancerous or not? There are so many.

Thank you for your question. A dermatologist evaluates the skin utilizing an instrument called a dermatoscope, which allows the provider to evaluate lesions under higher magnification. Through experience and education, a dermatologist assesses lesions for any alarming characteristics that may indicate a malignancy, but no one knows if a lesion is cancerous unless a biopsy is performed. I highly recommend you consult with a board certified dermatologist to have your skin evaluated and to answer further questions. Take care and good luck.