Most cysts are benign, or non-cancerous, but some may be malignant (cancerous). A cyst may be malignant if it contains solid matter. A benign cyst is typically filled with fluid and air. To make a diagnosis, the doctor may order a CT scan or perform a biopsy, which is a procedure in which the doctor takes a tissue sample and sends it to a laboratory.
Dermatologists can treat cancerous cysts with Mohs micrographic surgery, a precise surgical technique in which the skin doctor removes and examines thin layers of cancer-containing skin until only cancer-free tissue remains.