Photodynamic therapy (PDT) is another treatment used for superficial squamous cell carcinoma. The treatment can be used for some skin cancers on the scalp and face with minimal damage to surrounding areas. PDT combines light and liquid drugs to destroy skin cancer cells. The drug is a topical agent that is photosensitizing towards cancer cells, which means the cells will become sensitive to light. This topical agent is applied to the skin and allowed time for absorption, and after about an hour, a strong light or laser is illuminated directly onto the medicated area to activate the agent and subsequently destroy the cancer cells. The area may swell, redden, and cause discomfort during recovery. Flaking and peeling may also result. Patients should avoid the sun for at least forty-eight hours after the procedure because the light may activate the photosensitizing drug and cause bad sunburns. Photodynamic therapy is not recommended for invasive squamous cell carcinoma.