Papule: A skin lesion that is small, solid, and raised.

Pathologist: A doctor who identifies diseases by studying cells and tissues under a microscope.

PDQ Information System: Physician Data Query. PDQ is an online database developed and maintained by the National Cancer Institute. It is designed to make the most current, credible, and accurate cancer information available to health professionals and the public.

Peripheral: Away from the center. Refers to the areas away from the center of the body or body part. The hands are peripheral to the shoulder. The toes are peripheral to the knees.

PET Scan: A diagnostic test that produces images of the body by detecting radiation emitted from a radioactive substance that is administered to the patient.

Phase I: A clinical trial of a molecule that has already passed all or most of the preclinical requirements. Phase I trials are aimed mainly at determining the maximal tolerated dose in healthy normal volunteers. If the drugs are known to be toxic or have predictable serious ADRs, these studies are often done in patients with the disease to be treated, eg, cancer chemotherapy or antiviral therapy in acquired immunodeficiency syndrome (AIDS) patients; in other words, Phase I is conducted in Phase II patients for obvious ethical motives. Patients are often housed for these studies in special Clinical Research Centers run by academic medical centers or clinical research organizations (CROs).

Phase II: Clinical trials of a substance that has successfully passed through (all or parts of) Phase I and preclinical trials. Phase II trials are aimed mainly at determining a minimal effective dose on the basis of either a pharmacologic criterion that serves as a surrogate outcome, such as arterial pressure, serum cholesterol levels, viral load or bone density, or an actual therapeutic benefit such as pain relief by an analgesic.

Phase III: The phase of study of a drug after it has passed through Phases I and II.The major method in this phase is the comparative controlled clinical trial that compares the new drug with a placebo or a reference medication. As in Phase II, the endpoints of the study can be either surrogate markers or actual therapeutic endpoints. These studies are generally larger and longer than Phase II trials. The goal is to amass sufficient efficacy and safety data to gain marketing authorization in at least one indication.

Pigment: A substance that gives color to tissue. Pigments are responsible for the color of skin, eyes, and hair.

Plastic surgeon: A surgeon who specializes in reducing scarring or disfigurement that may occur as a result of accidents, birth defects, or treatment for diseases, such as melanoma.

Plastic surgery: The repair or replacement of malformed, injured, or disfigured tissues of the body, usually by transplanting living skin.

Platelet count: A test that measures the number of platelets in blood. Platelets are necessary for normal blood clotting (hemostasis). Most importantly, they aggregate (clump) together to plug small holes in damaged vessels. They also activate factor VIII (a component of the coagulation cascade) and release phospholipids necessary for the coagulation cascade.

Prognosis: A prediction of the probable course of the disease.