p53 gene: A gene that normally helps to suppress tumors, researchers have found that, when mutated, the p53 gene increases a woman’s risk of developing breast cancer.

Paclitaxel (brand name, Taxol): See Taxol.

Paget’s disease of the nipple: A rare form of breast cancer that begins in the milk passages (ducts) and spreads to the skin of the nipple and areola. This affected skin may appear crusted, scaly, red, or oozing. The prognosis is generally better if these nipple changes are the only sign of breast disease and no lump can be felt.

Palliative treatment: Therapy that relieves symptoms, such as pain, but is not expected to cure the disease. Its main purpose is to improve the patient’s quality of life.

Palpable: Able to be felt.

Palpation: The examination of the breasts by manually feeling for breast lumps. A palpable mass in the breast is one that can be felt.

Parenchyma: The functional tissue of an organ. In the breast, it is the glandular tissue, as opposed to fatty or stromal (connective) tissues.

Partial mastectomy: See mastectomy.

Pathologist: A physician who specializes in examining, diagnosing, and classifying diseases by conducting laboratory tests (such as examining tissues and cells under a microscope). The pathologist determines whether a lump is benign or cancerous.

Pectoral muscles: Muscles attached to the front of the chest wall and upper arms. The larger one is called pectoralis major, and a smaller one is called pectoralis minor. Because these muscles are next to the breast, breast cancer may occasionally spread to the pectoral muscles.

Pectoralis muscle: The main chest wall muscle that is underneath the breast tissue.

Per os (PO): By mouth, orally.  Denotes a medication or treatment given orally.

Pathology: Branch of science that deals with all aspects of disease, specifically the microscopic examination of body tissue to look for evidence of disease.

Peripheral neuropathy: A condition of the nervous system that usually begins in the hands and/or the feet with symptoms of numbness, tingling, and/or weakness.  Can be caused by certain anticancer drugs.

Permanent section: Preparation of tissue for microscopic examination. The tissue is soaked in formaldehyde, processed in various chemicals, surrounded by a block of wax, sliced very thin, attached to a microscope slide and stained. This usually takes 1-2 days. It provides a clear view of the specimen so that the presence or absence of cancer can be determined. (See also frozen section).

Phyllodes tumors: Breast tumors that may be found in the glandular and stroma (connective) tissues of the breast. Phyllodes tumors are usually benign but on very rare occasions, they may be cancerous.  Also spelled phylloides.

Physician: A licensed medical doctor or doctor of osteopathy (DO) who typically participates in additional training (a residency) after medical school to specialize in a more limited field of practice.

Placebo: An inert or inactive substance that is not distinguishable from the active substance.  Placebos are often used in clinical trials to compare the effects of a given treatment with no treatment.

Plastic surgeon: A physician with advanced training in cosmetic surgery.  Plastic surgeons may surgically reconstruct a woman’s breast after mastectomy (breast removal).

Platelets: Type of blood cells that help stop bleeding.

Pleomorphic: Having many or various shapes. These terms often describe microcalcifications which can indicate ductal carcinoma in situ (DCIS), an early stage breast cancer.

Ploidy: A measure of the amount of DNA contained in a cell. Ploidy is a characteristic (marker) that helps predict how aggressive a cancer is likely to be. Cancers with the same amount of DNA as normal cells are called diploid and those with either more or less than that amount are aneuploid. About two-thirds of breast cancers are aneuploid.

Polymorphic: Having an irregular shape or various shapes. This term often describes microcalcifications which can indicate ductal carcinoma in situ (DCIS), an early stage breast cancer.

Postmenopause: Term used to describe the time in a woman’s life after menopause.

Precancerous: Abnormal changes in cells that indicate a higher than normal risk of developing into cancer.  (See also premalignant). 

Predisposition: Susceptibility to a disease that can be triggered under certain conditions. For example, some women have a family history of breast cancer and are therefore predisposed (but not necessarily destined) to develop breast cancer.

Premalignant: Abnormal changes in cells that may, but do not always, become cancer. Also called precancerous.

Premenopause:   Term used to describe the time in a woman’s life before menopause.

Prevalence: A measure of the proportion of persons in the population with a particular disease at a specified time.   

Primary cancer: The site where cancer begins. Primary cancer is usually named after the organ in which it originates (for example, cancer that originates in the breast is always breast cancer even if it metastasizes to other organs, such as bones or lungs).

Progesterone: A female sex hormone released by the ovaries during every menstrual cycle to prepare the uterus for pregnancy and stimulate milk production (lactation) in the breast.

Progesterone receptor assay: A laboratory test done on a piece of the breast cancer to determine whether the cancer depends on progesterone for growth. Progesterone receptors are tested along with estrogen receptors for more complete information on the hormone sensitivity of a cancer, and how best to treat it. (See also estrogen receptor assay).

Prognosis: A prediction of the course of disease; the outlook for the cure of the patient. For example, a woman with breast cancer that was detected early and received prompt treatment generally has a good prognosis.

Prolactin: A hormone released from the pituitary gland that prompts milk production (lactation).

Prophylactic mastectomy: See mastectomy.

Prosthesis: An artificial form, such as a breast prosthesis, that can be worn under the clothing after a mastectomy. (Plural: prostheses).

Protocol: A formalized outline or plan such as a description of what type of  treatments a patient will receive and exactly when each should be given.