HUMAN BIOLOGY

432 Chapter 22

medical imaging
Methods such as MRI,
X-rays, and ultrasound that
are used to obtain an inter-
nal view of the body or its
parts.

tumor marker A substance
produced by a cancerous
cell or by normal cells when
cancer is present.

medical imaging can reveal the site

and size of tumors

Medical imaging includes methods such as PET scanning,

MRI (magnetic resonance imaging), X-rays, ultrasound,

and CT (computed tomography). Unlike a standard X-ray,

an MRI scan can reveal tumors that are obscured by bone,

such as in the brain (Figure 22.11).

You may remember from Section 2.2 that radioactive

tracers (substances with a radioisotope attached to them)

are another important tool for diagnosing cancer. A doctor

administers the tracer, then uses a tracking device such as

a PET scanner to see where the tracer ends up in the body.

For example, thyroid cancer can be diagnosed using a tracer

that includes a radioactive isotope of iodine (Figure 22.12).

Radioactively labeled monoclonal antibodies, which

home in on tumor antigens, are useful for pinpointing the

location and sizes of tumors of the colon, brain, bone, and

some other tissues. A DNA probe (radioactively labeled

DNA) can be used to locate mutated genes, such as the

p53 tumor suppressor gene, or genes associated with some

inherited cancers. This type of screening can allow people

with increased genetic susceptibility to make medical and

lifestyle choices that may reduce their cancer risk. The

procedure is expensive, however, and relatively few people

have insurance that covers it.

Cancer screening and diagnosis

Figure 22.11 MRI scanning is a noninvasive tool

for diagnosing cancer. The patient is placed in

a chamber that is surrounded by a magnet. The

machine produces a magnetic field in which nuclei

of common atoms in the body align and absorb

energy. A computer analyzes the information and

uses it to generate an image of soft tissues. (Will &

Deni McIntyre/Science Source)

n Early and accurate diagnosis of cancer is important to

maximize the chances that a cancer can be cured.

n Links to Gene mutation 21.2, dNA profiling 21.10

Routine screening is important for people with a family

history of cancer or whose risk is elevated for some other

reason, including simply getting older. Table 22.3 lists some

recommended cancer screening tests.

Blood tests can detect

chemical indications

of cancer

To confirm or rule out cancer, vari-

ous types of tests can refine the diag-

nosis. Blood tests can detect tumor

markers, substances produced by

specific types of cancer cells or by

normal cells in response to the can-

cer. For example, as we noted earlier,

the hormone HCG is a marker for certain cancers. Prostate-

specific antigen, or PSA, can be a useful marker for detect-

ing prostate cancer, and markers have been identified for

ovarian cancer, bladder cancer, some forms of leukemia,

liver cancers, and others.

22.5

Table 22.3 Recommended Cancer Screening Tests

Test or

Procedure Cancer Sex Age Frequency

Breast self-

examination

Breast Female 201 Monthly

Mammogram Breast Female 40–49

501

Every 1–2 years

Yearly

Testicle self-

examination

Testicular Male 181 Monthly

Sigmoidoscopy Colon Male,

Female

501 Every 3–5 years

Fecal occult

blood test

Colon Male,

Female

501 Yearly

Digital rectal Prostate,

colorectal

Male,

Female

401 Colorectal: Yearly

examination

Prostate: Yearly up

to age 75

Pap test Uterus,

cervix

Female 181 and all

sexually active

women

Every other year until

age 35; yearly thereafter

Pelvic

examination

Uterus,

ovaries,

cervix

Female 18–39

401

Every 1–3 years w/ Pap

Yearly

General

checkup

Male,

Female

20–39

401

Every 3 years

Yearly

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