Definition

The rectum is the portion of the large bowel that lies
in the pelvis, terminating at the anus. Cancer of the rectum
is the disease characterized by the development of
malignant cells in the lining or epithelium of the rectum.
Malignant cells have changed such that they lose normal
control mechanisms governing growth. These cells may
invade surrounding local tissue or they may spread
throughout the body and invade other organ systems.

Description

The rectum is the continuation of the colon (part of
the large bowel) after it leaves the abdomen and descends
into the pelvis. Anatomically, it is divided into equal
thirds; the upper, mid, and lower rectum.

The pelvis and other organs in the pelvis form
boundaries to the rectum. Behind, or more accurately,
posterior to the rectum is the sacrum (the lowest portion
of the spine, closest to the pelvis). Laterally, on the sides,
the rectum is bounded by soft tissue and bone. In front,
the rectum is bounded by different organs in the male and
female. In the male, the bladder and prostate are present.
In the female, the vagina, uterus, and ovaries are present.

The upper rectum receives its blood supply from
branches of the inferior mesenteric artery from the
abdomen. The lower rectum has blood vessels entering
from the sides of the pelvis. Lymph, a protein-rich fluid
that bathes the cells of the body, is transported in small
channels known as lymphatics. These channels run with
the blood supply of the rectum. Lymph nodes are small
filters through which the lymph flows on its way back to
the blood stream. Cancer spreads elsewhere in the body
by invading the lymph and vascular systems.

When a cell or cells lining the rectum become
malignant, they first grow locally and may invade partially
or totally through the wall of the rectum. The tumor
here may invade surrounding tissue or the organs that
bound it, a process known as local invasion. In this
process, the tumor penetrates and may invade the lymphatics
or the capillaries locally and gain access to the
circulation in this way. As the malignant cells work their
way to other areas of the body, they again become locally
invasive in the new area to which they have spread. These
tumor deposits, originating in the primary tumor in the
rectum, are then known as metastasis. If metastases are
found in the regional lymph nodes, they are known as
regional metastases. If they are distant from the primary
tumor, they are known as distant metastases. The patient
with distant metastases may have widespread disease,
also referred to as systemic disease. Thus the cancer
originating in the rectum begins locally and, given time,
may become systemic.

By the time the primary tumor is originally detected,
it is usually larger than 0.39 in (about 1 cm) in size and
has over a million cells. This amount of growth itself is
estimated to take about three to seven years. Each time
the cells double in number, the size of the tumor quadruples.
Thus like most cancers, the part that is identified
clinically is later in the progression than would be
desired and screening becomes a very important endeavor
to aid in earlier detection of this disease.

Passage of red blood with the stool, (noticeable bleeding
with defecation), is much more common in rectal cancer
than that originating in the colon because the tumor is
much closer to the anus. Other symptoms (constipation
and/ or diarrhea) are caused by obstruction and, less often,
by local invasion of the tumor into pelvic organs or the
sacrum. When the tumor has spread to distant sites, these
metastases may cause dysfunction of the organ they have
spread to. Distant metastasis usually occurs in the liver,
less often to the lung(s), and rarely to the brain.

There are about 36,500 cases of rectal cancer diagnosed
per year in the United States. Together, colon and
rectal cancers account for 10% of cancers in men and
11% of cancers in women. It is the second most common
site-specific cancer affecting both men and women.
(Lung cancer is the first affecting both men and women,
breast is the leader in women and prostate the leader in
men.) About 8,500 people died from rectal cancer in the
United States in 2000. In recent years the incidence of
this disease is decreasing very slightly, as has the mortality
rate. It is difficult to tell if the decrease in mortality
reflects earlier diagnosis, less death related to the actual
treatment of the disease, or a combination of both factors.
Cancer of the rectum is felt to arise sporadically in
about 80% of those who develop the disease. 20% of
cases are felt to have genetic predisposition that ranges
from familial syndromes affecting 50% of the offspring
of a mutation carrier, to a risk of 6% when there is just a
family history of rectal cancer occurring in a first-degree
relative. Development of rectal cancer at an early age
suggests a genetically transmitted form of the disease as
opposed to the sporadic form.

Causes and symptoms

Causes of rectal cancer are probably environmental
in the sporadic cases (80%), and genetic in the hereditypredisposed
(20%) cases. Since malignant cells have a
changed genetic makeup, this means that in 80% of
cases, the environment spontaneously induces change. In
those born with a genetic predisposition, they are either
destined to get the cancer, or it will take less environmental
exposure to induce the cancer. Exposure to agents in
the environment that may induce mutation is the process
of carcinogenesis and is caused by agents known as carcinogens.
Specific carcinogens have been difficult to
identify; dietary factors, however, seem to be involved.
Rectal cancer is more common in industrialized
nations, and dietary factors are thought to be related to
this observation. Diets high in fat, red meat, total calories,
and alcohol seem to predispose. Diets high in fiber
are associated with a decreased risk. The mechanism for
protection by high-fiber diets may be related to less
exposure of the rectal epithelium to carcinogens from the
environment as the transit time through the bowel is
faster with a high-fiber diet than with a low-fiber diet.
Age plays a definite role in the predisposition to rectal
cancer. Rectal cancer is rare before age 40. This incidence
increases substantially after age 50 and doubles
with each succeeding decade.

There is also a slight increase risk for rectal cancer
in the individual who smokes.

Patients who suffer from an inflammatory disease of
the colon known as ulcerative colitis are also at increased
risk.

In regards to genetic predisposition, on chromosome
5, there is a gene called the APC gene associated with
familial adenomatous polyposis (FAP) syndrome. There
are multiple different mutations that occur at this site, yet
they all cause a defect in tumor suppression that results
in early and frequent development of colon cancer. This
genetic aberration is transmitted to 50% of offspring and
each of those affected will develop colon or rectal cancer,
usually at an early age. Another syndrome, hereditary
non-polyposis colon cancer (HNPCC), is related to
mutations in any of four genes responsible for DNA mismatch
repair. In patients with colon or rectal cancer, the
p53 gene is mutated 70% of the time. When the p53 gene
is mutated and ineffective, cells with damaged DNA
escape repair or destruction, allowing the damaged cell
to perpetuate itself. Continued replication of the damaged
DNA may lead to tumor development. Though
these syndromes (FAP and HNPCC) have a very high
incidence of colon or rectal cancer, family history without
the syndromes is also a substantial risk factor. When
considering first-degree relatives, history of one with
colon or rectal cancer raises the baseline risk of 2% to
6%, the presence of a second raises the risk to 17%.
The development of polyps of the colon or rectum
commonly precedes the development of rectal cancer.

Polyps are growths of the rectal lining. They can be unrelated
to cancer, pre-cancerous, or malignant. Polyps,
when identified, are removed for diagnosis. If the polyp,
or polyps, are benign, the patient should undergo careful
surveillance for the development of more polyps or the
development of colon or rectal cancer.

Symptoms of rectal cancer most often result from
the local presence of the tumor and its capacity to invade
surrounding pelvic structure: