Early Gut Reactions

The small and large intestines are frequently irradiated during abdominal or pelvic treatments. Areas of fixed bowel, such as the rectum or the retroperitoneal parts, are particularly susceptible as they are unable to move significantly within the peritoneal cavity.

Stomach

Microscopic Structure

The stomach is lined by a columnar epithelium with numerous gastric pits. Each pit leads to submucosal gastric gland, which are large and located throughout the submucosa of the stomach. The columnar cells of the surface epithelium produce mucous; they have a lifespan of about 3 days. The cells of the glands have much longer lifespans (hundreds of days).

Response to Radiation

The first response usually seen is nausea. Nausea after radiation of the stomach is not well understood; there is evidence that radiation can activate chemoreceptors in the stomach itself, or alternatively cause release of neurotransmitters that directly affects the emesis centre of the brainstem.
The precursor cells of the surface epithelium are the target cell for early reactions. Loss of dividing capacity leads to loss of this superficial layer; this can cause symptoms of dyspepsia. Doses over 40 Gy can lead to acute ulceration of the gastric mucosa with potential serious complications. Ulceration can also occur as a late effect, but this due to vascular insufficiency of the mucosa rather than direct killing of the surface epithelial precursors.

Small Intestine

Microscopic Structure of the Small Intestine

The small bowel is formed of villi with intervening crypts. Stem cells are found in the base of the crypts, where they undergo assymetrical division, producing a new stem cell and a differentiating cell. The differentiating cells gradually migrade up the crypt, onto the villi and are shed from the tips of the villi. The time for cells to ascend from the crypt to the villus is about 2 - 4 days.

Early Intestinal Reactions

Death or disablement of the crypt stem cells from radiation leads to loss of the intestinal villi within several days. This leads to several problems:

  • Poor absorption of gut contents, causing diarrhoea
  • Risk of sepsis

Nausea may also occur following radiation of the small intestine; the mechanism is also unclear.

Large Intestine

Microscopic structure

The large intestine contains crypts but no villi; therefore, there are no villi to be lost if the crypt stem cells are disabled. Radiation can still cause changes in intestinal absorption, causing diarrhoea or rarely constipation. Ulceration of the large bowel can also occur, with risks of sepsis.
The rectum, which is often irradiated during pelvic treatments, often develops acute side effects. These include pain, urgency and rectal bleeding. Haemorrhoids are likely to recur or get worse in patients with a past history of this condition; I can't find any good reason for this but perhaps due to increased blood flow in the area.

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