Neuroendocrine Tumors of the Rectum

Rectal NETs represent the third largest group of gut NETs, with about 13 to 20% of all carcinoid tumors. They are associated with noncarcinoid tumors in about 20% of cases, so that complete colonic evaluation is essential in all patients with rectal carcinoid tumors.10,59 Moreover, routine screening examinations with proctosigmoidoscopy in asymptomatic patients have revealed an increased number of previously undetected small (< 1 cm) rectal carcinoid tumors.59 Rectal tumors are derived from enterochro-

maffin cells that demonstrate argentaffin silver reaction in only 20% and a positive argyrophil reaction in up to 70% of cases. Rectal NETs generally present as small, mobile, submucosal, polypoid, or sessile nodules or focal areas of submucosal thickening identified after a bleeding episode. They can be separated into two groups: small solitary tumors measuring < 1 cm in about 80% of cases and larger lesions with the possibility of metastases. These tumors have a marked variability in staining with S-100 and Leu-7, in contrast to the positive reaction exhibited by tumors of the foregut and midgut. Immunohisto-chemical identification of somatostatin, glicentin, pancreatic polypeptide, peptide YY, enkephalin, endorphin, and serotonin has been described. The distribution and presence of immunoreactivity for several different markers are often uneven. This may indicate the development of multiclonal lesions, in which additional genetic derangements are prone to occur, causing more aggressive disease in some tumor cells in individual patients.10,59 Only rare tumors express somatostatin receptors that yield positive results on scintigraphic scans.

Approximatively 50% of patients with rectal tumors are asymptomatic. Other patients mainly present with rectal bleeding, constipation, rectal syndrome, or rectal pain. Carcinoid syndrome is very rare because it is very unusual for the tumors to release serotonin into the circulation, despite their capacity to synthesize this amine. About 75% of the lesions are within 8 cm of the anal verge and are possible to reach with digital palpation. Luckily, only about 14% of patients with rectal carcinoids present with metastasis. Local excision or transanal resection is recommended for tumors measuring < 1 cm in diameter because these tumors are at low risk for recurrence or metastasis (< 2%). For tumors between 1 and 2 cm in size (10% of cases) without evidence of lymph node metastasis, wide excision with a meticulous evaluation to exclude muscular invasion is usually recommended. Transanal endosonography may be particularly useful in this intermediate group to assess tumor extension. In doubtful cases, these intermediate tumors should be considered as tumors measuring > 2 cm. Patients with tumors > 2 cm (10% of cases) or with muscular invasion or lymph node

Table 18-3. SURVIVAL RATE OF GASTROINTESTINAL NEUROENDOCRINE TUMOR

Location

5-Year Survival (%)

Thymus

Constipation Prescription

Constipation Prescription

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