Chapter 117: Rare GI Tumors

INTRODUCTION

In this section we discuss oncologic entities of the lower gastrointestinal (GI) tract, comprising the appendix, colon, rectum, and anus, which are less frequently encountered in surgical practice. These pathologies are distinct tumors that are related only by their common location in the lower GI tract and the infrequency of their occurrence. The lower GI tract tumors to be discussed, in descending order of frequency, include anal melanomas, carcinoids, gastrointestinal stromal tumors (GISTs) and other sarcomas, and retrorectal tumors. Owing to the rarity of these tumors, exact incidences are difficult to determine. The incidence of anorectal melanoma is estimated at 1.7 cases per 100,000 population per year based on a review of cancer registries in the United States.¹ There are approximately five new cases of carcinoids diagnosed per year for every 100,000 individuals in the United States. Twenty-five percent of these occur in the lower GI tract (5% in the appendix, 5% to 7% in the colon, and 14% in the rectum), for an estimated incidence of 1.25 per 100,000 per year.² The overall incidence of GISTs is estimated at 1 to 2 per 100,000, with only about 5% in the lower GI tract.³ The incidence of colonic sarcomas has been reported at 0.04 per 100,000 per year, with appendiceal and rectal sarcomas also being incidentally reported.⁴ Retrorectal tumors are so rare that an overall incidence has not been determined; however, it is reported that between one and six patients are diagnosed annually at major referral centers.^5,6 Though all of these lower GI tract tumors are rare, a thorough understanding of their management, as well as a high clinical suspicion for the diagnoses in the correct clinical setting, is essential for the surgical oncologist.

ANAL MELANOMAS

Background

Anal melanoma is a rare subset of mucosal melanoma which represents only 1% to 2% of lower GI tumors, only 1% to 2% of melanomas overall, and fewer than 25% of all mucosal melanomas.^7-9 One-third of anorectal melanomas arise in the anal canal, 42% occur in the rectum, and the remainder are considered to have an indeterminate site of origin.¹⁰ Anal melanoma disease was first described in 1857 by W.D. Moore in the case of a 65-year-old man.¹¹ It remains a disease primarily of the elderly, with a mean age of diagnosis of 60 to 70 years and a female predominance.^10,12-15 HIV infection is a likely risk factor and perineural invasion has been shown to be an independently poor prognostic indicator.^16,17

Varying histologic subtypes of this disease were described in a study out of Memorial Sloan Kettering Cancer Center, which described 62 consecutive patients with anorectal melanoma treated at their institution from 1984 to 2003. They described 44% as being of an epithelioid subtype, 31% mixed type, and ...