WBR0940

Author	PageAuthor::William J Gibson (Reviewed by Serge Korjian)
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Anatomy
Sub Category	SubCategory::Gastrointestinal
Prompt	[[Prompt::A 65-year-old man presents to his primary care physician for blood in his stools. He reports no pain with passing stool, but he states he has lost 10 pounds over the past 2 months. Physical exam and diagnostic sigmoidoscopy reveal a single mass in the distal anal canal. The mass is biopsied and shows tumor invasion of the basement membrane. Which of the following sites would be the most likely next site of spread of the tumor?]]
Answer A	AnswerA::Superficial inguinal lymph nodes
Answer A Explanation	AnswerAExp::Below the pectinate line, the rectum is drained by the superficial inguinal lymph node.
Answer B	AnswerB::Inferior mesenteric lymph nodes
Answer B Explanation	[[AnswerBExp::The mesenteric lymph nodes drain the gut. The superior mesenteric lymph nodes drain all parts of the gut proximal to the sigmoid colon. The inferior mesenteric lymph nodes drain lymph from the sigmoid colon to the rectum (above the pectinate line).]]
Answer C	AnswerC::Para-aortic lymph nodes
Answer C Explanation	AnswerCExp::The para-aortic lymph nodes drain the testes and the lower gastrointestinal tract. The superficial inguinal lymph node is a more proximal site of lymphatic drainage for the rectum than the para-aortic lymph nodes.
Answer D	AnswerD::Liver
Answer D Explanation	[[AnswerDExp::While the liver is the most common site of metastasis in colon cancer patients, cancer cells are more likely to spread to the lymphatics before metastasis. Furthermore, cancers below the pectinate line are more likely squamous cell carcinomas than colorectal adenocarcinomas.]]
Answer E	AnswerE::Lung
Answer E Explanation	[[AnswerEExp::While lung is the second most common site of metastasis in colon cancer patients, cancer cells are more likely to spread to the lymphatics before metastasis. Furthermore, cancers below the pectinate line are more likely squamous cell carcinomas than colorectal adenocarcinomas.]]
Right Answer	RightAnswer::C
Explanation	[[Explanation::The patient in this vignette most likely has anal squamous cell carcinoma (SCC). The dentate or pectinate line divides the upper 1/3 and lower 2/3 of the anal canal. The line marks the transition from columnar epithelium to squamous epithelium; and demarcates the meeting point of ectoderm (distal anal canal) and endodermal hindgut (proximal anal canal). Cancers of the anal canal are relatively uncommon and account for 2% of colorectal cancers. Anal SCC progresses from a locally confined lesion, to a cancer that invades the basement membrane, to lymphatic spread and finally metastasis. This set of steps is the basis for the TNM staging system. Because of the location of this particular tumor, the most likely site of spread from the primary site is via the lymphatics to the superficial inguinal lymph nodes. Above the pectinate line, the tumor would most likely be a colorectal adenocarcinoma and would first reach the mesenteric lymph nodes. Educational Objective: The most likely route of spread of a distal rectal tumor (squamous cell carcinoma most common) is via the lymphatics to the superficial inguinal lymph nodes. References: Bullard Dunn KM, Rothenberger DA. Colon, Rectum, and Anus. In: Brunicardi F, Andersen DK, Billiar TR, Dunn DL, Hunter JG, Matthews JB, Pollock RE. eds. Schwartz's Principles of Surgery, 10e. New York, NY: McGraw-Hill; 2014.]]
Approved	Approved::Yes
Keyword	WBRKeyword::Lymph node, WBRKeyword::Anus, WBRKeyword::Anal, WBRKeyword::Colon, WBRKeyword::Cancer, WBRKeyword::Rectum, WBRKeyword::Lymphatic, WBRKeyword::Lymph
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