View source for WBR0940

{{WBRQuestion
|QuestionAuthor=William J Gibson
|ExamType=USMLE Step 1
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|MainCategory=Anatomy
|MainCategory=Anatomy
|SubCategory=Gastrointestinal
|Prompt=A 65-year-old male presents to his primary care physician for blood in his stools.  He reports no pain associated with passing stool, and has lost 10 pounds over the past 2 months.  The physician orders a colonscopy, which reveals a single mass in the rectum.  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?
|Explanation=The patient in this vignette most likely has squamous cell carcinoma.  While colorectal carcinoma is the most common carcinoma of the digestive tract, the pectinate line divides the mucosal epithelium of the colon and the upper 2/3 of the rectum from the squamous epithelium of the lower 1/3 of the rectum. Cancer 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. 

References:
First Aid 2013 page 192, 315

Mcgraw S, Thakkar J, Mehta D. Inguinal lymph node metastasis of colon cancer. Indian J Med Paediatr Oncol. 2011;32(3):168-70.
|AnswerA=Superficial inguinal lymph nodes
|AnswerAExp=Correct: From below the pectinate line, the rectum is drained by the superficial inguinal lymph node.
|AnswerB=Inferior mesenteric lymph nodes
|AnswerBExp=Incorrect:  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).
|AnswerC=Para-aortic lymph nodes
|AnswerCExp=Correct:  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.
|AnswerD=Liver
|AnswerDExp=Incorrect:  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.
|AnswerE=Lung
|AnswerEExp=Incorrect: 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.
|RightAnswer=C
|WBRKeyword=Lymph node, Anus, Anal, Colon, Cancer, Rectum, Lymphatic, Lymph
|Approved=Yes
}}