5.5 Abdomen, Pelvis, GI & GU Sites: Anatomy & Metastatic Patterns

Esophageal Cancer

The esophagus is divided into cervical, upper/mid/lower thoracic, and gastroesophageal (GE) junction segments. Location drives histology and lymphatic spread patterns that the therapist must recognize when reviewing a simulation CT or a portal image.

• Squamous cell carcinoma (SCC) predominates in the cervical and upper/mid thoracic esophagus and is linked to tobacco and alcohol use.
• Adenocarcinoma predominates at the distal esophagus and GE junction, typically arising from Barrett esophagus (chronic reflux-induced columnar metaplasia).
• Siewert classification describes GE-junction tumors by their center relative to the junction (Type I, II, III) and guides whether treatment follows an esophageal or gastric paradigm.
• Lymphatic drainage is longitudinal and unpredictable — cervical tumors can drain to mediastinal nodes and distal tumors to celiac nodes, so elective nodal volumes are often generous along the length of the esophagus rather than site-restricted.
• Standard management for locally advanced disease is trimodality therapy: neoadjuvant chemoradiation followed by esophagectomy (CROSS regimen), or definitive chemoradiation alone when surgery is not an option (cervical esophagus, medically inoperable patients).

Gastric Cancer

The stomach is subdivided into the cardia, fundus, body, antrum, and pylorus. Gastric adenocarcinoma is the dominant histology. Nodal drainage follows the perigastric chains (lesser and greater curvature) before reaching celiac, splenic, and porta hepatis nodes. Because the stomach is a mobile, fluid- and air-filled organ, daily volume and position vary, which complicates radiotherapy planning. The typical paradigm is surgery followed by adjuvant chemoradiation (INT-0116 regimen) for resected, node-positive or margin-positive disease, targeting the tumor bed and regional nodal basins.

Pancreatic Cancer

Roughly 60–70% of pancreatic adenocarcinomas arise in the head of the pancreas, where the tumor can compress the distal common bile duct as it passes through the pancreatic head — producing painless obstructive jaundice, dark urine, and pale (acholic) stools. This anatomic relationship is a frequent testable point: head-of-pancreas tumors present with jaundice because of biliary obstruction, not liver metastasis.

Resectability is determined by tumor contact with the superior mesenteric artery (SMA), superior mesenteric vein (SMV), celiac axis, and portal vein:

Head tumors are treated with a Whipple procedure (pancreaticoduodenectomy) when resectable; chemoradiation is used for borderline-resectable/locally advanced disease to attempt downstaging, and stereotactic body radiation therapy (SBRT) is increasingly used for locally advanced disease given the tight margins from bowel and duodenum.

Hepatocellular Carcinoma (HCC) and Liver Metastases

HCC arises almost exclusively in a background of chronic liver disease — cirrhosis from hepatitis B/C, alcohol use, or non-alcoholic fatty liver disease. Treatment selection depends on tumor burden and underlying liver function, quantified by the Child-Pugh score (bilirubin, albumin, INR, ascites, encephalopathy). SBRT and proton therapy are used for inoperable, well-selected lesions; whole-liver tolerance is limited, so highly conformal, hypofractionated approaches that spare uninvolved parenchyma are preferred over broad-field liver irradiation.

Colorectal Cancer

Colon and rectal cancers are grouped together but managed differently because of anatomy. The rectum is the distal ~15 cm of the large bowel and is fixed within the bony pelvis, surrounded by the mesorectum (fatty tissue containing perirectal lymph nodes). Because the rectum is fixed and adjacent to the bladder, sacrum, and (in men) prostate, local recurrence risk is higher than in the mobile colon — this is why radiation therapy has a defined role in rectal cancer but essentially no role in colon cancer.

• Total mesorectal excision (TME) is the surgical standard, removing the rectum with an intact mesorectal envelope to minimize circumferential margin positivity.
• Neoadjuvant (preoperative) chemoradiation is standard for locally advanced (T3–T4 or node-positive) rectal cancer. Goals are tumor/nodal downstaging, improved resectability, sphincter preservation, and lower local recurrence — delivered before surgery because pelvic tissue tolerates radiation better preoperatively and tumor oxygenation/response is improved.
• Lymphatic drainage is to mesorectal and internal iliac nodes; low rectal/anal-margin tumors also drain to inguinal nodes.

Anal Canal Cancer

Anal canal SCC is strongly associated with HPV infection. Unlike most GI malignancies, anal cancer is managed with definitive chemoradiation as first-line therapy rather than surgery — the Nigro protocol (concurrent 5-fluorouracil and mitomycin-C with radiation) achieves organ preservation, sparing the patient a permanent colostomy that abdominoperineal resection (APR) would otherwise require. Because lymphatic drainage below the dentate line passes to the inguinal nodes, elective inguinal nodal coverage is a required part of the treatment volume, distinguishing anal canal planning from rectal cancer planning.

Bladder Cancer

Most bladder tumors are urothelial (transitional cell) carcinoma. Staging separates non-muscle-invasive disease (managed with transurethral resection ± intravesical therapy) from muscle-invasive disease, where options are radical cystectomy or trimodality bladder-preservation therapy: maximal transurethral resection of bladder tumor (TURBT) followed by concurrent chemoradiation, reserving cystectomy for salvage. Because the bladder is a distensible, mobile organ, simulation and daily treatment require a consistent bladder-filling protocol (typically a comfortably full or consistently empty bladder per department protocol) to minimize day-to-day volume and position variation.

Renal Cell Carcinoma (RCC)

RCC has historically been considered relatively radioresistant, so surgery (partial or radical nephrectomy) remains the primary treatment for localized disease. SBRT is an emerging option for medically inoperable patients or oligometastatic disease, exploiting high per-fraction doses to overcome the tumor's relative radioresistance.

Metastatic Spread Patterns — Quick Reference

Recognizing these organotropic patterns helps the therapist anticipate staging workup findings (e.g., liver protocol CT/MRI for colorectal cancer, bone scan for prostate cancer) and understand why certain sites are imaged during simulation and follow-up.