Gastrointestinal Cancers
Core concepts
Colorectal cancer: know the adenoma-carcinoma sequence, mismatch repair/MSI testing for both prognosis and immunotherapy eligibility, and RAS/BRAF testing before anti-EGFR antibodies (panitumumab, cetuximab) in metastatic disease. Adjuvant chemotherapy duration and regimen depend on stage and risk. Gastric and esophageal cancers hinge on HER2 status, PD-L1, and MSI, with perioperative chemotherapy or chemoradiation for locally advanced disease. Hepatocellular carcinoma is managed by liver function, tumor burden, and portal invasion, with sorafenib/lenvatinib and atezolizumab-bevacizumab as systemic options. Pancreatic cancer: resectability determines surgery plus adjuvant chemotherapy versus palliative gemcitabine/nab-paclitaxel or FOLFIRINOX. Neuroendocrine tumors and anal squamous cell carcinoma (chemoradiation with mitomycin/5-FU) round out the domain.
Traps
Questions test RAS wild-type requirement before anti-EGFR therapy, the difference between adjuvant and metastatic regimens, and whether MSI-high disease should receive immunotherapy. Watch for confusion between neoadjuvant and adjuvant intent in rectal cancer and for the role of capecitabine/oxaliplatin neurotoxicity.
Rule of thumb
In metastatic colorectal cancer, sequence is: molecular testing (RAS, BRAF, MSI) → side of tumor → treatment. Right-sided tumors respond poorly to anti-EGFR therapy.
Worked example
A patient with left-sided, RAS wild-type metastatic colon cancer: anti-EGFR antibody plus chemotherapy is appropriate; if RAS mutant, use bevacizumab-based therapy instead.