Diagnostic Oncology and Pathology
Core concepts
Know how tissue is obtained (core biopsy, FNA, excisional), when it is adequate, and the role of immunohistochemistry in determining origin (e.g., cytokeratin, TTF-1, CDX2, PAX8, GATA3). Understand grading and staging systems (TNM, Gleason, Nottingham), margin status, and the difference between grading and staging. Recognize cancer of unknown primary workup and when to pursue molecular profiling. Know the limitations of cytology versus histology, frozen section use, and the importance of adequate tissue for biomarker testing (PD-L1, HER2, molecular panels). Liquid biopsy is emerging but not a universal replacement for tissue. Understand screening principles: sensitivity, specificity, positive predictive value, and lead-time bias.
Traps
Questions test whether a test result changes management, the meaning of a negative predictive value, and the difference between screening and diagnostic testing. Watch for overinterpretation of a single IHC marker and for confusion between grade and stage.
Rule of thumb
Before ordering a test, ask: will a positive or negative result change what I do? If not, do not order it.
Worked example
A patient with adenocarcinoma of unknown primary: targeted IHC panels and molecular profiling can identify a treatable primary or actionable alteration, but exhaustive testing without management impact should be avoided.