• BTC (intrahepatic &extrahepatic cholangiocarcinoma, gallbladder cancer) is rare but increasinglyactionable with routine NGS + RNA fusion testing (e.g., FGFR2 fusions, IDH1mutations, HER2 amplification/overexpression, BRAF V600E, NTRK fusions, KRASG12C, RET fusions). ^{[1][5][6][10]}
• Chemo-immunotherapy hasimproved first-line overall survival (adding a PD-1/PD-L1 inhibitor togemcitabine/cisplatin). ^[2][3]
• Guidelines recommend testingearly so results can guide today’s plan and the next line. ^[1][4]

1) Why precision medicine matters in BTC

BTCs aren’t one disease. Molecularprofiling shows distinct, targetable drivers by subtype—e.g., FGFR2 fusions andIDH1 mutations are enriched in intrahepatic cholangiocarcinoma (iCCA), whileHER2 alterations are more common in gallbladder cancer (GBC) and parts ofextrahepatic CCA. ^[1][5]

2) First-line therapy has shifted: add immunotherapy

Two large randomized trials changed the 1Llandscape for unresectable/metastatic BTC:

• TOPAZ-1: adding a PD-L1inhibitor to gemcitabine/cisplatin improved overall survival with durablebenefit on update. ^[2]
• KEYNOTE-966: adding a PD-1inhibitor to the same chemotherapy backbone also improved overall survival. ^[3]
•  Implication: For most 1Ladvanced BTC, chemo-IO is now a common backbone while molecular results informsubsequent (and sometimes parallel) decisions. ^[1]

3) What to test—and when

In advanced or metastatic BTC,comprehensive molecular profiling is recommended at or near the time ofdiagnosis to inform treatment planning, including subsequent-line therapy andclinical trial considerations. ^[1][4]

High-value biomarkers in BTC

• FGFR2 fusions/rearrangements(iCCA ~9–15%; RNA assays increase detection). ^[1][5]
• IDH1 mutations (iCCA often~10–15%). ^[1]
• HER2 (ERBB2)amplification/overexpression or activating mutations (notably in GBC/eCCA). ^[1]
• Less frequent but actionable:BRAF V600E, NTRK fusions, KRAS G12C, RET fusions, MSI-H/dMMR, TMB-High. ^[1]

Circulatingtumor DNA (ctDNA) testing may be considered when tumor tissue is unavailable orinadequate, and for assessment of molecular evolution or emerging resistance;however, tissue-based testing remains the preferred approach when feasible. ^[1][4]

4) Mapping biomarkers to strategies (mechanism-focused)

• FGFR2 fusion/rearrangement(mostly iCCA): consider FGFR pathway inhibition; confirm fusions with RNA-basedmethods where possible. ^[1][5]
• IDH1 mutation (iCCA):IDH1-directed strategies are established; research continues onsynthetic-lethality approaches. ^[1]
• HER2amplification/overexpression or mutation (esp. GBC/eCCA): multipleHER2-directed strategies under study; status may carry prognostic value. ^[1]
• BRAF V600E, NTRK fusions, KRASG12C, RET fusions: tumor-agnostic targeted approaches exist; prevalence is lowbut clinically meaningful when present. ^[1]
• MSI-H/dMMR or TMB-High: whenpresent, these biomarkers may support consideration of immune checkpointinhibitors in selected clinical contexts, particularly in later-line settings. ^[4]
• Always cross-check the latestguideline version for line-of-therapy placement and evidence levels. ^[1]

References

1. NationalComprehensive Cancer Network. NCCN Clinical Practice Guidelines in Oncology:Biliary Tract Cancers. Version 2.2025.
2. Oh DY, HeAR, Bouattour M, et al. Durvalumab or placebo plus gemcitabine and cisplatin inparticipants with advanced biliary tract cancer (TOPAZ-1): updated overallsurvival from a randomised phase 3 study. Lancet Gastroenterol Hepatol.2024;9(8):694-704. doi:10.1016/S2468-1253(24)00095-5
3. Kelley RK,Ueno M, Yoo C, et al. Pembrolizumab in combination with gemcitabine andcisplatin compared with gemcitabine and cisplatin alone for patients withadvanced biliary tract cancer (KEYNOTE-966): a randomised, double-blind,placebo-controlled, phase 3 trial. Lancet. 2023;401(10391):1853-1865.doi:10.1016/S0140-6736(23)00727-4
4. Mosele MF,Westphalen CB, Stenzinger A, et al. Recommendations for the use ofnext-generation sequencing (NGS) for patients with advanced cancer in 2024: areport from the ESMO Precision Medicine Working Group. Ann Oncol.2024;35(7):588-606. doi:10.1016/j.annonc.2024.04.005
5. Zhang X,Bai Q, Wang Y, et al. FGFR2 fusion/rearrangement analysis in intrahepaticcholangiocarcinoma using DNA/RNA-based NGS and FISH. Virchows Arch.2025;487(5):1103-1115. doi:10.1007/s00428-025-04067-9

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