Biliary Tract Cancer: Acting on a Fourteen-Month-Old HER2 Result
A HER2-positive result that is technically still true doesn't guarantee it's still accurate — and in biliary cancer, that gap between true and current can decide the next line of therapy.
L.F., a 59-year-old woman who ran a small tailoring shop out of her home for over twenty years, was diagnosed with gallbladder adenocarcinoma fourteen months ago after months of vague right- upper-quadrant discomfort she'd attributed to her diet. Imaging at diagnosis already showed liver metastases, and the biopsy of her gallbladder primary — the only tissue ever sampled — came back HER2 IHC 3+, strongly positive, alongside PD-L1 testing that was negative. She completed eight cycles of gemcitabine, cisplatin, and durvalumab, tolerated it reasonably well, and had a partial response that lasted ten months before this month's scan showed new hepatic lesions and a rising CA 19-9. She has otherwise been well — no jaundice, eating normally, still doing light alterations for neighbors between appointments — but the growth on imaging is real and the team wants to move to second-line therapy without unnecessary delay.
Zanidatamab, a HER2-targeted bispecific antibody, received accelerated approval for previously treated HER2-positive biliary tract cancer based on HERIZON-BTC-01, a single-arm trial in which HER2 IHC 3+ patients had a response rate above fifty percent — a genuinely strong signal in a disease with few second-line options. But HER2 expression in biliary tract cancer is not a fixed property the way it often behaves in breast cancer: real discordance between primary and metastatic sites, and loss of HER2 positivity over time and after prior therapy, has been documented in this disease specifically. L.F.'s only HER2 result is fourteen months old, drawn from her original gallbladder primary, not from the liver lesions that are progressing right now — raising a genuine question of whether that result still describes the tumor she actually has today. Her ten-month partial response is the detail that sharpens this rather than softening it: eight cycles of effective therapy is precisely the sustained selective pressure under which HER2 loss has been described, so the very thing that bought her those ten months is what makes her original result least likely to still hold. HERIZON-BTC-01 enrolled patients whose IHC 3+ status was confirmed on contemporaneous tissue; nobody yet knows whether L.F. belongs to that population.
Clinic visit, three weeks after progression scan
Her IHC 3+ result is real, and HERIZON-BTC-01 showed a response rate over fifty percent in exactly this population — that's an unusually strong second-line signal for biliary tract cancer. She has confirmed progression, both radiographic and biochemical, and I don't want to spend weeks re-testing tissue when we already have a documented positive result and a drug approved specifically for it.
The response rate is real, but it was measured in patients whose HER2 status was confirmed on contemporaneous tissue, not on a fourteen-month-old sample from a site that isn't even the one progressing now. HER2 discordance between primary and metastatic biliary tumors, and loss of HER2 positivity after prior systemic therapy, is specifically documented in this disease — not a generic caution I'm importing from another cancer type. If her liver lesions have lost HER2 expression, zanidatamab has essentially nothing to work against, and she'd be exposed to infusion-related and cardiac monitoring burden for a drug that can't do what the trial data promise.
I'm not disputing that the original result was accurate when it was drawn — I'm disputing that fourteen-month-old tissue from a different lesion tells us what today's tumor actually expresses.
The new dominant lesion is a straightforward percutaneous target — no bowel loops in the way, good acoustic window, same-day procedure. That changes the calculus: this isn't a case where biopsy means a real delay or a real risk. Biopsy it this week, and start a bridging cycle of her prior chemotherapy backbone while we wait on the result, so the clock both of you are worried about keeps moving either way.
Agreed: percutaneous biopsy of the new hepatic lesion this week, with one bridging cycle of gemcitabine/cisplatin started the same day rather than waiting on results, and zanidatamab initiated promptly if HER2 positivity is confirmed on current tissue.
Not fully agreed: the medical oncologist's underlying discomfort with the delay was noted explicitly rather than resolved — if the biopsy is technically difficult or non-diagnostic, the group has not yet decided whether to proceed empirically on the original result or pursue a second biopsy attempt, and that decision was deliberately deferred rather than forced today.