Germline Variants, Liquid Biopsies, and Screens That Actually Save Lives

Drew, I had three cancer-related conversations in one week and they felt completely different. A woman with a PALB2 result, a man with a positive multi-cancer blood test and a negative workup, and a smoker asking about whole-body MRI. Where do I even start?

Start with what kind of question each one actually is. The PALB2 result is a germline question — germline meaning a DNA variant present in every cell since conception, inherited, not acquired. That has a management pathway. The other two don't, yet.

So the PALB2 woman — she's asking if she needs surgery. What does the evidence actually say?

PALB2 is a DNA repair gene. A pathogenic variant — meaning a change that disrupts protein function — roughly doubles lifetime breast cancer risk, putting most carriers into the forty-to-sixty percent range. NCCAP guidelines recommend risk-reducing discussions including enhanced MRI surveillance and the option of bilateral mastectomy, same framework as BRCA2. That's guideline-recommended, not experimental.

But her test came from a direct-to-consumer lab. Does that change anything?

It means confirm it first. Send confirmatory clinical-grade sequencing before any management decision. DTC labs vary in variant interpretation. Once confirmed, the result is actionable. Refer to a cancer genetics clinic — they'll address penetrance, family implications, and the surgical conversation properly.

Okay. Now the multi-cancer blood test — signal detected, upper GI origin, CT and endoscopy negative. Patient is terrified. What do I tell her?

Tell her the truth: this test has detection data, not mortality data. It finds signals. We do not yet have a randomized trial showing that acting on those signals reduces cancer deaths. The PATHFINDER study demonstrated feasibility. The SYMPLIFY study showed a positive predictive value around forty percent in a symptomatic UK population. Neither was powered for mortality.

So a negative CT and scope with a positive signal — what's the right move?

Reasonable surveillance, document clearly that the standard workup was negative, and do not chase with PET or repeat imaging without a specific clinical indication. The signal may represent a true early cancer, a false positive, or a pre-malignant state we don't have a protocol for. NCCN has no guideline here yet. NHS England is running SUMMIT, which may give us mortality data eventually.

And the smoker asking about whole-body MRI — thirty-five pack-years, still smoking. He's read it finds things other scans miss.

Whole-body MRI has no mortality evidence in general-population cancer screening. What does have mortality evidence for lung cancer in exactly his population is low-dose CT — the NLST and NELSON trials both showed reduced lung cancer mortality in heavy smokers. He qualifies under USPSTF criteria: fifty to eighty years old, twenty or more pack-years, currently smoking or quit within fifteen years.

So I redirect him from the MRI he wants to a low-dose CT he might not know about.

Exactly. And the bigger conversation is cessation. Smoking cessation reduces lung cancer incidence more than any screen. That's not a platitude — the NLST data showed benefit even in continuing smokers, but the absolute risk reduction from quitting dwarfs the screening benefit.

The evolutionary framing in this tutorial — the idea that cancer is decades of accumulated mutations selected round by round — does that actually change how I counsel patients, or is it just explanatory?

It's clinically useful because it explains why germline variants matter so much more than somatic ones for screening decisions. If you're born with one PALB2 copy already non-functional, every round of selection starts one step ahead. It also explains why metabolic and inflammatory exposures — obesity, chronic liver inflammation, smoking — aren't soft lifestyle factors. They increase mutation rate and clonal expansion rate. The biology is causal, not associative.

If there's one thing you want me to hold onto from all three of these patients, what is it?

Detection is not the same as mortality benefit. A test that finds cancer earlier is not automatically a test that helps patients live longer. Demand that distinction every time a result or a technology lands in your clinic.