[woke_neoliberal]

You are correct - meaningful amounts of DNA are only shed when the tumor breaks the basement membrane.

You can see a breakout at the bottom of this poster [1] from Grail (developing a methylation based liquid biopsy) showing sensitivity per-cancer and per-stage.

Sensitivity increases as stage increases, as expected, and cancers like Leukemia are well detected. IIRC, the stage classifications vary across different tissue types as to when the cancer breaks through the basement membrane - would be compelling if sensitivity of the assay rises in accordance.

[1] https://grail.com/wp-content/uploads/2020/12/ASCO_2019_CCGA2...

[killjoywashere]

Yeah, now break out the prevalence of each of their 21 cancer types in the population. The overall cancer prevalence is only 0.4%, so, b-school math (not to be used in actual calculations) is 0.02%. With a 99% specificity and a 80% sensitivity, you're going to get a lot of false positives and a lot of false negatives. Oy vey.