It should still be compared to differences in other resources being transported to the tissue, see my answer below. I am not a fan of microplastics, I don’t try to discredit their health effects. It is just that this much information does not help much. I understand that causation would be much harder to prove, but at least one should try to prove for instance that ratio of cancerous to healthy tissue microplastics is much higher than the same ratio other for other stuff that tissues generally transport by blood vessels. This would atleast show that there is an extraordinary relation between the tissue and microplastics in the context of cancer. It could still be causation on the other direction, such as “maybe tissue structure of prostate cancer allows it to absorb microplastics more than other types of resources” but even that would be a useful piece of information.