(Another layperson here.) That's a great point @wellness100 , but I also understand the anxiety — a doctor will have the opportunity to learn from a mistake and do better next time, but their patient might not.

That's one reason I'm a big fan of team approaches like we see at dedicated Cancer Centres or specialised clinics. They have a range from doctors with new ideas but little experience to doctors with old ideas but lots of experience (and everything in-between), together with specialised nurses and technicians to make sure the doctors don't stray too far off track.

Obviously misdiagnoses still happen. They have no absolutely-reliable way to predict how any given cancer will progress or how any given patient will respond to a treatment, and no way at all to detect individual cancer cells until they start to form tumours, so diagnosis has to be a combination of scientific analysis and (highly-educated) guesswork. However, through pooling a treatment team's knowledge and experience, I believe those misdiagnoses will happen less often and patient outcomes will improve.