Hi Jonny 

Sorry to read your news. I have no medical training to be sure, but i would say at the least further tests on the shadow back in October 21 should have taken place at the time to fully investigate and establish WHAT it actually was.

I will tell you obvious lung cancer is usually referred to as a mass and a respiratory consultant can tell by an xray if it's a cancer (that was my experience anyway, but my tumour was 6.5 cm) 

Shadow could be infection, TB etc that said Dr Google also says maybe lung cancer.

You'll need to know if he actually has lung cancer that has spead to brain, or at the least precisely what that shadow is before you can establish if a spread could have been prevented.

In other words if the brain is  a metastatic spread from somewhere else (primary cancer) such ss the lung.

Hope this helps, best of luck to you and yours on this journey.

Regards