Hi, Jane!
PCI is a tough decision because there just isn't enough long-term information. We'd all like to know how people fare 5-10 years out from it but...well, nobody seems to have those stats available.
The summary of what I've read is, 60% of lc patients will get brain mets (can't remember if that's all lc or just sclc). PCI reduces brain mets by about 50%. So, even with PCI, about 30% will still get brain mets. And PCI may make brain mets harder to treat.
My mom will probably opt against PCI for a couple of reasons. She is most likely going to have stem-cell replacement (which means super-doses of chemo, and one of the drugs used will cross the blood-brain barrier). Also, the hospital where she is being treated has a state-of-the-art radiology center, so if she needed treatment for brain mets, she could get the best available quickly. With those factors, Mom is leaning away from PCI.
There are long-term survivors in both the PCI and non-PCI groups. All you can do is make the decision your dad feels most comfortable with.