Brain radiation for sclc - 1270677

My husband was diagnosed with sclc with mets to liver January 2015. He had 4 rounds of carboplatin/etoposide with a very good response after 2 rounds, not much more after 4. He received 10 days of PCI and chest irradiation. After 6 weeks his CT showed lung and lymph node stable with progression in the liver. As we prepared to begin 2nd round chemo with topotecan we did a brain MRI to satisfy requirements for a clinical trial. One 3mm lesion was noted. We did not expect that so soon after PCI. Now we are having gamma knife this week for that lesion. My question is can WBR be used down the road after PCI and gamma knife and if so, how long must one wait before doing so. We don't know when this lesion appeared since no MRI was done since early 2015.