My first question is: What is the reason you'd want to bill an E/M code during the global period for 77427?
If there is a new metastatic site that wasn't part of the initial treatment plan, you can bill an E/M to address the new metastasis. There are some specific limited circumstances where you could bill the E/M visit in that 90-day global period.
However, you can't bill an E/M code for a routine post-treatment follow-up in the global period. My providers will occasionally have a patient come back in after 3 weeks - I don't bill those visits. Occasionally a patient will be scheduled too early, for example 85 days after treatment ended - I don't bill those visits either.