As a heme/onc attending, average compensation is in the $500,000 range per year. More than that is easily achievable.
That's ~9 years of opportunity cost to go back into training.
That's $4.5 million (gross) you are leaving on the table. Don't forget the ability to tax shelter a decent bit of this along with matching options for retirement plans.
As a resident/fellow, we can estimate an average salary of about $70k a year over those 9 years. That's a total of $630k (gross). Typically the investment options are very limited and in my experience, residencies usually don't offer 401k/403b matching options.
This can't be a purely financial decision but that is a lot of money to leave on the table. Heme/onc is a pretty great field.
In demand, being involved with cutting edge therapies, etc.