how can I mount 12-R .iso file?

Mon Jan 21 11:57:12 UTC 2019

the iso file has report a corrupt GPT table and I can not mount the
iso file as usual as cd9660:

#:mdconfig ./FreeBSD-12.0-RELEASE-amd64-disc1.iso
md1
# gpart show md1
=>      3  1743095  md1  GPT  (851M) [CORRUPT]
        3       29    2  freebsd-boot  (15K)
       32       48       - free -  (24K)
       80     1600    1  efi  (800K)
     1680  1741418       - free -  (850M)

It seems the 850M contain the cd contain,but how can I get the content?

the checksum matches.