How reliable is a frozen section for diagnosing cancer?

Pretty good: Pretty good at diagnosing cancer, but not as good as the permanent sections that take a day or two, as they need chemical stains to be more accurate, and to tell what sub-type of cancer.

Reliable with cautio: Frozen sections are meant to provide critical information about lesions, during surgery, that may alter the course of surgical procedure. In expert hands these are very reliable, but not a substitute for "permanent" sections. Pathologists are usually conservative with frozen sections and will defer an opinion if they are not sure of the diagnosis on frozen section.

Very. But it: Is considered preliminary until the permanent sections are fixed, stained and read. Rarely the diagnosis is revised finding things not seen on frozen.