Can cancer be diagnosed with blood tests? are biopsies necessary?

This depends on the cancer and the type of blood test. Leukemia might be found on a random CBC, but other forms of cancer would not. Some specialized cancer marker tests are available but rarely used unless a doc is looking for that specific cancer. Most would not compose a cancer treatment plan without a confirmatory biopsy.

Unfortunately, for solid cancers biopsies are needed as a tissue diagnosis is critical in formulating a proper treatment plan as well as prognosis