In therapy related leukemia after radiotherapy, given that notionally i have sen latency periods between 2 and 9 years, what is going in during that period assuming that the damage is done at time of expodure. presumably during that time fbc is ok?

One theory is that the gene mutation that caused the leukemia was present long before the radiotherapy but the radiotherapy allowed the emergence or expression of that gene allowing onset of leukemia from months to years. This is a different theory from the conventional belief that the radiation caused DNA damage to genes allowing new malignant transformation of certain bone marrow cells.