Hi Tedro
I'm no expert at this, but here's my attempt at a short answer to your question:
The levels of CML in one's body can be measured in terms of blood counts, bone marrow and then at the molecular level. The very first thing that doctors look for in a responding patient is called complete haematological remission (CHR), which means that all blood counts return to normal. Most of us are diagnosed with high white blood counts, often going hand in hand with low red counts and low or high platelets. So CHR is when all these return to normal.
Cytogenic remission is the next important level and refers to how much CML can be detected in the bone marrow. This level is also called CCyR by doctors and stands for Complete Cytogenic Response. When you are in CCyR, it means that there is no leukaemia detected in the bone marrow. However, since a bone marrow biopsy is a painful and invasive procedure, the doctors don't usually do a second one to test that there is no CML in the marrow - instead, they check what your BCR-Abl level is by performing a PCR test. This is much less invasive than a bone marrow biopsy and, if this test shows less than 1%, this roughly corresponds to no leukaemia in the bone marrow.
The third step is molecular response, which is called MMR, MR4, MR4.5, etc. This level is also tested by a PCR test on the blood. The definitions are as follows:
MMR means that there is 0.1% or less BCR-Abl in the blood test.
MR4 means that there is 0.01% or less.
MR4.5 means that there is less than 0.0032%.
So you are right: MR4 is way better than cytogenic remission in terms of numbers, because CCyR is 1% and MR4 is 0.01% - in other words, someone who has responded to MR4 can be said to have 100 times less disease than someone who is "only" in CCyR. However, all the articles I have read have said that CCyR is the "gold standard" of response and that that is the most important milestone to reach - there is no survival benefit in reaching lower levels, except that patients who stay in sustained levels of MR4 / MR4.5 might be able to stop the drugs. CCyR remains a very good place to be!
If I've gotten anything wrong, I'm sure that someone will correct me, but I hope that this explanation helps.
Best wishes
Martin
