Tell your doctor that Dr. Jorge Cortes, one of the top 4 or 5 specialists in CML research and now is the director of the Georgia cancer center (U.S.) is the one who prescribed 20 mg to me and told me that since I had strong myelosuppression, that starting low and working up (if needed) is the way to proceed. That should give your doctor some measure of confidence. I did not come up with 20 mg on my own. I was guided by an expert. I am sharing what I have learned and experienced with you. Share what you have now learned with your doctor.
All she needs to do is monitor you! There's no mystery here. You should be having weekly CBC blood tests (to track blood counts) and monthly PCR checks to verify response. When you start 20 mg dose, within a week you will know if myelosuppression is still an issue. If it is, you stop and monitor blood counts. Then re-start again when blood counts recover and monitor. You may have to do this two or three times, maybe more, hopefully, no more. But once you stabilize (i.e. neutrophils or platelets stay above critical, but may be below normal), you will continue taking your low dose. Then in one months time, you test PCR (or FISH depending) and compare to previous level. If PCR dropped - you should dance and have wine and celebrate. You have arrived. Then you continue 20 mg as YOUR personal treatment dose. This is what happened for me - so it's possible eh?
Now then - if your PCR doesn't drop or instead rises, you can move to a higher dose at 40 mg. You may experience myelosuppression again, but likely not as severe (remember, your normal blood system is continuing to recover while your leukemic system is getting strangled and can't compete). And if myelosuppression is severe enough to stop therapy, restart back at 20 mg. Your priority goal is to get your normal blood system adapted first while continuing to have some dasatnib in your system. As long as PCR is stable during this phase you are good. And if on 40 mg you test and no severe myelosuppression and PCR in one month's time is trending down, 40 mg is your dose. I doubt it will be higher - maybe 50 mg, but not likely any higher than that. 100 mg is out of the question - that's toxic for your personal situation (toxic = myelosuppression and related).
This is not rocket science. I wish doctors worked with their patients based on what is now known about CML. You are likely learning more about CML in your particular case than your doctor knows. Think about that. All she has to do is work with your personal situation. We KNOW CML is slow in chronic phase - this is why the new standard for pregnant CML patients is to stop therapy and tolerate PCR rising while the pregnancy is taken to term. There is NO case I know of where a pregnant women stopped therapy in order to have her child, did not successfully resume treatment post pregnancy). That's NINE MONTHS. So, tell your doctor, imagine you are pregnant - what would she do? You know the answer. You have nine months to get your dose right. I have a strong suspicion you are going to do great on 20 mg.
Note: if your doctor tells you she is concerned about "resistance" developing at such a low dose, get a new doctor. CML is not a bacteria or viral infection.