Although Prof Saglio may very well be correct in calling these muscle pains, this is not entirely clear to me from the description. I will put a caveat on my discussion as I have very limited (almost no) experience in pediatrics and TKIs. Imatinib therapy can be complicated by 3 basic types of pains, at least in adults. In the acute phase and usually lasting a few weeks at the start of therapy, is fairly severe bone type pain, that usually resolves when blood counts normalize and can usually but not always be controlled with an NSAID. On rare occasions, this pain can go on for several months and on even rarer occasions does not seem to resolve so long as imatinib is continued. More chronically, and I do not think this is the issue here, there are 2 types of pain. The first is the cramping that Prof Saglio discusses. This can be controlled with good hydration, exercise of affected muscle, sometimes calcium or magnesium supplements and most effectively with tonic water (quinine) preferably sugar-free. The second is chronic myalgia/arthralia type pain that usually responds to ibuprofen. It is concerning if suboptimal therapeutic doses of imatinib need to be used as the impact on response and possible disease progression/mutation development become issues. I would go back to 200mg per day and slowly increase the dose if stable using a protocol of increasing one day per week to 300mg waiting a week or two to be sure there is no significant pain and then increasing a second day, etc until you get to a therapeutic dose. As mentioned, monitor response. If sub-optimal, a second TKI with a different side effect profile may be helpful. If milestones are being met and improving at lower doses as sometimes occurs, I would be less concerned that the dose is not "textbook". If symptoms and/or inadequate response persist even with a drug switch, then start thinking BMT before disease progression occurs.