The investigators try to save or "rescue" normal cells from the side effects of methotrexate by giving a drug called folinic acid. Folinic acid is started 24 hours after methotrexate and given regularly until methotrexate levels are really low and not dangerous to normal cells anymore. Despite this rescue, side effects are still a problem and many patients are not well enough to receive subsequent chemotherapy on time. Almost half of the planned chemotherapy cycles are not given on time due to methotrexate side effects.