In this study the patient will receive 4 courses of high-dose methotrexate. High-dose methotrexate is normally given at weekly intervals, in blocks of two.The first two courses will be given on weeks 1 & 2; the second two courses on weeks 4 & 5. Two courses will be given with folinic acid rescue (standard high-dose methotrexate), and the other two will be given with glucarpidase rescue as well as folinic acid. This will enable us to compare whether there is any difference in side effects with and without glucarpidase and also how quickly patients recover from them.