In this phantom study, we tried to compare matching dosimetry differences between half-blocking of Co-60 and asymmetric collimation of the 4 MV photons during craniospinal irradiation. The dose distributions are compared and discussed. Firstly, some gaps with different sizes are left between cranial and spinal field borders. Secondly, the fields are overlapped in the same sizes. We irradiate the films located in water-equivalent solid phantoms with Co-60 and 4MV photon beams. This study indicates that the field placement errors in +/- 1 mm are acceptable for both Co-60 and 4 MV photon energies during craniospinal irradiation with half-beam block technique. Within these limits the dose variations are specified in +/- 5%. However, the setup errors that are more than I mm are unacceptable for both asymmetric collimation of 4 MV photon and half-blocking of Co-60. (C) 2004 Elsevier Ltd. All rights reserved.