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Uludağ Üniversitesi Tıp Fakültesi Radyasyon Onkolojisi AD., Bursa Purpose: Craniospinal irradiation is a complex treatment technique using adjacent irradiation fields. Cranium is usually treated with parallel opposed lateral whole brain portals while spinal region is treated with either one or two fields depending on the length of the spine. Patients are generally treated in prone position, which is mostly uncomfortable for them. Therefore, immobilization of the patients during treatment session is extremely important to be able to avoid hot or cold spot formation at the junction of the fields and to avoid inhomogeneities originating from the irregularities of the body contour. In the present study, our aim was to investigate the effects of immobilization systems used in our department on dose distribution and maintenance of optimal set-ups during the whole treatment period and compare them in terms of preparation time, ease of application and labor consumption. Material and Methods: Craniospinal irradiation has been applied to 17 patients in our department between January 1996 and December 1998. Customized plaster casting technique was used in 6 patient while "alpha Cradle" immobilization device was used in 11. Computerized planning was performed in all cases. 5°-gantry angle, 10°-collimator angle and 6.5°-treatment couch angle were given, at the junction of cranial and spinal fields. Using "Moving Junction" technique, fields were moved at every 12 Gy. At the junction of cranial and spinal fields, asymmetric collimation was used to avoid overlapping. Results: Median dose inhomogeneity was 110% within spinal fields in patients immobilized with customized plaster casting system, while it was 107% when "Alpha Cradle" immobilization system was used. Accuracy of the radiation fields was evaluated using port films. Immobilization with "Alpha Cradle" device was found to be better in terms of preparation time and ease of application although it was twice as expensive as customized plaster casting system. Conclusion: Immobilization using "Alpha Cradle" device can be achieved more rapidly and easier than customized plaster casting system. Dose homogeneity was appeared to be better although the difference did not reach statistical significance. Reduction in set up errors can be achieved more easily using "Alpha Cradle" immobilization system. Keywords :