A dissertation submitted in accordance with the requirements for the degree of Master of Science in the Faculty of Science at theUniversity of Witwatersrand, Johannesburg, 2012. / AIM: This studied aimed at analysing the set-up error (margin) of head and neck cases, treated in a vacuum formed acrylic shell with an in-house immobilisation system. METHODS AND MATERIALS: Two population groups were studied, namely virtual simulated and intensity modulated radiotherapy (IMRT) cases. All cases were treated with the in-house immobilization system, which located centrally, but not longitudinally, to the treatment couch. Verification of the couch position, other than the isocentric angle, was not activated. The virtual simulated cases consisted of two lateral fields with a matched anterior neck field. The borders of these fields were chosen by the radiation oncologist. The IMRT cases were planned by a medical physicist and consisted of 6-9 fields of 3-4 intensity levels each. Digitally reconstructed radiographs (DRRs) of the 2 lateral fields and the anterior neck field for the virtual simulated cases, and the 2 lateral and anterior composite fields at the same isocentre for the IMRT cases, were printed and represented the ideal patient position. On the first day of treatment, megavoltage verification films were taken of the treated or positioning fields respectively. These verification films were compared to the DRRs and approved by a radiation oncologist. The absolute bed position in the vertical (Y), lateral (X) and longitudinal (Z) directions at the time of film approval, was used as the reference or ideal position. The absolute readings of the couch position that were captured daily over the course of treatment were then compared to the initial couch position to give an indication of the systematic and random errors. One linear accelerator was used in this study and weekly mechanical quality control (QC) was performed on it. RESULTS: The total number of daily fractions (F) studied in this thesis was 5644 and 600 for virtual simulated and IMRT cases respectively. The systematic error of this population was 4.7 and 4.4 mm for the virtual simulated and IMRT cases respectively. This compares well with published results using a similar immobilisation system. The random error of this population was 7 mm and 6.1 mm for the virtual simulated and IMRT cases respectively. This is three times larger than the results reported in the literature (using a similar immobilization device). CONCLUSION: Offline monitoring of couch position provides insight into setup margins and this can contribute to realistic institutional planning target volumes. Better results were obtained in the IMRT cases and this could be due to the requirement for weekly verification imaging. Lack of radiation therapist vigilance and insufficient training were most likely responsible for the individual cases with systematic variations of larger than 3 cm. The data confirm that the immobilisation system can be located to a fixed position on the tabletop; this will allow online verification of absolute bed position for treatment and further decrease the chance of incorrect setup.