Introduction: Conformal radiotherapy demands an accurate and reproducible treatment set up technique therefore the purpose of this study was to determine whether the current techniques were suitable for use with conformal treatment plans and to establish whether the current verification procedure was adequate.

Method: 30 patients undergoing radical radiotherapy were recruited to each technique to be studied. Megavoltage images (MVI's) were taken twice during week one and then at weekly intervals. All study images were assessed after the patient had completed the treatment course using Masthead PIPS software. Field placement errors were recorded in x, y and z directions where appropriate. Patient characteristics were recorded to establish whether there was any relationship between reproducibility and patient size.

Results: Head and neck: 26.6% patients had one or more errors >5 mm, 3.3% had consistent errors >5 mm throughout treatment. Poor fit of the beam direction shell was associated with errors >5 mm. Chest and abdomen: The technique was modified after 20 patients due to the large number of errors exceeding 10 mm. A significant (p < 0.01) reduction in the number of errors was in the modified technique group, this group also had a significantly (p < 0.01) reduced range of errors in the y direction. Pelvis: Significantly more errors were seen in the y and z directions using the sinmed kneeblock. A link between field placement errors and patient size was found.

Conclusion: Imaging on the first fraction is not sufficient to determine whether a set up error is likely to occur consistently during treatment, at least two sets of images are required. The study has shown that technique development is necessary for the chest, abdomen and pelvis prior to the introduction of conformal radiotherapy.