Influence of a vac-fix immobilization device on the accuracy of patient positioning during routine breast radiotherapy

Continued use of basic planning and treatment techniques, in contrast to th e improved methods implemented at many other anatomical sites, has emphasiz ed the need for improved breast dosimetry. Any future technique delivering a superior three-dimensional dose distribution will be of maximum benefit i f set-up errors are minimized. To determine the influence of vacuum moulded bag (vac-fix) immobilization on routine breast radiotherapy, 17 patients r eceived half their radiotherapy fractions using our standard breast board t echnique and half using a vac-fix device positioned on the breast board. Tr eatment accuracy and reproducibility were assessed for each technique using daily electronic portal imaging and were analysed in terms of random and s ystematic translational and rotational displacements of treatment fields wi th respect to corresponding simulation images. In addition, patients comple ted a short questionnaire aimed at determining which technique they preferr ed. Results showed that random errors for the two techniques did not differ significantly. Approximately 80% of random translations recorded were less than 3 mm and 80% of random rotations were less than 1.5 degrees. Systemat ic errors showed some improvement with the vac-fix system. In the anteropos terior direction, approximately 80% of systematic errors were less than 4 m m for both techniques, but in the superoinferior direction the 80% point wa s reduced from 5.0 mm for the standard set-up to 2.7 mm for treatment in va c-fix. For rotational systematic errors, the corresponding value dropped fr om 1.8 degrees for the standard setup to 1.1 degrees in vac-fix. Therefore, for many patients. additional use of a vac-fix device improved the transfe r of the planned set-up from simulator to treatment unit. Additionally, ans wers to the questionnaire indicated that patients generally favoured the va c-fix system over use of the breast board alone. In conclusion, however, in troduction of vac-fix immobilization for all patients was not thought justi fied as the improvements demonstrated are not likely to be clinically signi ficant with the present treatment technique.