Pulsed brachytherapy: a modelled consideration of repair parameter uncertainties and their influence on treatment duration extension and daytime-only "block-schemes".

OBJECTIVES: The radiobiological modelling of all types of protracted brachytherapy is susceptible to uncertainties in the values of tissue repair parameters. Although this effect has been explored for many aspects of pulsed brachytherapy (PB), it is usually considered within the constraint of a fixed brachytherapy treatment time. Here the impact of repair parameter uncertainty is assessed for PB treatments of variable duration. The potential use of "block-schemes" (blocks of PB pulses separated by night-time gaps) is also investigated. METHODS: PB schedule constraints are based on the cervical cancer protocols of the Royal Marsden Hospital (RMH), but the methodology is applicable to any combination of starting schedule and treatment constraint. Calculations are performed using the biologically effective dose (BED) as a tissue-specific comparison metric. The ratio of normal tissue BED to tumour BED is considered for PB regimens with varying total pulse numbers and/or "block-schemes". RESULTS: For matched brachytherapy duration, PB has a good "window of opportunity" relative to the existing RMH continuous low dose rate (CLDR) practice for all modelled repair half-times. The most clear-cut route to radiobiological optimisation of PB is via modest temporal extension of the PB regimen relative to the CLDR reference. This option may be practicable for those centres with scope to extend their relatively short CLDR treatment durations. CONCLUSION: Although daytime-only "block-scheme" PB for cervical cancer has not yet been employed clinically, the possibilities appear to be theoretically promising, providing the overall (external beam plus brachytherapy) treatment duration is not extended relative to current practice, such that additional tumour repopulation becomes a concern.

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 the improved methods implemented at many other anatomical sites, has emphasized the need for improved breast dosimetry. Any future technique delivering a superior three-dimensional dose distribution will be of maximum benefit if set-up errors are minimized. To determine the influence of vacuum moulded bag (vac-fix) immobilization on routine breast radiotherapy, 17 patients received half their radiotherapy fractions using our standard breast board technique and half using a vac-fix device positioned on the breast board. Treatment accuracy and reproducibility were assessed for each technique using daily electronic portal imaging and were analysed in terms of random and systematic translational and rotational displacements of treatment fields with respect to corresponding simulation images. In addition, patients completed a short questionnaire aimed at determining which technique they preferred. 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. Systematic errors showed some improvement with the vac-fix system. In the anteroposterior direction, approximately 80% of systematic errors were less than 4 mm for both techniques, but in the superoinferior direction the 80% point was reduced from 5.0 mm for the standard set-up to 2.7 mm for treatment in vac-fix. For rotational systematic errors, the corresponding value dropped from 1.8 degrees for the standard set-up to 1.1 degrees in vac-fix. Therefore, for many patients, additional use of a vac-fix device improved the transfer of the planned set-up from simulator to treatment unit. Additionally, answers to the questionnaire indicated that patients generally favoured the vac-fix system over use of the breast board alone. In conclusion, however, introduction of vac-fix immobilization for all patients was not thought justified as the improvements demonstrated are not likely to be clinically significant with the present treatment technique.