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

Abstract

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.

Radiation repair models for clinical application

“…exposure, but the same modelling principles can be applied to pulsed brachytherapy, 25,26,35 permanent implants, 32,33,36,37 and biologically targeted radiotherapy. [38][39][40][41] Some summary points are: (a) The fractional amount of SLD repair occurring in successive time intervals is not always constant-the repair taking place in successive time intervals may reduce with increasing time after irradiation, particularly in some normal tissues.…”

The implementation of a PDR 3D-guided gynaecological brachytherapy service in a UK centre

“…In response to this, risk assessments are produced for these patients to ensure nursing staff are made aware of any issues in order to reduce this risk as much as possible. Developments with biphasic radiobiological modelling are underway with a possible move towards a shorter ‘office hours’ schedule making use of an increased dose per hourly pulse to reduce the number of hours total treatment time 9 . This would involve an increase in dose rate to around 1·5 Gy/hour which is closer to the dose rate achieved from Caesium 137, however, due to the higher instantaneous dose rate of the PDR and the associated radiobiological uncertainties, care must be taken with implementation of the schedule.…”

“…Developments with biphasic radiobiological modelling are underway with a possible move towards a shorter 'office hours' schedule making use of an increased dose per hourly pulse to reduce the number of hours total treatment time. 9 This would involve an increase in dose rate to around 1?5 Gy/hour which is closer to the dose rate achieved from Caesium 137, however, due to the higher instantaneous dose rate of the PDR and the associated radiobiological uncertainties, care must be taken with implementation of the schedule. Daytime scheduling might be of benefit for these patients, enabling closer supervision of the treatment by brachytherapy staff, and reducing the total treatment time for this patient group.…”