Summary:
This thesis presents a generalisation of a methodology which quantifies the
radiobiological consequences of dose gradient effects in brachytherapy
applications. The methodology uses the linear-quadratic (LQ) formulation to
identify an equivalent Biologically Effective Dose (BEDeq) which, if applied
uniformly to a specified tissue volume, would produce the same net cell
survival as that achieved by a given non-uniform brachytherapy application.
Multiplying Factors (MFs), which enable the equivalent BED for an enclosed
volume to be estimated from the BED calculated at the dose reference surface,
have been calculated and tabulated for both spherical and cylindrical geometry.
The main types of brachytherapy [high dose-rate (HDR), low dose-rate (LDR) and
pulsed (PB)] have been examined for a range of radiobiological
parameters/dimensions. Equivalent BEDs are consistently higher than the BEDs
calculated at the reference surface by an amount which depends on the treatment
prescription (magnitude of the prescribed dose) at the reference point. MFs are
closely related to the numerical BED values, irrespective of how the original
BED was attained (e.g. via HDR, LDR or PB). Thus, an average MF can be used for
a given prescribed BED as it will be largely independent of the assumed
radiobiological parameters (radiosensitivity and /) and standardised look-up
tables may be applicable to all types of brachytherapy treatment. This analysis
opens the way to more systematic approaches for correlating physical and
biological effect in several types of brachytherapy and for the improved
quantitative assessment and ranking of clinical treatments which involve a
brachytherapy component.
Keywords:
Brachytherapy, Radiobiological modelling, Dose gradient effects, Linear qudratic model
