Biological effect of different IMRT delivery techniques: SMLC, DMLC, and helical tomotherapy.
PURPOSE
Intensity-modulated radiotherapy (IMRT) is delivered using a variety of techniques with differing temporal dose characteristics. Spatial dose metrics are generally used to evaluate treatment plan quality. However, the use of this information alone neglects the effects of the significant differences in dose delivery duration and dose accumulation patterns, both of which can impact cell survival. This study uses the linear-quadratic model with dose protraction corrections to evaluate the biological effectiveness of different IMRT delivery techniques, including fixed gantry IMRT in SMLC (step-and-shoot) and DMLC (sliding window) modes and a rotational IMRT technique (helical tomotherapy) for the treatment of prostate and head/neck sites.


METHODS
The temporal dose pattern was measured using a small volume ion chamber (A1SL--0.057 cm3) to calculate the protraction factor, and biological equivalent dose (BED) was calculated for a range of repair half-times and alpha/beta ratios. The treatment BED is compared to an ideal delivery of the target prescription dose, in which dose is delivered instantaneously (G(t0) = 1), to evaluate loss in biological effectiveness due to protraction in delivery. In the case of a conventional prescription, the loss in biological effectiveness was further evaluated using published tumor control probability (TCP) data.


RESULTS
With SMLC and DMLC IMRT delivery, for both prostate and head/neck, the expected additional loss in BED is about 1% compared to 3D CRT, which corresponds to a predicted 2%-3% reduction in TCP. For tomotherapy, the prostate BED loss is smaller in comparison to 3D CRT; hence, the authors expect a TCP increase of the order of 2%-3%. The aforementioned differences are due to the dose accumulation time.


CONCLUSIONS
While it is theoretically possible to compensate for changes in biologically effective dose, this would be hindered by large uncertainties in parameters used for such calculations; therefore, it is advantageous to irradiate target volume elements as rapidly as possible. The results of this study indicate that temporal dose delivery pattern is an important component in determining the biological effects of IMRT treatment.