The dosimetric consequences of inter-fractional patient movement on three classes of intensity-modulated delivery techniques in breast radiotherapy

Background and purpose: A comparison between three classes of intensity-mod ulated delivery techniques was undertaken to examine the dosimetric consequ ences of using a multileaf collimator (MLC) reshaped on each imaged fractio n as opposed to compensators designed on the first day of treatment potenti ally giving a treatment technique whose accuracy is thus degraded by moveme nt. Materials and Methods: The effects of inter-fractional patient movement for a cohort of six breast patients were studied. Five treatment techniques we re evaluated, two using a compensator, two using multiple static fields (MS F) and one using a dynamic multileaf collimator (DMLC). The compensated tec hniques consisted of (i) the use of compensators designed on day 1 only and used each fraction thereafter and (ii) the use of a compensator redesigned for each imaged fraction. The two MSF techniques were (i) a four-field-com ponent design and (ii) a method where the fluence interval between the MLC field components was set so they were equivalent to the compensator ('quant ized' MSF-MLC). The final technique investigated was the DMLC. Plans were p roduced for each of the five methods and a paired t-test was used to assess the reduction in the breast volume outside the dose range 95-105% between sets of pairs of techniques. An on-line correction strategy was simulated t o determine the number of treatments that required intervention. The action levels were calculated using the difference between the volume outside the dose range 95-105% calculated for treatments where the DMLC was designed o n day 1 only and for each imaged Fraction. Differences of greater than 2%, greater than 5% and greater than 10% were investigated. Results: Thirty-five plans were evaluated for each technique. Results showe d that a statistically significant mean reduction in the volume of the brea st outside the dose range 95-105% could be achieved if the compensators wer e designed on each imaged fraction rather than on day 1 only (P = 0.0045). When the comparison was made between the 'quantized' MSF-MLC and the techni que where the compensators were designed on day 1 only, a statistically sig nificant mean reduction in the volume of the breast tissue outside the dose range 95-105% was not achieved (P = 0.21). Comparison of the DMLC techniqu e to the technique where the compensators were designed on day 1 only resul ts in a statistically significant mean reduction in the volume outside the dose range 95-105% (P = 0.024). This corresponds to a mean reduction in the volume outside 95-105% dose of 1.94%. The 2% action level showed the great est reduction in the volume outside 95-105% dose and intervention was only required in approximately one-third of the treatments investigated. Conclusions: Redesigning MSFs for each imaged fraction did not provide a st atistically significant mean reduction in the volume outside the dose range 95-105%. However, using the DMLC technique creates a statistically signifi cant mean reduction in the volume outside the dose range 95-105%. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.