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

Abstract

Background and purpose: A comparison between three classes of intensity-modulated delivery techniques was undertaken to examine the dosimetric consequences of using a multileaf collimator (MLC) reshaped on each imaged fraction as opposed to compensators designed on the first day of treatment potentially giving a treatment technique whose accuracy is thus degraded by movement.Materials and methods: The effects of inter-fractional patient movement for a cohort of six breast patients were studied. Five treatment techniques were evaluated, two using a compensator, two using multiple static fields (MSF) and one using a dynamic multileaf collimator (DMLC). The compensated techniques 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-component design and (ii) a method where the fluence interval between the MLC field components was set so they were equivalent to the compensator (‘quantized’ MSF-MLC). The final technique investigated was the DMLC. Plans were produced 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 to 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 on 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 showed that a statistically significant mean reduction in the volume of the breast outside the dose range 95–105% could be achieved if the compensators were 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 technique where the compensators were designed on day 1 only, a statistically significant 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 technique to the technique where the compensators were designed on day 1 only results 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 greatest 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 statistically significant mean reduction in the volume outside the dose range 95–105%. However, using the DMLC technique creates a statistically significant mean reduction in the volume outside the dose range 95–105%.