Dosimetric evaluation of compensation in radiotherapy of the breast: MLC intensity modulation and physical compensators

Background and purpose: Electronic portal images may be used to design the compensation required to maximise dose uniformity in the breast from opposed tangential beams. Materials and methods: Four methods of implementing the desired compensation have been studied: a simple wedge, a physical compensator in conjunction with a wedge; one open field plus four shaped multi-leaf-collimated (MLC) fields, and one wedged field in conjunction with three shaped MLC fields. Evaluation was performed using thermoluminescent dosimeters (TLDs) placed inside a phantom which was designed to mimic the human breast. The measured results are compared with both the prediction of the in-house compensation design software and with the dose predicted by the GE Target II planning system. The implications of each method for the time taken to plan and deliver treatment were analysed. Results: The dose inhomogeneity, as measured at seven points in the central plane was greatest for the simple wedge (root mean square (rms) = 4.5%) compared to an open field plus four shaped MLC fields (rms = 2.2%), a wedged field plus three shaped MLC fields (rms = 3.3%), and the physical compensator (rms = 2.4%). The times required to plan and prepare these treatments varied considerably. The standard wedged treatment required under 15 min; both MLC-based and the physical compensator treatments required = 50 min. Differences of treatment delivery times were up to 8 min. Conclusions: These results indicate that the dose inhomogeneity can be reduced by beam intensity modulation designed using EPIDs