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 f rom opposed tangential beams. Materials and methods: Four methods of i mplementing the desired compensation have been studied: a simple wedge , a physical compensator in conjunction with a wedge; one open field p lus four shaped multi-leaf-collimated (MLC) fields, and one wedged fie ld in conjunction with three shaped MLC fields. Evaluation was perform ed using thermoluminescent dosimeters (TLDs) placed inside a phantom w hich 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 syst em. The implications of each method for the time taken to plan and del iver treatment were analysed. Results: The dose inhomogeneity, as meas ured 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 f our shaped MLC fields (rms = 2.2%), a wedged field plus three shaped M LC fields (rms = 3.3%), and the physical compensator (rms = 2.4%). The times required to plan and prepare these treatments varied considerab ly. The standard wedged treatment required under 15 min; both MLC-base d and the physical compensator treatments required approximate to 50 m in. Differences of treatment delivery times were up to 8 min. Conclusi ons: These results indicate that the dose inhomogeneity can be reduced by beam intensity modulation designed using EPIDs. (C) 1997 Elsevier Science Ireland.