Effects of Geometrical Uncertainties on Whole Breast Radiotherapy: A Comparison of Four Different Techniques

PURPOSE: The purpose of this study was to quantify the target coverage, homogeneity, and robustness of the dose distributions against geometrical uncertainties associated with four whole breast radiotherapy techniques. METHODS: The study was based on the planning-computed tomography-datasets of 20 patients who underwent whole breast radiotherapy. A total of four treatment plans (wedge, field-in-field [FIF], hybrid intensity-modulated radiotherapy [IMRT], and full IMRT) were created for each patient. The hybrid IMRT plans comprised two opposed tangential open beams plus two IMRT beams. Setup errors were simulated by moving the beam isocenters by 5 mm in the anterior or posterior direction. RESULTS: With the original plan, the wedge technique yielded a high volume receiving ≥107% of the prescription dose (V(107); 7.5%±4.2%), whereas the other three techniques yielded excellent target coverage and homogeneity. A 5 mm anterior displacement caused a large and significant increase in the V(107) (+5.2%±4.1%, p<0.01) with the FIF plan, but not with the hybrid IMRT (+0.4%±1.2%, p=0.11) or full IMRT (+0.7%±1.8%, p=0.10) plan. A 5-mm posterior displacement caused a large decrease in the V(95) with the hybrid IMRT (-2.5%±3.7%, p<0.01) and full IMRT (-4.3%±5.1%, p<0.01) plans, but not with the FIF plan (+0.1%±0.7%, p=0.74). The decrease in V(95) was significantly smaller with the hybrid IMRT plan than with the full IMRT plan (p<0.01). CONCLUSION: The FIF, hybrid IMRT, and full IMRT plans offered excellent target coverage and homogeneity. Hybrid IMRT provided better robustness against geometrical uncertainties than full IMRT, whereas FIF provided comparable robustness to that of hybrid IMRT.