Evaluation of RayStation robust optimisation for superficial target coverage with setup variation in breast IMRT

When planning breast IMRT, the distance of the CTV from the patient external surface is often less than the PTV margin required, presenting difficulties for ensuring CTV coverage. Several techniques have been proposed to ensure coverage in this scenario, one of which is robust optimisation; a technique that simultaneously optimises a plan in multiple geometries representing the worst case setup error expected. A range of plans were created utilising opposed tangential beams and these differing planning techniques, and were delivered and computed at 5 and 10 mm offsets perpendicular to the beam axes. The accuracy of dose computation was verified with a scintillator and film, and the surface dose coverage was evaluated for each of the plans in the offset positions. When 10 mm robust optimisation was used the CTV minimum, maximum and mean dose at the 5 and 10 mm offset locations were all within 3 % of those at the no offset setup. Robust optimisation was found to be comparable to other established planning methods for ensuring coverage of the breast CTV with setup variations.