Planning target volume (PTV) expansions for lumpectomy cavity boost and partial breast irradiation can be large. In this study, 44 patients received lumpectomy boost using MR-guidance with cine images during treatment delivery. The mean maximum anterior/posterior and superior/inferior intrafraction cavity motion was 1.4 mm and 1.2 mm, respectively. During >90% of intrafraction cine images, cavity motion was <5 mm, suggesting PTV margins as small as 5 mm are adequate when using 3D cross-sectional image guidance. What’s less certain is alignment prior to beam on, however, so traditional PTV margins should probably be larger if utilizing orthogonal or portal imaging. | Merfeld, Pract Radiat Oncol 2022


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