Before CT-planning, radio-opaque wire was used to delineate clinically palpable breast tissue with the patient in Angiotensin 1/2 (1-6) supine position. The supine VBH CT-planning procedure has been described previously . For free-breathing prone CT-planning, patients were positioned on an Orfit AIO Solution® prone breast board (ORFIT Industries, Wijnegem, Belgium) (see Fig. 1). A marker (tattoo) was placed ipsilaterally in the posterior axillary line and aligned axially with a posterior midline marker using lateral lasers. A second posterior marker was placed 15 cm inferiorly to the primary posterior marker, in line with sagittal lasers. CT data (Philips Medical Systems, UK) were acquired without contrast for both scans (2 mm slices, C6 to below diaphragm). Both scans were performed in one CT-planning session, with patients dismounting the couch between scans. Photographs of patient positioning were taken for both techniques to aid treatment setup. The time taken to complete each CT was recorded, from the time the patient mounted the CT couch to the time at which they dismounted the couch. After completing both scans, patients and radiographers completed validated questionnaires to assess comfort and satisfaction respectively (see Figs. S1 and S2) .