On board imaging system

For each treatment, a single isocentric field setup was used: WBRT with two tangential half‐beams, and WBRT‐LN either with a four‐field oblique parasternal photon technique⁽¹⁶⁾ or a three‐field partly wide tangential technique.⁽¹⁷⁾ The latter techniques share the use of two tangential breast quarter‐beams and a nearly anterior half‐beam for MS LN irradiation (i.e., the MS‐field). Irrespective of the field setup, the PTV margins prescribed in the EORTC 22922‐10925 trial⁽¹⁾ were applied (i.e., 10 mm around the breast and 5 mm around the LNs). The treatments were delivered on a Varian Clinac 2100C/D or a Varian TrueBeam linear accelerator, both equipped with an amorphous silicon electronic portal image detector (EPID), aSi1000, and an On‐Board Imaging system (Varian Medical Systems, Inc., Palo Alto, CA).