Technical Note: Development of 3D‐printed breast phantoms for end‐to‐end testing of whole breast volumetric arc radiotherapy

End‐to‐end testing of a new breast radiotherapy technique preferably requires realistic phantom geometries, which is challenging to achieve using currently commercially available solutions. We have developed a series of three‐dimensional (3D)‐printed breast phantoms, with ionization chamber and radiochromic film inserts, which can be attached to a commercial anthropomorphic thorax phantom. A contoured left breast from a patient’s planning CT was mapped onto a CT of the CIRS E2E thorax phantom (CIRS Inc.) and cropped to fit the surface. Four versions of the breast were 3D printed, containing a cavity for an ionization chamber and slits for radiochromic film insertion in the three cardinal planes, respectively. The phantoms were fully compatible with surface scanning technology used for setup. The phantoms were validated using a whole‐breast volumetric modulated arc therapy protocol with a simultaneous integrated boost to the tumor bed (VMAT‐SIB). Six patient plans and one original plan on the breast phantom were verified with planar portal imaging, point dose, and film measurements in the MultiCube phantom and planar γ‐analysis using ArcCHECK diode array. Six patient plans were recalculated on the breast phantom (hybrid plans) and delivered with point dose and film measurements with 3% (local)/2 mm γ‐analysis. One complete end‐to‐end test on the breast phantom was performed. All plan quality verifications had point dose differences below 2.4% from the calculated dose and γ‐agreement scores (γAS) > 87.3% for film measurements in the MultiCube, portal dosimetry, and ArcCHECK. Point dose differences in the 3D‐printed phantoms were below 2.6% (median −1.4%, range −2.6%; 0.3%). Median γAS was 96.4% (range 80.1%–99.7%) for all film inserts. The proposed 3D‐printed attachable breast dosimetry phantoms have been shown to be a valuable tool for end‐to‐end testing of a new radiotherapy protocol. The workflow described in this report can aid users to create their own phantom‐specific breast 3D‐printed phantoms.