An observational study of post-operative volumetric modulated arc radiotherapy (VMAT) of breast cancer by immobilizing patients on All-in-One (AIO™) solution breast and lung board without using thermoplastic mask: risk of collision of gantry with couch exists if acquisition of Cone Beam Computed Tomography (CBCT) is attempted for set-up verification

Background Automatic cone beam computed tomography (CBCT)-based image matching for set-up verification is recommended as compared with 2D image match for post-operative local/loco-regional radiotherapy of breast cancer patients by volumetric modulated arc therapy (VMAT) technique. However, in a supine position, off-midline peripheral body clinical target volume (CTV) of unilateral breast cancer patients immobilized on breast and lung board of All-in-One (AIO) positioning system may necessitate augmented movement of the couch in 'x' and 'z' axes thereby raising the risk of collision of x-ray sources/detector system with couch. Methods and material VMAT was planned by a pair of partial arcs for the whole target volume for seven consecutive post-operative breast cancer patients (five post-mastectomy and two post-breast conservation patients). Tattoo-based set-up by the shift of treatment table inx-,y-, andz-axis as determined by treatment planning system followed by x-rays with planar image acquisition and online 2D image matching with Digitally reconstructed radiographs (DRRs) was performed for set-up verification. In-room 360 degrees rotation of x-ray source and detector system of a linear accelerator (linac) was performed before x-ray planar image acquisition. Result Completion of 360 degrees in-room rotation of x-ray source and detector system of linac around the machine isocentre was not possible in six out of seven patients due to the possibility of collision of the gantry with contralateral side of the couch. Conclusion Performing CBCT for generating 3D images for computed tomography (CT) reconstruction may not be practical for patient set-up verification of post-operative radiotherapy of unilateral breast cancer patients positioned supine on breast and lung board.