Pre-operative partial breast irradiation may decrease the side effects and inconvenience of long course post-operative whole breast radiotherapy for patients with early breast cancer. Magnetic resonance guided radiotherapy using an integrated MR Linac could allow improved soft tissue visualization of the intact tumour and therefore allow improved accuracy and ultra hypofractionation in suitable patients. However, radiation delivered on an MR Linac, particularly the Elekta Unity, requires consideration of physical issues that are not relevant to a conventional linac. The electron return effect (ERE) can increase...

Plans were generated for a single 21Gy fraction using seven, nine and eleven field (step and shoot, 7MV flattening filter free) beam arrangements in the prone position on conventional planning CT scans. This work was a pilot studyprior to recruiting patients for the proposed MR Linac trial and received low risk ethics approval. Target volumes were generated: Gross tumour volume (GTV): visible tumour Clinical target volume (CTV): GTV + 15mm excluding skin by 5mm and pectoralis muscles Planning target volume (PTV): CTV + 3mm Organs...

Plans were generated for eight patients (four of each laterality). All of the final plans met the dosimetric criteria aims for the study (Table 1&2). [Fig 1][Fig 2] On average, the seven-field arrangement required more segments but less monitor units than the other two arrangements. It was also estimated to be the fastest to deliver (by the planning system), taking 634 seconds on average. PTV D98 coverage of 21Gy was achieved for all plans and maximum doses ranged from 125-130%. Conformity and gradient indices met...

It is feasible to generate and optimise plans that meet study protocol constraints for single fraction 21Gy pre-operative breast radiotherapy using the 7MV FFF step and shoot beam model for the MR Linac. We found that increasing the number of fields slightly increases the estimated treatment delivery time but can improve OAR dosimetry, particularly with lower doses away from the target. This trade off will need to be considered for clinical application of this technique. The specific effects of ERE and ESE will be addressed...

Charaghvandi RK, van Asselen B, Philippens ME, Verkooijen HM, van Gils CH, van Diest PJ, et al. Redefining radiotherapy for early-stage breast cancer with single dose ablative treatment: a study protocol. BMC Cancer. 2017;17(1):181. Raaijmakers AJ, Raaymakers BW, Lagendijk JJ. Integrating a MRI scanner with a 6 MV radiotherapy accelerator: dose increase at tissue-air interfaces in a lateral magnetic field due to returning electrons. Phys Med Biol. 2005;50(7):1363-76. Raaijmakers AJ, Raaymakers BW, van der Meer S, Lagendijk JJ. Integrating a MRI scanner with a 6...