CBCT (Cone-Beam CT)-Based Online Adaptive Radiotherapy in the Post-operative Oesophageal Cancer Patient.

We present a case of a patient having cone-beam CT (CBCT)-based online adaptive radiotherapy (oART) on Ethos Therapy after oesophagectomy and gastric pull-up. This case report aims to demonstrate that daily oART is a viable treatment option for post-oesophagectomy patients. The patient's radiotherapy plan was generated on the Ethos system using an eight-field intensity-modulated radiation therapy (IMRT) plan imported from the Eclipse planning system. The patient was prescribed 50.4Gy/28# over 5.5 weeks with concurrent carboplatin and paclitaxel. There were no changes to our department's standard clinical tumour volume (CTV) and planning tumour volume (PTV) margins due to the novel nature of the case. For each fraction of daily oART, the heart, right and left lungs, oesophagus and trachea were Ethos-contoured 'Influencer' structures which were reviewed and edited at the console. Minor edits were required to each structure. The adaptive plan was chosen for each fraction due to showing clear dosimetric benefits to both PTV coverage and organ at risk (OAR) doses. Lung doses were significantly reduced with the course mean lung dose reduction of 8.1% and the Lung-CTV V20 reduction of 4.4%. The treatment duration from the start of CBCT acquisition to the end of beam delivery averaged 19 minutes and 12 seconds. The patient was able to tolerate the time on the bed without incident within the scheduled 30-minute appointment slot. Our case demonstrates that significant inter-fraction anatomy changes occur in patients having radiotherapy after oesophagectomy. Without the use of adaption, we have seen that there would be clinically significant undercoverage of target volumes due to sub-optimal dose distribution. We plan to evaluate the imaging available in Ethos dose monitoring to establish if these margins could be reduced in patients treated with online adaptive radiotherapy.