Pelvic and Thoracic Cancer Treated With Online Adapted Radiotherapy: A Prospective Registry-based Phase-II Trial

Background: Conventional Radiotherapy (Image Guided Radiotherapy, IGRT) requires a CT-based treatment planning process a priori. During this process, a treatment plan is calculated, which then is applied to the patient with a linear accelerator on a daily basis, possibly using image guidance to account for variability in patient position. However, daily changes of the anatomy of targets and organs at risk (OARs) can only be addressed by applying additional safety margins, resulting in larger irradiated volumes and possibly higher toxicity.

A promising and innovative technique for margin and in consequence toxicity reduction is online Adaptive Radiotherapy (oART) using daily imaging to create a "plan of the day" aligned to the actual anatomy by means of artificial intelligence (AI) and with the patient on the treatment couch. This approach is especially promising in the pelvic region due to the high anatomic variability, e.g. caused by peristalsis or volume changes of bladder and rectum. Furthermore, the application of oART in the field of thoracic tumors is being evaluated. oART is an already established form of treatment which is used regulary everyday in clinical practice and is not a study-related intervention. A direct comparison of both techniques IGRT vs. oART is missing.

Methods: This prospective and retrospective registry-based trial will include patients with pelvic or thoracic tumors with an indication for radiotherapy treated with oART or IGRT. For the primary endpoint and the secondary clinical endpoints, the trial will compare oART versus IGRT, for technical endpoints the trial will compare the real oART scenario with two virtual (hypothetical) control scenarios as follows:

• oART - Applied treatment: AI-based daily treatment plan creation, adapted to daily anatomy
• Virt 1 - Hypothetical dose distribution of initial treatment plan, based on the initial planning CT
• Virt 2 - Hypothetical dose distribution of initial treatment plan, applied to the anatomy of the day as depicted by cone-beam CT at the treatment machine