With online-adaptive radiotherapy (ART), the radiotherapy plan can be adapted to the anatomical changes before each radiotherapy fraction. The basis for adaptation is a cone-beam CT with the patient on the treatment table at the start of each radiotherapy fraction. Adaptive radiotherapy is a further development of image-guided radiotherapy (IGRT), which is now the basic standard in radiotherapy. Here, a cone beam CT is also performed at the start of each fraction, but this is only used to position the patient in relation to the radiation field arrangement of the initial radiation plan approved for the series, so that predefined landmarks match the target position in the planning CT as closely as possible. However, the radiotherapy plan itself cannot be adapted to deformations in the body during IGRT, e.g. due to different bowel and bladder fillings. IGRT is currently carried out on all linear accelerators at the radiotherapy clinic. For online adaptive radiotherapy (ART), which allows the radiotherapy plan to be adapted to the anatomical deformities of the day online on the treatment table, the ETHOS® radiotherapy system from Varian-Siemens has been available at the radiotherapy clinic for a year as a medical device with handling authorisation from the district government. This therapy device enables on-line adaptive plan adjustment within 10-20 minutes.

In patients with lung carcinoma who receive radiotherapy preoperatively to reduce the size of the tumour before surgery or alternatively curative, high-dose radiotherapy instead of surgery, there are also deformations of the thorax from fraction to fraction. For example, the depth of breathing during deep inspiration radiotherapy can change depending on the shape of the day. Ventilation disturbances caused by the tumour may increase or decrease, pleural effusions may occur or tumour shrinkage may occur. Furthermore, the strength of the various respiratory muscles can vary from day to day, as can the intra-abdominal pressure.

Currently, ART for lung carcinomas is carried out as part of patient care in the radiotherapy clinic at the University Hospital Essen. Patients are irradiated during repeated breath-holding manoeuvres in deep inspiration. The pure irradiation time takes about 1.5 minutes. The breath-holding manoeuvre is practised several times before the daily irradiation. An optical surface detection device is available for this purpose, which is used to compare the body surface in the patient's thorax area on the treatment table with the target surface to be achieved during radiotherapy planning, which took place days before the current fraction. If the target surface is reproducibly achieved, a conebeam CT is taken to determine the current tumour extent and the deformations occurring in comparison to the dose distribution to be applied. For this purpose, the normal tissues heart, lung and tracheal tree are automatically contoured by the Ethos therapy system in the cone-beam CT. A specialist physician and a physicist are present at the Ethos device during this process. These organ contours are used to determine the extent of the current target volume and the adaptive radiotherapy plan is adjusted to the new target volume. If the current tumour extent is not sufficiently covered by the initial radiotherapy plan planned for the series with its planned tolerances, or if the surrounding normal tissue is exposed to too much radiation, then the online adaptive radiotherapy plan for the current treatment is selected by the specialist. Otherwise, the initial radiotherapy plan is used by the planning CT as for IGRT.

The aim of this study is to treat 30 patients in ART readiness on the Ethos therapy device over 10 radiotherapy fractions. The dose distribution is always compared with the adaptive and the initial series plan on the anatomy of the day by the specialist and medical physics expert. The ART plan is then applied with dosimetric advantages. The aim of this study is to prospectively analyse in how many patients safety margins around the clinical target volume can be reduced with ART compared to IGRT. After the first 7 of the 10 fractions to be administered in ART readiness, the dose is accumulated over these fractions. The percentage of patients who benefit from ART will be determined prospectively. The criterion for a relevant benefit from ART is the effective uniform dose (EUD) in the target volume with the accumulated dose distribution across the 7 fractions. If the EUD is below 95% of the target dose with IGRT and above 95% of the target dose with ART, then an advantage is determined with ART. Such dose accumulations accompanying a radiotherapy series were not possible in the past. Patients with a benefit from ART receive ART for the further fractions of the series, while the others continue to be treated with IGRT.