The study aims to develop an improved method to tailor the geometric safety margin when treating with stereotactic body radiotherapy (SBRT), to avoid tumour cells being missed during irradiation and to limit the exposure of normal tissues as much as possible. Dynamic magnetic resonance imaging (MRI) is assumed to represent the tumour motion during the respiratory cycle more realistically, compared to 4-dimensional computed tomography (4D CT), thanks to its high temporal resolution and the possibility to image the motion over a longer period of time. Patients receiving SBRT for a pulmonary target located below the level of the carina are included in the study, and the hypothesis will be tested through a comparison of the tumour amplitude measured with each modality. Furthermore, the predictive value of the image series acquired at treatment planning, for the motion patterns observed at each treatment fraction, will be evaluated through repeated 4D CBCT acquisitions.

In the current study, a 2D-cine MRI examination will be performed, in addition to the 4D CT examination that is standard for patients receiving radiotherapy for tumours in the thorax or abdomen. A subgroup of the study participants (referred to as the 4D CBCT subcohort) will also be required to perform a 4D CBCT examination at three treatment sessions. The main cohort consists of 95 subjects and the 4D CBCT subcohort of 55 subjects. The study does not involve any change in the treatment routine; rather, the study images are collected for retrospective analysis. Following each imaging session, the subject should fill out an anxiety questionnaire and their pulse will be measured, to give an indication of any anxiety which might influence the breathing pattern of the current session.

Clinical follow-up will be performed retrospectively by reviewing the participants' medical records. Progress, health status and treatment-related toxicity will be evaluated, up to 10 years after completed treatment.

The study objectives are:

• To evaluate dynamic MRI for characterising the respiratory-related tumour motion, compared to the current standard modality of 4D CT.
• To evaluate the variation in breathing motion at different times points.
• To evaluate the clinical outcome.

Inclusion Criteria:

• Accepted for SBRT treatment of one or more caudally situated lung targets (NSCLC or metastases), i.e. below the carina.
• Age ≥ 18 years
• Adequate clinical condition to maintain immobilised position for at least 30 minutes
• Adequate Swedish skills and cognitive function to understand study information and to fill in the questionnaire

Exclusion Criteria:

• Pregnancy or breastfeeding, or planned pregnancy or breastfeeding, during treatment
• Contraindication for MRI examination
• Otherwise incapable of participating in study