The purpose of this study is to quantify the variation in tumour middle position during a course of stereotactic body radiotherapy (SBRT), and thereby be able to design radiotherapy margins that takes into account the full motion span throughout an entire course of SBRT.

Lung tumours move with respiration. This must be considered when designing margins for radiotherapy. This movement can be quantified by fluoroscopy or 4DCT. It is possible to identify a tumour middle position for planning. This middle position will vary from day to day and the extent of this variation is not fully known. The aim of this study is to quantify the variation in tumour middle position during a course of stereotactic body radiotherapy (SBRT), and thereby be able to design margins for patients that take into account the full motion span throughout an entire course of SBRT Patients: 15 consecutive patients with inoperable low stage lung cancer or solitary metastases to the lung (1-2) referred for SBRT - 45 Gy/3 fractions.

Methods: A gold coil will be implanted into the lung tumour one week before the planning. At planning and all treatment days supplementary 4DCT of thorax and two orthogonal fluoroscopy sessions will be performed. Tumour motion in the superior-inferior, medio-lateral and cranio-caudal direction will be measured and variation in amplitude and baseline for the tumour motion will be reported.

Perspective: By examining the variation in tumour movement it will be possible design margins for SBRT, accounting for the full tumour motion span and minimizing the risk of geographical miss and thereby optimizing the chance for local tumour control.

Inclusion Criteria:

• tumor > 6 cm
• no more than 2 tumours
• histological proven non small celled lung cancer
• signed Informed Consent

Exclusion Criteria:

• Serious bleeding disorder
• Performance status 3-4
• Tumour close to large vessels (judged by interventional radiologist)