Background and purpose We analysed the positional and structural stability of a long-term biodegradable liquid fiducial marker (BioXmark) for radiotherapy in patients with locally advanced lung cancer. Material and methods Markers were injected via endoscopic- or endobronchial ultrasound in lymph nodes and reachable primary tumours. Marker volume and Hounsfield Units (HU) changing rates were estimated using breath-hold CBCT. Inter-fraction variation in marker position relative to gross tumour volume (GTV) position was established, as well as the inter-fraction variation in mediastinal marker registration relative to a carina registration through the treatment. Results Fifteen patients were included and 29 markers analysed. All markers that were in situ at planning were visible through the treatment. Mean HU was 902 ± 165 HU for lymph node and 991 ± 219 HU for tumour markers. Volume degradation rates were −5% in lymph nodes and −23% in primary tumours. Three-dimensional inter-fraction variation for marker position relative to the GTV position was −0.1 ± 0.7 mm in lymph nodes and −1.5 ± 2.3 mm in primary tumours. Inter-fraction variations in marker registration relative to carina registration were −0.4 ± 1.2 mm in left–right, 0.2 ± 2.0 mm in anterior–posterior and −0.5 ± 2.0 mm in cranio-caudal directions. Conclusions The liquid fiducial markers were visible and stable in size and position throughout the treatment course.