ESTRO 2025 Congress Report I Teaching lecture

This teaching lecture, given by Dr Jenny Bertholet, a medical physicist from Switzerland, provided an overview of the evolution, current state, and future directions of image guidance in radiotherapy, with a focus on cone beam CT (CBCT), MR guidance, and surface-guided radiotherapy (SGRT).

Since the early 2000s, image guidance has become a significant area of focus in radiotherapy, with the integration of X-ray imaging, particularly kV and MV imaging, into treatment machines. CBCT has emerged as a key technology that provides high-quality 3D and 4D volumetric imaging, although it has introduced challenges such as increased radiation dose and lower image quality compared with diagnostic CT. Early guidance strategies, including offline protocols, were designed to correct for large, systematic errors. The introduction of daily online image-guided radiotherapy (IGRT) proved beneficial, particularly for prostate cancer, improving biochemical control and reducing toxicity.

While it is common to apply couch shifts in radiotherapy, there has been a noticeable change towards replanning to address anatomical changes that cannot be corrected by simple shifts, such as variations in body contour, weight loss, organ filling, and tumour response. Replanning or plan adaptation, whether on- or offline, involves the acquisition of images, assessment of the need for changes, and adjustment of the treatment plan accordingly. Online replanning, in particular, consolidates imaging, quality assurance, and treatment delivery into a single session, requires therefore advanced imaging and precise dose optimisation.

CBCT continues to be a widely used imaging modality as its use benefits from advances in hardware, such as larger detectors, better scatter grids, and faster gantry rotation. These improvements have led to enhanced image quality and reduced numbers of motion artefacts. However, CBCT faces challenges, particularly in soft-tissue contrast in areas such as the abdomen and pelvis, and CBCT-based online replanning workflows  are resource-intensive.

MR imaging, known for its superior soft-tissue definition, offers promising potential in radiotherapy. However, the integration of MR with treatment delivery systems presents significant challenges due to the interaction between magnetic fields and treatment machines. Some MR-guided systems are already in clinical use, . offering high-quality diagnostic images but also facing challenges such as coil technology limitations, longer adaptation times, and high cost.

SGRT uses light patterns projected onto the patient and cameras to monitor surface motion. While it does not replace traditional imaging, SGRT offers advantages for patient setup and monitoring, including tattoo-less and mask-less setups, which are particularly beneficial for certain cases, such as paediatric patients.

Future developments in image guidance are likely to be focused on the incorporation of functional and biological imaging, including technologies such as PET, MR, thermal, and Cherenkov imaging. There is also a push for integration with proton therapy and advancements in treatment delivery methods, such as upright therapy. Automation, artificial intelligence, and improved workflows will play crucial roles in streamlining the image-planning and quality assurance processes, and hence enable more efficient adaptive replanning. In conclusion, while CBCT and MR guidance remain central to current radiotherapy practices, the future will probably see more personalised, functional, and biologically informed treatment strategies, along with advancements in technology and emerging treatment modalities.

Dr Vania Batista,

Medical physicist

Heidelberg University Hospital, Germany