Endorsed by ESTRO

15-18 March 2026, Ascona, Switzerland

This workshop has been set up to focus on the development of quantitative methods taken from statistics, artificial intelligence, mathematical modelling, and medical image processing to support and improve the definitions of CTVs.

The use of intensity-modulated and image-guided radiotherapy allows us to deliver radiation precisely to tumours and to conform the dose distribution to target volumes that are of complex shapes. Nowadays, the largest uncertainty is often the definition of the target volume. Radiotherapy planning distinguishes three target volumes:

• the gross tumour volume (GTV), which comprises the macroscopic tumour mass;
• the clinical target volume (CTV), which contains microscopic extensions of the tumour; and
• the planning target volume (PTV), which compensates for geometric uncertainty.

The definition of the GTV has been improved with the development of biomedical imaging techniques such as novel PET tracers and functional MRI. PTV margins have been reduced through the application of modern image guidance. Arguably, the definition of the CTV has made the least progress. GTV definition amounts to delineating an abnormal tumour mass that is visible in images. In contrast, the CTV is invisible in the sense that it consists of tissue that appears normal in function and that may or may not be infiltrated by the tumour. Therefore, the CTV definition is conceptually different from that of the GTV and is based on knowledge of the routes of tumour progression, which are anatomically defined.

In 2019, a think tank of the ESTRO physics committee identified four Grand Challenges for medical physicists in radiation oncology. One of them was to improve CTV definition [1]. Subsequently, the working group named "Target definition transformed from an art to a science", which is led by Jan Unkelbach (University of Zurich, Switzerland) and Vincent Grégoire (Léon Bérard Cancer Centre, Lyon, France), was formed [2].

The March workshop will bring together physicists, computer scientists and clinical radiation oncologists. It will offer to researchers working on CTV definition the opportunity to join the ESTRO working group. The format will be similar to that of ESTRO physics workshops, with most of the time allocated for discussion.

If you are interested in attending the workshop, please send a one-page motivational statement to ctv.workshop.2026@usz.ch, outlining your CTV-related research and providing a reference to prior work.

The deadline for applications is 15 November 2025. Applicants will be informed by 30 November.

Workshop dates: 15-18 March 2026

Venue: Monte Verità Conference Centre, Ascona, Switzerland

Organisers: Professors Unkelbach, Grégoire, Thomas Bortfeld (Massachusetts General Hospital and Harvard Medical School, USA) and Jennifer Dhont (Institut Jules Bordet, Brussels, Belgium)

Further information: https://www.physik.uzh.ch/en/groups/unkelbach/events.html

[1] https://doi.org/10.1016/j.radonc.2020.10.001

[2] https://doi.org/10.1016/j.radonc.2020.10.002