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The microscopic tumour extension, encompassed in the clinical target volume (CTV), has been extracted from relatively small series of thoroughly investigated tumour resection specimen. As a rule of thumb, depending on the tumour site and the underlying histology, the CTV may range from 5 mm up to 2-3 cm. In times when the identical radiation treatment plan was delivered throughout the entire course of treatment, this CTV concept may have been adequate. However, with the ever increasing capabilities of anatomical and functional tumour response assessment during treatment, treatment plan adaptation is mainstream nowadays. Moreover, highly conformal photon and in particular particle treatment plans, the latter exploiting the steep dose gradients, are offered to patients in order to reduce the normal tissue complication probability. In these situations, the gross tumour volume (GTV) is derived from the per-treatment imaging and the margins for the planning target volume (PTV) are maintained. However, the behaviour of the microscopic tumour extension and thus the CTV is entirely unclear.

In this presentation, I will highlight the current status of the CTV as well as research projects aimed at unravelling the possible biological rationale for adapting the CTV during the course of treatment.