The invention relates to an interstitial ultrasound ablation device (5) for being inserted into tissue surrounding, for instance, a tumor. The interstitial ultrasound ablation device comprises an arrangement (7) of ultrasound units (17, 18) for a focal ablation treatment which is modifiable from a folded configuration, to be used while inserting the ultrasound ablation device into the tissue, to an unfolded configuration, to be used after the ultrasound ablation device has been inserted into the tissue, such that the unfolded arrangement of ultrasound units is next to the tumor. This ultrasound ablation device can be easily positioned close to the tumor, where the unfolded arrangement of ultrasound units provides a large ultrasound emission area allowing for a very good focusing of the ultrasound on the tumor. This can lead to a very effective ablation of the tumor with only very few or no unwanted side effects.