Reduction of the planning target volume with daily online adaptive radiotherapy in bladder cancer

External radiation therapy for bladder cancer requires large planning target volumes (PTVs) due to the daily anatomy of the bladder. Online adaptive radiotherapy (oART) can reduce the PTV by considering daily anatomical changes.

We performed oART in 8 patients with muscle-invasive bladder cancer between June 10, 2022, and April 14, 2023, on an Ethos linear accelerator (Varian, Palo Alto, USA). Using the 496 cone-beam computed tomography (CBCT) images of the fractions, we retrospectively compared the differences in volumetric changes between oART and image-guided and intensity-modulated radiotherapy (IGRT/IMRT). According to our local protocol, for oART, a patient-specific PTV margin was created based on the intrafractional clinical target volume (CTV) changes observed during the first three fractions.

The average duration of treatment was 14.8 min (range 7-49 min). The average volume of the PTV with oART and IGRT/IMRT was 296.8 cm³ (range 114.5-810.4 cm³) and 416.5 cm³ (range 188.2-991.3 cm³), respectively, representing a 30% reduction with oART. This new technique resulted in an average reduction of 43.9% in the volume of unnecessarily irradiated healthy tissues. Geometrical miss of the CTV occurred in 13 fractions with IGRT/IMRT, with an average of 9.4 cm³ of missed volume (range 0.4-56.4 cm³, standard deviation [SD] 15.73), for oART in 7 fractions, with an average missed volume of 4 cm³ (range 0.4-21.2 cm³, SD: 7.6).

The use of patient-specific margins in oART allows for reduction of the PTV and dose to healthy tissues while achieving equal or better target coverage.