Session Item

In current clinical practice CTV-PTV margins of 1.5-2.0 cm are used for image-guided radiotherapy (IGRT) for bladder cancer patients to compensate for inter-fractional variations of bladder filling. This relatively large margin results in a substantial dose to the surrounding organs at risk (OARs). CBCT-guided online adaptive radiotherapy (oART) can be used to correct for inter-fractional bladder filling variations, resulting ultimately in a treatment with smaller CTV-PTV margins. The aim of this study was to analyze the value of oART for bladder cancer patients in terms of target coverage and bladder filling variations.

Between July and October 2021, three bladder cancer patients were clinically treated with CBCT-guided oART using Ethos therapy (Varian Medical Systems, Palo Alto, CA, US). Two patients were treated with radiotherapy alone using a fractionation scheme of 20×2.75 Gy. One patient was treated with chemoradiotherapy using a fractionation scheme of 33×2 Gy. For all patients a reference 9- or 12-field IMRT plan was created on an empty bladder planning CT, applying a CTV-PTV margin of 5 mm in lateral and 8 mm in superior-inferior and anterior-posterior direction. During the adaptive workflow on couch, two treatment plans were created based on the OAR and CTV contours on the CBCT. The scheduled plan is the recalculated reference plan and the adapted plan is the re-optimized reference plan on the daily CBCT anatomy. Both plans were compared based on coverage of the PTV (V95%≥99%). Moreover, volume differences of the PTV on the daily CBCT relative to the planning CT were analyzed.

Volume differences of the PTV on the daily CBCT relative to the planning CT and dose differences between adapted and scheduled plans are plotted in Figure 1. The volume of the PTV on the daily CBCT was smaller compared to the planning CT in 51/73 fractions (69.9%). Even so, the adapted plan was chosen in all fractions because of increased coverage of the PTV (V95%) with respect to the scheduled plan. The adapted plan showed a coverage of ≥99% in all cases, whereas for the scheduled plan this was only in 2/73 cases, with a mean coverage of 94.6%±3.3% (range 84.3% – 99.1%). Figure 2 shows the CBCTs of two separate fractions in which the volume of both PTVs were comparable, whereas the shape of the PTVs was completely different.

Target coverage was superior in the adapted plans as compared to the scheduled plans, even in cases with decreased target volume when the scheduled plan would be deemed sufficient. Therefore shape changes of the target volume play an important role, which cannot be corrected for using an IGRT or plan of the day approach. Further analysis with more patients is ongoing.