Copenhagen, Denmark
Onsite/Online

ESTRO 2022

Session Item

Implementation of new technology and techniques
7002
Poster (digital)
Physics
Correction of target shape changes in bladder cancer patients using online adaptive radiotherapy
Lisanne Zwart, The Netherlands
PO-1673

Abstract

Correction of target shape changes in bladder cancer patients using online adaptive radiotherapy
Authors:

Lisanne Zwart1, Liselotte ten Asbroek1, Erik van Dieren1, Judith Dasselaar1, Francisca Ong1

1Medisch Spectrum Twente, Radiotherapy, Enschede, The Netherlands

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Purpose or Objective

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.

Material and Methods

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.

Results

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.

Conclusion

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.