Comparison of Library of Plans with two daily adaptive strategies for whole bladder radiotherapy

Abstract

Background and purpose: Whole bladder radiotherapy is challenging due to inter- and intrafraction size and shape changes. To account for these changes, currently a Library of Plans (LoP) technique is often applied, but daily adaptive radiotherapy is also increasingly becoming available. The aim of this study was to compare LoP with two magnetic resonance imaging guided radiotherapy (MRgRT) strategies by comparing target coverage and volume of healthy tissue inside the planning target volume (PTV) for whole bladder treatments.

Methods and materials: Data from 25 MRgRT lymph node oligometastases treatments (125 fractions) were used, with three MRI scans acquired at each fraction at 0, 15 and 30 min. Bladders were delineated and used to evaluate three strategies: 1) LoP with two plans for a 15 min fraction, 2) MRgRT_15min for a 15 min fraction and 3) MRgRT_30min for a 30 min fraction. The volumes of healthy tissue inside and bladder outside the PTV were analyzed on the simulated post-treatment images.

Results: MRgRT_30min had 120% and 121% more healthy tissue inside the PTV than LoP and MRgRT_15min. For LoP slightly more target outside the PTV was found than for MRgRT_30min and MRgRT_15min, with median 0% (range 0-23%) compared to 0% (0-20%) and 0% (0-10%), respectively.

Conclusions: Taking into account both target coverage and volume of healthy tissue inside the PTV, MRgRT_15min performed better than LoP and MRgRT_30min for whole bladder treatments. A 15 min daily adaptive radiotherapy workflow is needed to potentially benefit from replanning compared to LoP.

Keywords: Bladder cancer; Daily online adaptive radiotherapy; Library of Plans; MR-guided radiotherapy.

Fig. 1

Illustration of the LoP strategy and the two MRgRT strategies (MRgRT_30min and MRgRT_15min). 1) illustrates the LoP strategy. a) The bladder was delineated on the planning CT and two PTVs were constructed (PTV LoP Tight and PTV LoP Wide). b) Based on Bladder₁ the most suitable PTV was selected (in this case PTV LoP Wide). c) MRI₂ was considered the simulated post-treatment image. Evaluation of PTV LoP Selected was done with Bladder₂. 2) illustrates the MRgRT_30min workflow. d) A PTV was constructed from Bladder₁ with population-based margins. e) This PTV was evaluated with the MRI₃ as simulated post-treatment image using Bladder₃. 3) illustrates the MRgRT_15min workflow. f) Similar to the MRgRT_30min, a PTV was constructed from Bladder₁. g) Evaluation was now done with MRI₂ as simulated post-treatment image using Bladder₂.

Fig. 2

Simulated volume of healthy tissue inside PTV and missed target volume in post-treatment image (MRI₂ for LoP and MRgRT_15min, MRI₃ for MRgRT_30min) for whole bladder treatment using three treatment strategies MRgRT_30min, MRgRT_15min and LoP based on 25 treatments of 5 fractions. Data shown for 107 fractions (18 were excluded due to too full or too empty bladder filling for LoP). ** indicates significance of p < 0.01, by two-tailed Wilcoxon matched-pairs signed-rank test a) The volume of healthy tissue inside the PTV in the post-treatment image. b) The volume of bladder outside the PTV in the post-treatment image. c) Relative volumes of the bladder (in %) outside the PTV in the post-treatment image.

Fig. 3

Two examples shown in the sagittal plane, illustrating exceptions; one or more of the strategies led to part of the bladder being outside the PTV in the simulated post-treatment image. Time of the image indicated in top right corner. a–d) Example 1. Planning CT (a) and T1 weighted images (b–d). The selected LoP PTV (yellow) was sufficient in MRI₁, but missed a part (29 cm³) of the bladder in MRI₂ (indicated by the yellow arrow in c). (e–h) Example 2. Planning CT (a) and T2 weighted images (f–h). The PTVs of all three strategies missed a part of the bladder in the simulated post-treatment MRI. For LoP and MRgRT_15min: 17 cm³ and 28 cm³ of the Bladder₂ was outside the PTV respectively (indicated by the yellow arrow in g). For the MRgRT_30min workflow: 100 cm³ of the Bladder₃ was outside the PTV (indicated by the yellow arrow in h).