Adaptive radiation therapy for bladder cancer: a review of adaptive techniques used in clinical practice

Abstract

Significant changes in the shape, size and position of the bladder during radiotherapy (RT) treatment for bladder cancer have been correlated with high local failure rates; typically due to geographical misses. To account for this, large margins are added around the target volumes in conventional RT; however, this increases the volume of healthy tissue irradiation. The availability of cone beam computed tomography (CBCT) has not only allowed in-room volumetric imaging of the bladder, but also the development of adaptive radiotherapy (ART) for modification of plans to patient-specific changes. The aim of this review is to: (1) identify and explain the different ART techniques being used in clinical practice and (2) compare and contrast these different ART techniques to conventional RT in terms of target coverage and dose to healthy tissue: A literature search was conducted using EMBASE, MEDLINE and Scopus with the key words 'bladder, adaptive, radiotherapy/radiation therapy'. 11 studies were obtained that compared different adaptive RT techniques to conventional RT in terms of target volume coverage and healthy tissue sparing. All studies showed superior target volume coverage and/or healthy tissue sparing in adaptive RT compared to conventional RT. Cross-study comparison between different adaptive techniques could not be made due to the difference in protocols used in different studies. However, one study found daily re-optimisation of plans to be superior to plan of the day technique. The use of adaptive RT for bladder cancer is promising. Further study is required to assess adaptive RT versus conventional RT in terms of local control and long-term toxicity.

Keywords: Adaptive; bladder; cone beam computed tomography; conventional; radiation therapy.

Figure 1

Schematic diagram of method used to obtain articles for the review.

Figure 2

A schematic diagram showing the different adaptive radiation therapy techniques described in the studies reviewed.

Figure 3

A schematic diagram of the technique used to create a composite adaptive radiotherapy plan. The five top nodes represent CBCT images taken during week 1 and the resulting node represents the composite plan produced using the CBCT images to create a plan that is more representative of a patient's bladder shape and size. CBCT, cone beam computed tomography; CTV, clinical target volume.

Figure 4

A schematic diagram of the process to create an online individualised plan library for the ‘plan of the day’ adaptive technique using CBCT images from week 1 of treatment. This creates a library of plans with small, medium and large planning target volumes for each patient. CBCT, cone beam computed tomography; CTV, clinical target volume.