Bladder cancer radiotherapy margins: a comparison of daily alignment using skin, bone or soft tissue.

Abstract Aims To determine the clinical target volume (CTV) to planning target volume (PTV) margins required for bladder coverage when using skin or bony or soft tissue matching on a daily basis. Materials and methods Twenty-seven patients with T2–T4 transitional cell carcinoma of the bladder were treated with daily online adaptive image-guided radiotherapy using cone beam computed tomography. All daily CTVs were contoured by a single observer. A retrospective comparison of coverage of the bladder CTV using skin, bone and soft tissue matching was conducted. Results With the skin set-up, bladder CTV coverage with a margin of 0.5, 1.0, 1.5, 2.0 and 2.5 cm was 0, 19, 56, 93 and 96%, respectively. For the daily set-up based on bone, the respective coverage was 0, 41, 63, 89 and 96%. For soft tissue set-up based on the geometric centre of the bladder, coverage was 52, 89, 96, 100 and 100%, respectively. Conclusions Based on coverage of the CTV, the required CTV to PTV margins are smaller for the daily online soft tissue set-up compared with skin or bone.