Whilst the geometric margins for bladder motion have been assessed, the difference between the dose coverage of the target, and the dose reduction to the pelvic normal tissue for different margins have not been previously evaluated. The aim of this study was to compare the margins required for dosimetric coverage of the planning CTV using daily positioning based on either skin tattoos, or bony landmarks, or soft tissue centre of the bladder. Finally, the effect on normal tissue dose of each of the three positioning...

Four plans were generated on each planning CT dataset using the CTV with 5, 10, 15, 20mm PTV margins using a 3D-conformal four-field technique. Setup data based on skin, bone and soft tissue to the bladder on pre-treatment CBCT were recorded. From 10 bladder cancer patients, 316 CBCTs were evaluable, and each CBCT was fused to the planning CT dataset using the isocentre corresponding to each of the three pre-treatment matching conditions. The target was contoured on each CBCT and called CTV of the day...

The mean D95 with CTV to PTV margins of 5,10,15,20mm for skin set up was 89.4, 93.0, 97.2, 98.6; for bone 88.8, 92.6, 96.7, 98.6; and for soft tissue 96.3, 98.6, 98.7, 99.5. The mean D100 with CTV to PTV margins of 5, 10, 15, 20mm for skin was 77.3, 84.5, 92.0, 95.1; for bone 75.9, 82.2, 90.7, 93.5; and for soft tissue 87.0, 91.9, 96.6, 98.4. With soft tissue matching, the mean(SD) volume(cm3) of normal tissue receiving 5Gy with 5,10,15, 20mm margins was 3899(1022),...

Daily setup of bladder radiotherapy patients based on soft tissue registration improved target coverage when compared with setup based on external skin markers or bony anatomy. Furthermore, daily setup based on soft tissue registration decreased the volume of normal tissue irradiated with low doses compared with daily setup based on external skin or bony anatomy for clinically used CTV to PTV margins. Soft tissue registration allows reduction in margin sizes whilst still maintaining target coverage.

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