[Assessment of movements of the different anatomic portions of the bladder, implications for image-guided radiation therapy for bladder cancer]

Abstract

Purpose: To assess interfraction and intrafraction bladder wall movements in the different anatomic portions of the bladder.

Patients and methods: Six patients were treated for prostate cancer with conformal irradiation. Daily online cone beam computed tomography was performed for repositioning and an additional one was performed following irradiation once weekly. Four craniocaudal levels were defined to calculate movements amplitudes compared to the scanner tracking: level 1 at the bladder neck, level 2 at mid-height of the bladder, level 3 at mid-height of the dome, level 4 at the apex in a distended bladder. Bladder height was also measured.

Results: On 198 daily cone beam computed tomographies, radial bladder right/left/anterior/posterior wall displacements at level 2 were 0.08 ± 0.24, 0.11 ± 0.33, 0.16 ± 0.45 and 0.14 ± 0.50 cm and at level 3 0.07 ± 0.78, 0.18 ± 0.98, 0.43 ± 0.94 and 0.04 ± 1.02 cm. Dome and neck displacements were 0.08 ± 1.41 cm and 0.08 ± 0.64 cm. Seventeen cone beam computed tomographies were done following irradiation. Radial bladder right/left/anterior/halfway up the trine wall displacements at level 2 before and after irradiation were 0.02±0.18, 0.01±0.30, 0.09 ± 0.32 and 0.22 ± 0.42 cm and at level 3 0.27 ± 0.60, 0.37 ± 1.15, 0.18 ± 0.87 and 0.54 ± 1.68 cm.

Conclusion: Significant bladder wall displacements were observed on the anterior wall and upper portion of the bladder. Isotropic margins may not be sufficient to account for inter- and intrafraction bladder wall displacements at the latter levels. Tailored bladder anatomy-based anisotropic margins may be necessary to optimally spare the small intestine and to guaranty proper tumour coverage in case of bladder cancer. For upper bladder tumours, margins of over 2 cm would be necessary, which make them less adequate for external beam irradiation.