Image-guided radiotherapy of bladder cancer: bladder volume variation and its relation to margins

To control and account for bladder motion is a major challenge in radiotherapy (RT) of bladder cancer. This study investigates the relation between bladder volume variation and margins in conformal and image-guided RT (IGRT) for this disease.

The correlation between the relative bladder volume (RBV, defined as repeat scan volume/planning scan volume) and the margins required to account for internal motion was first studied using a series of 20 bladder cancer patients with weekly repeat CT scanning during treatment. Both conformal RT (CRT) and IGRT were simulated; in the latter translational movement of the bladder was accounted for by isocentre shifting. Further analysis of bladder volumes and margins was performed using a second series of eight patients with twice-weekly repeat CT scanning. In an attempt to control bladder volume variation these patients were given fluid intake restrictions on alternating weeks during treatment.

IGRT gave the strongest correlation between the RBV and margin size (R(2)=0.75; p<0.001). Using IGRT, isotropic margins >10mm were required in only 1% of the situations when the RBV1, whereas isotropic margins >10mm were required in 55% of the situations when the RBV>1. Less marked correlation was found using CRT (R(2) in the range 0.43-0.53, p<0.001) for four different methods used to assess the margins required in the six directions, although a strong correlation was found for the superior margin (R(2)=0.63; p<0.001). Fluid intake restriction gave a small reduction in both bladder volume (average absolute volume reduced from 126 to 121cm(3); RBV from 0.83 to 0.80) and bladder volume variation, but not sufficient to translate into margin reduction.

The study showed the potential for a large margin reduction in bladder RT if the bladder volume is controlled and this potential was even greater for IGRT. An attempt to control the bladder volume by restricting fluid intake prior to the treatment session failed to give any reduction in the margins required.