Organ movements and dose exposures in teletherapy of prostate cancer using a rectal balloon

During radiotherapy of localized prostate cancer, organ movements for the dose exposure of organs at risk like rectum, urinary bladder and urethra play, inter alia, a significant role. One possibility of internal organ stabilizing is offered by the usage of a rectal balloon during radiotherapy. The influence on organ movements and dose allocation of the organs at risk is unknown.

Twelve patients (Table 1) were characterized based on planning-CT's regarding organ movements and organ doses using a rectal balloon, inflated with 0 ml and 60 ml air. For the determination of the organ doses, three-dimensional conformal radiation plans (3-field-pelvis box) with a cumulative dose of 59.4 Gy were created, and the dose-volume-histograms for the anterior rectal wall, the posterior rectal wall, the rectal mucosa, the whole rectum, as well as the urinary bladder were compared (Figures 1 and 2).

The application of a 60 ml air-filled rectal balloon during each fraction of teletherapy led to significant organ movements of the anterior and posterior rectal wall and to a reduction of the transversal prostate diameter, as well as to a changed organ dose exposure of the organs at risk. A ventral shift of the anterior rectal wall (maximum 0.8 cm, mean 0.4 cm) was shown, as well as a dorsal shift of the posterior rectal wall (maximum 1.2 cm, mean 0.7 cm), associated with a transversal prostate diameter decrease (maximum 0.8 cm, mean 0.3 cm) (Table 2, Figure 3). The organ dose of the anterior rectal wall increased significantly (maximum 1.3 Gy, mean 0.5 Gy) during application of a rectal balloon, the one of the posterior rectal wall decreased significantly (maximum 18.6 Gy, mean 6.5 Gy). Related to the entire rectal mucosa and the rectum as a complete organ, a decrease of the maximum doses was shown (rectal mucosa: maximum 9.1 Gy, mean 3.0 Gy; rectum: maximum 9.4 Gy, mean 3.7 Gy). The organ dose of the urinary bladder did not show significant changes (Tables 3 and 4, Figures 4 to 7).

The application of a rectal balloon in teletherapy of localized prostate cancer leads to significantly changed dose exposition of organs at risk. The decreased dose exposure of the posterior rectal wall and the rectal mucosa is opposed by the higher organ dose of the anterior rectal wall. It has to be shown weather documented organ dose exposure is associated with short and long-term consequences.