Author: Neha Devi, Deon M. Dick 👨🔬
Affiliation: Jaeger Corporation 🌍
Purpose: To evaluate the impact of bladder volume variation on small bowel dose in prostate cancer patients with nodal involvement and assess the efficacy of a 'half-full bladder' guideline for minimizing bowel dose during radiation therapy.
Methods: A rigid registration was first performed between the planning CT and CBCT datasets, followed by a deformable registration to account for anatomical changes. With the help of RadFormation’s AutoContour tool, the organs at risk were delineated on the CBCT. The deformable registration's quality was quantified with the Dice Coefficient. The final approved structures were exported to the Eclipse treatment planning system, where the original prostate treatment plan was adapted to the CBCT structure set, and the dose was recalculated using the same monitor units (MUs). The dose to both the small bowel and bladder was evaluated.
Results: The patient's bladder volume decreased by 70% (434.8cc vs 131.5cc) between simulation and treatment, resulting in a compensatory shift of the small bowel into the void left by the reduced bladder. The Dice coefficients for the bladder and small bowel were 0.44 and 0.02, respectively, indicating minimal overlap between the deformed bladder and small bowel. This is consistent with the significant volume reduction of the bladder. Despite the substantial reduction in bladder volume, all bladder dose constraints were successfully met. However, the dose constraints for the small bowel were exceeded, indicating a failure to adequately protect the small bowel due to the unexpected shift in anatomical positioning.
Conclusion: This study shows impact of bladder volume changes on small bowel dose distribution. Although the 'half-full bladder' guideline maintained bladder dose constraints, it failed to prevent small bowel dose escalation. These findings suggest that the guideline may require refinement to account for individual bladder volume variations, highlighting the need for continuous monitoring and adjustments for daily treatment delivery.