In the era of dose escalation for localised prostate cancer, the dose–volume histogram (DVH) is integral to the assessment of rectum and bladder dose constraints. However, reliance on a single planning computerised tomography-based (P-CT) dose distribution may not account for variations in delivered dose that results from deformation of the prostate, bladder and rectum. This study uses cone-beam CT (CBCT) datasets from five patients to investigate the concordance between the dose prediction from the initial treatment plan and the dose delivered during treatment.

The intensity-modulated radiation therapy distribution used for treatment was superimposed on alternate day CBCT images for each patient. Dose metrics and absolute volumes for the prostate, rectum and bladder were extracted from the CBCT-based DVH. Differences in dose and volumes were compared with the P-CT values, and significance was tested using the Wilcoxon signed-rank test.

For all five case studies, the prostate dose coverage on CBCT plans was lower than predicted with an average reduction of 3% in mean dose. Significant differences in rectal volumes and dose were observed in two out of five and four out of five patients, respectively. Reductions in bladder volume and subsequent increases in dose were observed for three out of five patients.

The DVH from P-CT was unable to consistently predict the dose delivered to the bladder and rectum. The current bowel and bladder preparation protocols used at our institution did not eliminate variation in bladder and rectum volumes for the five patients included in this study.