Associations between volume changes and spatial dose metrics for the urinary bladder during local versus pelvic irradiation for prostate cancer.

Acta oncologica (Stockholm, Sweden)

PubMedID: 28401808

Casares-Magaz O, Moiseenko V, Hopper A, Pettersson NJ, Thor M, Knopp R, Deasy JO, Muren LP, Einck J. Associations between volume changes and spatial dose metrics for the urinary bladder during local versus pelvic irradiation for prostate cancer. Acta Oncol. 2017;1-7.
BACKGROUND
Inter-fractional variation in urinary bladder volumes during the course of radiotherapy (RT) for prostate cancer causes deviations between planned and delivered doses. This study compared planned versus daily cone-beam CT (CBCT)-based spatial bladder dose distributions, for prostate cancer patients receiving local prostate treatment (local treatment) versus prostate including pelvic lymph node irradiation (pelvic treatment).

MATERIAL AND METHODS
Twenty-seven patients (N?=?15 local treatment; N?=?12 pelvic treatment) were treated using daily image-guided RT (1.8?Gy@43-45 fx), adhering to a full bladder/empty rectum protocol. For each patient, 9-10 CBCTs were registered to the planning CT, using the clinically applied translations. The urinary bladder was manually segmented on each CBCT, 3?mm inner shells were generated, and semi and quadrant sectors were created using axial/coronal cuts. Planned and delivered DVH metrics were compared across patients and between the two groups of treatment (t-test, p?
RESULTS
Bladder volumes varied considerably during RT (coefficient of variation: 16-58%). The population-averaged planned and delivered DVH metrics were not significantly different at any dose level. Larger treatment bladder volumes resulted in increased absolute volume of the posterior/inferior bladder sector receiving intermediate-high doses, in both groups. The superior bladder sector received less dose with larger bladder volumes for local treatments (rs?±?SD: -0.47?±?0.32), but larger doses for pelvic treatments (rs?±?SD: 0.74?±?0.24).

CONCLUSIONS
Substantial bladder volume changes during the treatment course occurred even though patients were treated under a full bladder/daily image-guided protocol. Larger bladder volumes resulted in less bladder wall spared at the posterior-inferior sector, regardless the treatment received. Contrary, larger bladder volumes meant larger delivered doses to the superior bladder sector for pelvic RT but smaller doses for local treatments.