Dose deformation-invariance in adaptive prostate radiation therapy: implication for treatment simulations.

PURPOSE

To investigate dose deformation-invariance in adaptive prostate radiation treatment.

METHODS

A 19 patient prostate cancer-cohort with 8-13 CTs/patient is used. The 79.2 Gy plans are developed on the reference image using seven 6 and 18 MV intensity-modulated beams with identical RTOG 0126 objectives. Dose on the subsequent images is evaluated in two ways: (A1) Dose is recalculated on each image. (A2) The initially planned dose distribution is copied to each image. A2 assumes dose-invariance in the accelerator-coordinate-system. Effects of patient miss-alignment are simulated by 27 per-patient image shifts; 0 and ±10 mm in left-right, anterior-posterior and superior-inferior directions. The per-voxel dose differences for each patient image, total accumulated patient dose, and dose-volume metrics (CTV-D98 and -D90, bladder- and rectum-D50, -D35, -D25 and -D15) are used to compare A1 and A2.

RESULTS

The per-voxel mean percent difference in A1 and A2 dose over all patient images at 6 MV is (0.01±1.56)% and at 18 MV is (0.00±0.96)%. For 6 MV and 18 MV plans, the root-mean-square-percentage-error (RMSPE) in A2 over all patient image shifts are CTV-D98=0.94 and 0.55, CTV-D90=0.92 and 0.55, rectum-D50, -D35, -D25 and -D15=1.00, 0.96, 0.86, 0.80 and 0.84, 0.96, 0.92, 1.05; and bladder-D50, -D35, -D25, -D15=1.07, 0.88, 0.78, 0.72 and 1.61, 0.93, 0.67, 0.51. The dose differences are not correlated to the dice-similarity coefficients; with respective correlation-coefficients for CTV, rectum and bladder being -0.17, -0.17 and 0.081.

CONCLUSIONS

Assumption of shift- and deformation-invariant dose distributions on an average introduces <2% error in evaluated dose-volume metrics for 6 and 18 MV IMRT prostate plans. Use of invariant dose distributions has a potential to reduce online re-planning time and permit pre-planning based on tissue deformation models.