Author: Stella Flampouri, Edgar Gelover, William Andrew LePain, Roelf L. Slopsema, Alexander Stanforth, Mingyao Zhu π¨βπ¬
Affiliation: Emory Healthcare, Emory University π
Purpose: Little consideration was given to the change in dose rate (DR) observed when moving to pencil beam scanning (PBS). This study reports on DR in clinical prostate PBS plans. While dose distributions are homogeneous, the scanning delivery results in non-uniform DR distributions. A model was built to simulate PBS deliveries and examine variation in DR metrics for prostate treatment.
Methods: The scripting environment in the treatment planning system was used to construct a delivery simulator to calculate the dose rate of clinical plans. Based on measured delivery timing parameters and validated before use, the simulator computes 1. The maximum voxel DR, MaxDR and 2. The voxel dose-weighted dose average, MeanDR. DR wascalculated for 20 prostate patients, planned according to institutional standards. DR are reported per field and per plan. We perform a statistical analysis on the DR distribution of voxels within the structures, including the target, bladder, rectum, and several isodose volume (IDV) rings minus target.
Results: The metrics are reported for the CTV, nontarget irradiated volume, IDV75%-Target, and bladder. The variance of the metrics was statistically equal for all patients (Leveneβs Test). Voxels with the highest DR were located outside the CTV but within the for 18 cases. The remaining two had the highest DR within the IDV65%. For the cohort, the MaxDR to 0.5cc for the CTV, IDV75%-Target, and Bladder are 2.2Gy/s, Ο=0.4Gy/s, 3.7Gy/s, Ο=.6Gy/s, and 0.36Gy/s, Ο=0.17, respectively. For the cohort, the median of MeanDR was 0.60 Gy/s, Ο=0.07Gy/s, Ο=0.15Gy/s, and 0.70Gy/s, Ο=0.09Gy/s for the Target, Bladder, and IDV75%-Target, respectively.
Conclusion: Our analysis reveals variation within individual and between different plans. These values differ considerably compared to non-PBS delivery systems. This work provides an initial analysis on DR seen clinically in PBS. This work is a prerequisite for investigating potential correlations between DR and biological outcomes.