Optimal set of grid size and angular increment for practical dose calculation using the dynamic conformal arc technique: a systematic evaluation of the dosimetric effects in lung stereotactic body radiation therapy.

Abstract

PURPOSE: To recommend the optimal plan parameter set of grid size and angular

increment for dose calculations in treatment planning for lung stereotactic body

radiation therapy (SBRT) using dynamic conformal arc therapy (DCAT) considering

both accuracy and computational efficiency. MATERIALS AND METHODS: Dose

variations with varying grid sizes (2, 3, and 4 mm) and angular increments (2

degrees , 4 degrees , 6 degrees , and 10 degrees ) were analyzed in a thorax

phantom for 3 spherical target volumes and in 9 patient cases. A 2-mm grid size

and 2 degrees angular increment are assumed sufficient to serve as reference

values. The dosimetric effect was evaluated using dose-volume histograms, monitor

units (MUs), and dose to organs at risk (OARs) for a definite volume

corresponding to the dose-volume constraint in lung SBRT. The times required for

dose calculations using each parameter set were compared for clinical

practicality. RESULTS: Larger grid sizes caused a dose increase to the structures

and required higher MUs to achieve the target coverage. The discrete beam

arrangements at each angular increment led to over- and under-estimated OARs

doses due to the undulating dose distribution. When a 2 degrees angular increment

was used in both studies, a 4-mm grid size changed the dose variation by up to

3-4% (50 cGy) for the heart and the spinal cord, while a 3-mm grid size produced

a dose difference of <1% (12 cGy) in all tested OARs. When a 3-mm grid size was

employed, angular increments of 6 degrees and 10 degrees caused maximum dose

variations of 3% (23 cGy) and 10% (61 cGy) in the spinal cord, respectively,

while a 4 degrees increment resulted in a dose difference of <1% (8 cGy) in all

cases except for that of one patient. The 3-mm grid size and 4 degrees angular

increment enabled a 78% savings in computation time without making any critical

sacrifices to dose accuracy. CONCLUSIONS: A parameter set with a 3-mm grid size

and a 4 degrees angular increment is found to be appropriate for predicting

patient dose distributions with a dose difference below 1% while reducing the

computation time by more than half for lung SBRT using DCAT.