Purpose: The Elekta Unity MRLinac treatment planning system, Monaco, can optimize treatments for each fraction, tailoring the delivered radiation precisely to the intended region. Adaptive planning calculations may utilize larger dose grids for computational efficiency and bulk density for MRI adaptive planning. Although not generally a concern, these approximations must be evaluated for thoracic spine stereotactic body radiotherapy (SBRT) treatments observed at our institution given their small target size and heterogeneity.
Methods: Single fraction thoracic spine SBRT treatments were retrospectively analyzed for two patients using the Monte Carlo dose engine in Monaco and the patient’s simulation CT scan. Dose grid resolutions of 0.1 cm, 0.2 cm, and 0.3 cm were applied to investigate the calculation accuracy, and a second set of dose calculations were performed using bulk density assignments. Dose differences were evaluated for the target and for the nearby critical organs.
Results: Using bulk density overrides and a 0.3 cm dose grid resolution resulted in a planning target volume (PTV) coverage difference of 2.3% and 3.9% for case 1 (1.93 cc) and case 2 (57.03 cc) respectively, relative to the PTV coverage that used a 0.1 cm dose grid and the CT densities. Increasing the dose grid resolution generally led to an increase in maximum calculated dose. This was most notable for case 2, which exhibited a 15% difference in the maximum calculated dose (D 0.03cc) using a bulk density assignment and a 0.2 cm dose grid compared to using the CT densities with a 0.1 cm dose grid.
Conclusion: Plan adaption is a powerful tool to customize radiotherapy at the patient level, but numerous challenges still exist to improve treatment efficiency. Dosimetric differences have been seen when using smaller dose grid sizes and bulk density assignments. However, a larger cohort study is required to determine statistical significance.