Author: Anupama Chundury, Hyejoo Kang, John C. Roeske, Iris A. Rusu 👨🔬
Affiliation: Loyola University Medical Center, Department of Radiation Oncology, Stritch School of Medicine, Loyola University Chicago, Stritch School of Medicine Loyola University Chicago, Cardinal Bernardin Cancer Center, Loyola University Chicago 🌍
Purpose: To quantify the dosimetric impact of six degrees of freedom (6DoF) intrafractional errors on gross tumor volume (GTV) and planning target volumes (PTV) for single-isocenter multi-target stereotactic radiosurgery (SIMT-SRS) as a function of minimum GTV dose (DminGTV), volume of target and distance to isocenter.
Methods: Four spherical GTVs (0.5cc-3.0cc) and the 1mm-margin PTVs were used to generate two SIMT-SRS plans, delivering 20Gy to each PTV, with a DminGTV of 22Gy in one plan and 24Gy in other. A random number generator simulated 100 translations-rotations within the 1mm/10 intrafraction tolerance. Using an open-source Python package, all GTVs/PTVs were translated-rotated around the isocenter according to the generated 6DOF errors. The same translations-rotations were performed around different points in space to mimic target-to-isocenter distances from 0cm to 10cm. For each plan, the dose received by 99% of the displaced target volumes (D99GTV/PTV22Gy, D99GTV/PTV24Gy) were extracted and normalized to the undisplaced value. The average over 100 displacements of the above parameters (AvgD99GTV/PTV22Gy (%), AvgD99GTV/PTV24Gy (%)) was computed and the 95% confidence interval was calculated for each distance from isocenter.
Results: The AvgD99GTV (%) and AvgD99PTV (%) decreases with increasing distance from isocenter and decreasing target size, regardless of DminGTV. Plans with higher DminGTV experience larger relative dose coverage loss of GTVs/PTVs compared to plans with lower DminGTV for all target sizes and distances from isocenter. Nevertheless, the loss of absolute dose coverage of GTVs and PTVs was reduced for the plans with higher DminGTV.
Conclusion: In SIMT-SRS, the loss in GTV and PTV dose coverage due to 6DoF intrafraction motion is subject to the interplay effect between DminGTV, target volume and distance from isocenter. For the same PTV dose, plans delivering higher DminGTV are more robust to uncorrected 6DoF intrafraction errors without compromising the dose fall-off outside the target.