(Saturday, 3/30/2019)
Room: Exhibit Hall
Purpose: VMAT technique with full gantry rotations has become popular in spine radiosurgery (SRS). In most institutions, free-breathing CTs (FBCT) are utilized for treatment planning. However, for treatments at the level of diaphragm, the diaphragm motion may have an impact on dose delivery accuracy. The aim of this study was to evaluate the dosimetric impact of breathing motion in lower thoracic spine SRS.
Methods: Four representative patients with FBCT and 4DCT datasets were selected for this preliminary study. For each case, an 18Gy VMAT plan composing of 2-3 full arcs was generated. The dose distribution was re-calculated on average CT (AVGCT) and two extreme phases (PH0%CT and PH50%CT, to represent the possible scenarios of FBCT). In addition, each arc was divided into 64-155 sub-arcs based on total monitor units, dose-rate, and timestamp relative to the breathing trace. Dose distributions from sub-arcs were recalculated onto corresponding phased CT images. 4D plans, which were summations of sub-arcs doses, were generated to simulate the delivered dose. Dose-volume metrics evaluated were D95%, V(Rx) for PTV, and D10%, D0.035cc for cord.
Results: No correlation was observed between the dose-volume metrics calculated on FBCT and other images (PH0%CT, PH50%CT, AVGCT). When comparing doses calculated on FBCT, PH0%CT, and PH50%CT to AVGCT, the largest changes in D95% and V(Rx) were 1.0Gy and 40.7%, respectively. In contrast, the largest differences between 4D and AVGCT plans were 0.02Gy and 0.75%. For cord dose, average changes (±standard deviation) in D10% and D0.035cc were 0.2±0.2 and 0.3±0.3 Gy, respectively.
Conclusion: Due to diaphragm-induced motion, the use of FBCT for VMAT planning in lower thoracic spine SRS sometimes leads to large deviations between planned and delivered dose. Dose calculation on AVGCT was found to be consistent to that on 4DCT, thus planning on AVGCT is recommended if diaphragm is in arc’s path.
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