TU‐F‐17A‐08: The Relative Accuracy of 4D Dose Accumulation for Lung Radiotherapy Using Rigid Dose Projection Versus Dose Recalculation On Every Breathing Phase

Abstract

Purpose:To investigate the accuracy of 4D dose accumulation using projection of dose calculated on the end‐exhalation, mid‐ventilation, or average intensity breathing phase CT scan, versus dose accumulation performed using full Monte Carlo dose recalculation on every breathing phase.Methods:Radiotherapy plans were analyzed for 10 patients with stage I‐II lung cancer planned using 4D‐CT. SBRT plans were optimized using the dose calculated by a commercially‐available Monte Carlo algorithm on the end‐exhalation 4D‐CT phase. 4D dose accumulations using deformable registration were performed with a commercially available tool that projected the planned dose onto every breathing phase without recalculation, as well as with a Monte Carlo recalculation of the dose on all breathing phases. The 3D planned dose (3D‐EX), the 3D dose calculated on the average intensity image (3D‐AVE), and the 4D accumulations of the dose calculated on the end‐exhalation phase CT (4D‐PR‐EX), the mid‐ventilation phase CT (4D‐PR‐MID), and the average intensity image (4D‐PR‐AVE), respectively, were compared against the accumulation of the Monte Carlo dose recalculated on every phase. Plan evaluation metrics relating to target volumes and critical structures relevant for lung SBRT were analyzed.Results:Plan evaluation metrics tabulated using 4D‐PR‐EX, 4D‐PR‐MID, and 4D‐PR‐AVE differed from those tabulated using Monte Carlo recalculation on every phase by an average of 0.14±0.70 Gy, – 0.11±0.51 Gy, and 0.00±0.62 Gy, respectively. Deviations of between 8 and 13 Gy were observed between the 4D‐MC calculations and both 3D methods for the proximal bronchial trees of 3 patients.Conclusions:4D dose accumulation using projection without re‐calculation may be sufficiently accurate compared to 4D dose accumulated from Monte Carlo recalculation on every phase, depending on institutional protocols. Use of 4D dose accumulation should be considered when evaluating normal tissue complication probabilities as well as in clinical situations where target volumes are directly inferior to mobile critical structures.