Dosimetric comparison of AAA and Acuros XB algorithm for breast DIBH VMAT treatment
PO-1396
Abstract
Dosimetric comparison of AAA and Acuros XB algorithm for breast DIBH VMAT treatment
Authors: Deshpande|, Sudesh(1)*[sudeshpande72@gmail.com];Naidu|, Suresh(1);Chavan|, Kamal Nayan(1);Kannan|, v(1);
(1)P.D. Hinduja National Hospital, oncology, Mumbai, India;
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Purpose or Objective
Radiotherapy of chest wall and nodes involves complex anatomy and many times VMAT is preferred to get uniform dose distribution and to avoid junction problem. In order to limit the radiation dose to the heart respiratory gating technique such as deep inspiration breath hold (DIBH) is used. The lung density may substantially decrease when utilizing DIBH
We investigated the impact of change in lung density on dose distribution of VMAT plans. In the study, calculations performed with deterministic particle transport algorithm Acuros XB are evaluated against the model based Anisotropic Analytical Algorithm (AAA)
Material and Methods
CT data from 10 patients presenting left sided post mastectomy patients involving regional nodes were selected for the study. For all patients two scanning acquisition sets were available: the first patient with normal breath (free breathing, FB), the second obtained by scanning patients under deep inspiration breath hold. Gating and breath tracking during scanning were determined by means of the Respiratory Gating RPM system (Varian Medical System, Palo Alto, CA); adjacent slices with 2.5 mm thickness were acquired on a GE CT scanner.
The lung HU was determined for each CT scan as the average lung HU in a two dimensional rectangular region of interest (ROI) in axial plane
DIBH CT scan set was used for VMAT plans for all the patients. VMAT plans were generated with four partial arcs using Eclipse v13.6 with 6MV energy and calculated using AAA algorithm. Same plans were recalculated using Acuros XB algorithm. Two sets of plan were compared using PTV-Eval (5mm crop from body) for target coverage (V95%), Homogeneity index (HI). For ipsilateral lung V20Gy, Mean dose and V5Gy and heart mean dose were compared
Results
Lung HU variations between -796 and -861 and between -680 and -750 were observed for DIBH CT scan set and FB CT scan set respectively. PTV- Eval dose coverage was less by average 3% and average HI index increased by 12% (Std dev 4.7, P value 0.027) when plans were calculated using Acuros XB algorithm The V20Gy and mean lung dose deviated by less than 2% for both algorithms. Larger difference was observed for low lung dose V5Gy, the correction based AAA algorithm overestimated V5Gy. Average overestimation of V5Gy was 9.22 % (std dev 1.5 and P value 0.006). Mean Heart dose obtained by the both algorithms agreed within 3%.
Conclusion
Lung density decreases for DIBH CT scan. Model based AAA algorithm overestimate low lung dose V5 Gy for DIBH CT scan which is very important parameter for accepting VMAT plan for chest wall.