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Transit Dosimetry for VMAT Lung Dose Verification

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J Dumas

JL Dumas1*, E Biggs2 , L Sandra3 , S Krihli4 , S Helfre5 , A Mazal6 , (1) Institut Curie, Paris, France, (2) Paris Sud University, Le Kremlin Bicetre, France, (3) Institut Curie, Paris, France, (4) Institut Curie, Paris, France, (5) Institut Curie, Paris, France, (6) Institut Curie, Paris, France


SU-I-GPD-T-261 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The aim of this study was to determine the effect of 4DCT volume reconstruction method on patient transit dosimetry during lung Volumetric Modulated Arc Therapy (VMAT).

Methods: Mobile phantom 4DCT scans were acquired and processed. Target volume reconstruction was provided with Free breathing, Mean, Maximum Intensity Projection (MIP), Mid Position (MidP) and Mean Density Overrided models. Radiological depths were evaluated. Film dosimetry was used to evaluate differences between calculated and measured doses in the phantom. Transit dosimetry was performed and evaluated on the basis of dose differences for 20 points distributed in the medium and a gamma index for the same points with 5%-5mm tolerance for each volume reconstruction.

Results: Film dosimetry results showed that more than 99% points had a gamma index less than 1 for 3%-3mm for both MIP and MidP, in a 1cmx1cm square region of interest centered in the target. A 3cm diameter region of interest resulted in a lower dose at the edge of the target whatever the reconstruction. Transit dosimetry results showed dose discrepancies ranging between -11.84% and -3.41% and root mean squares ranging from 3.64 to 8.58, depending on volume reconstruction and breathing rate. Gamma mean 5%-5mm was less than 1 except for free breathing and Mean reconstruction and 3s breathing period. Radiological depths varied on a 1cm scale during movement, resulting in measured dose discrepancies between film and transit dosimetry as high as 6.3%.

Conclusion: This study shows a high dependence of 4D reconstruction method on both target dose and transit dosimetry. We therefore propose a novel approach for lung VMAT transit dosimetry evaluation on the basis of 4D reconstructions. The use of more realistic radiological depths may improve the results and the gamma index give a better and almost more complete evaluation.

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