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Measurement-Guided 4D VMAT Dose Reconstruction On An Arbitrary Homogeneous Dataset

V Feygelman

D Opp1, J Robinson2, B Nelms3, G Zhang1, V Feygelman1* (1) Moffitt Cancer Center, Tampa, FL (2) Department of Physics, University of South Florida, Tampa, FL (3) Canis Lupus LLC, Sauk County, WI

SU-E-T-348 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To develop and validate a VMAT QA tool that takes as input a 2D, low-density (~10 mm) empirical dose map from a commercial helical diode array, and outputs a high density, volumetric, time-resolved (4D) dose matrix on an arbitrary patient dataset. At first, the method validation is limited to a homogeneous 'patient'.

Methods: A VMAT treatment is delivered to a diode array (ArcCHECK, Sun Nuclear Corp., Melbourne, FL). 3DVH software (Sun Nuclear) derives the high-density volumetric dose using measurement-guided dose reconstruction (MGDR). MGDR cylindrical phantom results are then used to perturb the 3D TPS dose on the patient dataset, producing a semi-empirical volumetric dose grid. Four-dimensional dose reconstruction on the patient is also possible by morphing the sub-beam instead of composite dose. For validation, TG-119 structures and objectives were used. 3DVH and TPS cumulative point doses were compared to ion chamber in a cube water-equivalent phantom ('patient'). The shape of the phantom is different from the ArcCHECK and the targets were placed asymmetrically. Ion chamber dose sampled at 10Hz was compared to time-resolved 3DVH point doses. Coronal and sagittal absolute film dose distributions in the cube were compared with 3DVH and TPS.

Results: Across four TG-119 plans, the average PTV point dose difference in the cube between 3DVH and ion chamber is 0.0±0.9%. Average film vs. 3DVH gamma analysis passing rates are 88.6, 96.1, and 99.5% for 1%/2mm, 2%/2mm, and 3%/3mm criteria, respectively. 4D MGDR was also sufficiently accurate.

Conclusions: Even for a well-commissioned TPS, comparison metrics show on average better agreement between MGDR and measurement compared to MGDR and TPS on the arbitrary-shaped phantom ('patient'). The method requires no more accelerator time than standard QA, while producing more clinically relevant information. Validation in a heterogeneous thoracic phantom is under way, as is ultimate application to virtual motion studies.

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