Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_6_3175 - Variable Gantry and Dose Rate Conformal Arc for Lung and Liver SBRT

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Variable Gantry and Dose Rate Conformal Arc for Lung and Liver SBRT
S. Stathakis, R. L. Crownover, D. Saenz, N. Kirby, K. Rasmussen, and N. Papanikolaou; University of Texas Health San Antonio Cancer Center, San Antonio, TX

Purpose/Objective(s): The purpose of the present study is to ascertain whether the dose distribution delivered by DCAT exhibits as good of an agreement with the planned dose distribution as the VMAT delivery and if it can be used more efficiently, specifically for SBRT lung and liver treatments.

Materials/Methods: Nineteen (n=19) patients previously treated for lung and liver lesions with SBRT were chosen for this study. All patients’ organs at risk and targets were contoured by a single physician. All patient plans were optimized by the same dosimetrist using the a treatment planning system for a 6MV flattening filter free (FFF) photon beam. Two plans were optimized for each patient, one using VMAT and one using DCAT. For each plan two arcs of 225o arc span were used during optimization.

Results: Overall, all plans have achieved the planning objectives that were set prior to optimization. The MU for the DCAT plans were reduced by an average of 2.5 times when compared against the VMAT plans. The target coverage was more favorable for the VMAT plans (average values of PTVr=1.3 and DCAT PTVr=1.4 respectively), but this was not shown to be statistically significant. The patient specific plans were measured using the Sacndidos Delta4 phantom. The plans were delivered as planned. The measured 3D dose distributions were compared against the calculated ones using 2mm DTA and 2% dose difference for the gamma index calculation. More than 95% of the evaluated points were within the gamma index criteria for all plans. In this study we found that the larger differences between VMAT and DCAT plan quality metrics occurred when OARs were very close to or overlapped with the PTV, e.g. ribs in the case of lung or stomach in liver cases. In such instances, the DCAT plans were inferior to the respective VMAT plans in terms of quality and possibly further optimization would be necessary to improve them, but this was out of the scope of this study since we did not allow for preferential treatment of such plans. On the other hand, there were cases for targets far from OARs where the DCAT plans showed improved PTV coverage and OAR sparing than the respective VMAT plans.

Conclusion: DCAT delivery for lung and liver SBRT treatments is a viable alternative to VMAT plans. While maintaining plan quality, DCAT deliveries are on average 2.5 times faster. Both VMAT and DCAT plans were accurately delivered on a static IMRT QA phantom and had excellent gamma index passing rates.

Author Disclosure: S. Stathakis: None. R.L. Crownover: None. D. Saenz: None. N. Papanikolaou: Research Grant; BrainLab. site visitor for ACR Radiation Oncology Practice Accreditation; ACR. Speaker's Bureau; BrainLab. treasurer/secretary; Texas Radiological Society. medical physics advisory committee member; Texas Medical Board.

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