Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_6_3173 - Comparison of Treatment Planning Approaches for Spatially Fractionated Irradiation of Deep Tumors

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

Comparison of Treatment Planning Approaches for Spatially Fractionated Irradiation of Deep Tumors
K. Sheikh1, T. R. McNutt1, L. Bell1, J. Moore1, W. T. Hrinivich1, T. Teslow1, W. Laub1, Y. Yan2, J. W. Wong1, and J. J. Meyer1; 1Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD, 2The University of Texas Southwestern Medical Center, Dallas, TX

Purpose/Objective(s): Conventional GRID radiation therapy fractionates radiation in space. Spatially fractionated radiation therapy (SFRT) with a grid block has limitations in the treatment of deep tumors. Other treatment options may better allow for the application of the principles of SFRT to deep tumors. The purpose of this work was to compare the dosimetric and delivery time characteristics of 3D conformal-, volumetric arc therapy (VMAT)-, and TomoTherapy-based approaches for SFRT of deep targets.

Materials/Methods: Two virtual GRID phantoms were created consisting of 7 cylinders (1cm diameter) aligned longitudinally along the tumor in a honey-comb pattern, mimicking a conventional GRID block, with 2cm center-to-center spacing (GRID2cm) and 3cm center-to-center spacing (GRID3cm), all contained within a larger cylinder. These virtual phantoms consisted of: the tumor (the larger cylinder structure), the targets (the cylindrical structures inside the larger cylinder), and an avoidance structure created by extracting the targets from the tumor. The larger cylinder was 8cm and 10cm in diameter for the GRID2cm and GRID3cm, respectively. TomoTherapy, VMAT and multi-multileaf collimator (MLC) segment 3D conformal plans forming the grid pattern were created using commercially available software. The mean dose to the GRID target and the heterogeneity of the dose distribution (peak-to-valley and peak-to-edge dose ratios) inside the GRID target were obtained. The peak-to-valley ratio was determined as the mean dose delivered to the 7 cylinders to the mean dose delivered to the avoidance structure. The peak-to-edge ratio was determined as the mean dose delivered to the 7 cylinders to the mean dose delivered at the edge of the tumor (defined as a 2mm ring encompassing the edge). Prescriptions for all plans were set to deliver a mean dose of 15 Gy to the grid targets in one fraction.

Results: The mean doses inside the GRID2cm/GRID3cm target were 16.3±2.0Gy/16.6±2.0Gy, 15.0±0.4Gy/15.0±0.5Gy, and 15.0±0.4Gy/15.0±2.2Gy for the TomoTherapy, VMAT, and 3D conformal plans, respectively. The mean doses at the edge of the GRID2cm/GRID3cm cylinder were 7.0±3.2Gy/6.3±3.3Gy, 6.0±1.3Gy/6.8±0.7Gy, and 5.1±2.2Gy/4.6±2.2Gy for the TomoTherapy, VMAT, and 3D conformal plans, respectively. The peak-to-valley ratios for GRID2cm/GRID3cm were 1.7/1.9, 1.7/1.6, 2.1/2.3 for the TomoTherapy, VMAT, and 3D conformal plans, respectively. The peak-to-edge ratios for GRID2cm/GRID3cm were 2.3/2.6, 2.5/2.2, 2.9/3.3 for the TomoTherapy, VMAT, and 3D conformal plans, respectively. The delivery times for the GRID2cm/GRID3cm were 44min/27.5min, 16.5min/8min, and 15min/21min for TomoTherapy, VMAT, and 3D conformal plans, respectively.

Conclusion: Our results demonstrate that 3D conformal planning yield the highest peak-to-valley and peak-to-edge ratios with indication of shorter treatment times for this specific geometry.

Author Disclosure: K. Sheikh: Trainee; Johns Hopkins University. T.R. McNutt: Research Grant; Toshiba, Philips Radiation Oncology Systems, Elekta Oncology Systems. Patent/License Fees/Copyright; Sun Nuclear, Accuray-Tomotherapy. President Elect; AAPM-MAC. L. Bell: None. J. Moore: President Elect; MAC-AAPM. W.T. Hrinivich: Trainee; Johns Hopkins Hospital. T. Teslow: None. W. Laub: Chief Medical Physicist; Johns Hopkins Hospital. Y. Yan: None. J.W. Wong: Research Grant; Elekta. Consultant; LAP Laser. Chief Scientific Officer; JPLC Associates. Founder; JPLC Associates. Consultant; Xstrahl Medical. J.J. Meyer: Research Grant; Peregrine Pharmaceuticals, Inc. clinical trials support; D-fine Inc. Honoraria; UpToDate, Inc.

Khadija Sheikh, PhD, MS

Disclosure:
Employment
Johns Hopkins University: Physics Resident: Trainee

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