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Single segment non-coplanar beam optimization for gated lung Stereotactic- body Radiotherapy (SBRT) planning and delivery Julien Partouche, MSc, Tianming Wu, PhD, Karl Farrey, MSc, Joseph K. Salama, MD, Kamil M. Yenice, PhD Department of Cellular and Radiation Oncology, The University of Chicago Medical Center ABSTRACT RESULTS RESULTS Table 2. Dose to normal lung: mean lung dose excluding GTV (cGy), Fig. 1 Isodoses and dose volume histogram (DVH) comparison between 3D-CRT (left, solid line) and 1- Purpose: To investigate the potential benefits of non-coplanar large segmented volume of total lung excluding GTV that receives 20Gy or greater, V20, segment/DMPO (right, dashed line) plans for patients 5, 7 and 8. (DVH colors: PTV is brown, direct machine parameter optimization (DMPO) for primary and metastatic lung 13Gy or greater, V13 and 5Gy or greater Wholelung-GTV is red, spinal cord is green, and esophagus is blue) SBRT. To assess the delivery accuracy of DMPO beam segments to a gated target in a motion phantom. Method and materials: Ten patients with primary or metastatic lung lesions treated with SBRT according to our institutional IRB protocol were included in this study. Patients with oligometastases received doses of 3 x 10-14 Gy and primary NSCLC patients received 50-60 Gy in 3-10 fractions depending on the tumor size and location. 3D custom treatment plans used 10-12 non-coplanar 6MV beam arrangements with manually optimized beam MLC aperture and weight to meet the clinical goals. Inverse planning was done using the identical beam arrangements. Beam apertures were reset and optimized through the DMPO (Pinnacle, Philips) Figure 1. Conformality index (CI) comparison between 3D CRT and algorithm with one segment per beam and the minimum segment area of half of 1seg/DMPO, CI=volume of prescription isodose volume/PTV volume the maximum PTV cross-section in the BEV. DMPO plans were normalized at the same identical target coverage level as those for 3D plans. The optimization routine used the collapsed-cone convolution dose calculation after 10 successive iterations. 3D dose distributions, dose volume histograms were calculated for both 3D conformal and 1-segment/DMPO SBRT plans. Finally, a motion phantom with a lung-equivalent insert was fitted with a small tissue-equivalent material to represent a lung tumor surrounded by lung tissue. Gafchromic EBT films were fitted between the sections of this phantom and were irradiated with optimized single-segment beams to evaluate the dose in the lung, in the target and at the edge of the target. Results: One-segment/DMPO planning improved the conformality of SBRT over 3D CRT delivering on average 12%±7%, 8%±7%, and 2%±2% less dose to the lung at 20, 13, and 5Gy levels, respectively. The maximum dose to the heart, esophagus, and cord were comparable between the 1-segment DMPO and 3DCRT plans. Except for one case, there was no increase in the number of monitor units used with 1-segment/DMPO plans. One -segment/DMPO plans yielded a mean reduction of 17.1% in the normal mean lung dose over 3DCRT 1seg/DMPO VALIDATION plans. A dose to distance agreement of 3%/3 mm between calculation and film A rigid phantom (QUASAR respiratory Motion Phantom, Modus medical Devices measurement for a representative plan in a motion phantom with gating was Inc., London, Canada) was used to simulate one-dimensional respiratory induced verified at 99% of points within the fields. movement. A special lung-equivalent insert, was manufactured from cork material. The total cork cylinder was 18 cm long with a 7.70 cm diameter. A tissue-equivalent cylinder of 3 cm long and 1.90 cm diameter was located in the geometrical center Conclusion: Single segment beam DMPO can be used to improve SBRT planning of the cork cylinder, simulating a tumor in lung. for lung lesions to meet the planning goals in an effective manner. This approach allows for easily deliverable and verifiable beam apertures for gated beam delivery. The automation of our method is a good alternative to more traditional methods and offers significant dosimetric benefits. STUDY OBJECTS Table 1. Patient, lesion locations, PTV volumes, prescription dose per fraction and total fractions, and the number of beams employed in the plan. 5.0cm Coronal dose profile comparison measured (green) vs calculated (red) DISCUSSION •We investigated the possible advantages of non-coplanar DMPO Fig. 2 Optimized MLC apertures with DMPO algorithm for beams #1 to #11 for patient 8 algorithm for gated lung SBRT in terms of PTV coverage and normal lung dose volume reduction. •We observed up to 72cc reduction in normal lung receiving 13Gy or Contact greater, and conformality improvement of 20% between 3D CRT and 1seg/DMPO. Kamil M. Yenice, firstname.lastname@example.org •DVH and conformality index comparison between 3D CRT and Julien Partouche, email@example.com 1seg/DMPO plans shows that 1seg/DMPO allows us to improve PTV Department of Radiation and Cellular Oncology, 5758 S Maryland Avenue, conformality, and to increase the dose delivered inside the PTV while reducing the volume of normal lung receiving 20Gy or greater, 13Gy or Chicago, IL 60637, U.S.A. greater, and 5Gy or greater.
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