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Modeling Thermal Lesion Development in Bubble-Mediated HIFU


									                     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.

                                                                                         Coronal dose profile comparison measured (green) vs calculated (red)

                                                                                         •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
Kamil M. Yenice,
                                                                                         •DVH and conformality index comparison between 3D CRT and
Julien Partouche,                                         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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