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      Computerized triplet beam orientation optimization for MRI-guided Co-60 radiotherapy

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          Abstract

          Purpose:

          Magnetic resonance imaging (MRI)-guided Co-60 provides daily and intrafractional MRI soft tissue imaging for improved target and critical organ tracking. To increase delivery efficiency, the system uses three Co-60 sources at 120° apart, allowing up to 600 cGy combined dose rate at isocenter. Despite the potential tripling in output, creating a delivery plan that uses all three sources is considerably unintuitive. Here, the authors computerize the triplet orientation optimization using column generation, an approach that was demonstrated effective in integrated beam orientation and fluence optimization for noncoplanar therapies. To achieve a better plan quality without increasing the treatment time, the authors then solve a fluence map optimization (FMO) problem while regularizing the fluence maps to reduce the number of deliverable MLC segments.

          Methods:

          Three patients—one prostate, one lung, and one head and neck boost plan (H&N Boost)—were evaluated in this study. For each patient, the beamlet doses were calculated using Monte Carlo, under a 0.35 T magnetic field, for 180 equally spaced coplanar beams grouped into 60 triplets. The beamlet size is 1.05 × 0.5 cm determined by the MLC leaf thickness and step size. The triplets were selected using the column generation algorithm. The FMO problem was formulated using an L2-norm dose fidelity term and an L1-norm anisotropic total variation regularization term, which allows controlling the number of MLC segments, and hence the treatment time, with minimal degradation to the dose. The authors’ Fluence Regularization and Optimized Selection of Triplets (FROST) plans were compared against the clinical treatment plans (CLNs) produced by an experienced dosimetrist. PTV homogeneity, max dose, mean dose, D95, D98, and D99 were evaluated. OAR max and mean doses, as well as R50, defined as the ratio of the 50% isodose volume over the planning target volume were investigated.

          Results:

          The mean PTV D95, D98, and D99 differ by +0.04%, +0.07%, and +0.25% of the prescription dose between planning methods. The mean PTV homogeneity was virtually same with values at 0.8788 (FROST) and 0.8812 (CLN). R50 decreased by 0.67 comparing FROST to CLN. On average, FROST reduced D max and D mean of OARs by 7.30% and 6.08% of the prescription dose, respectively. The manual CLN planning processes required numerous trial and error runs. The FROST plans on the other hand required minimal human intervention.

          Conclusions:

          Efficient delivery of MRI-guided Co-60 therapy needs the output of multiple sources yet suffers from unintuitive and laborious manual beam selection processes. Computerized triplet orientation optimization improves both planning efficiency and plan dosimetry. The novel fluence map regularization provides additional controls over the number of MLC segments and treatment time.

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          Author and article information

          Journal
          Med Phys
          Med Phys
          MPHYA6
          Medical Physics
          American Association of Physicists in Medicine
          0094-2405
          October 2016
          26 September 2016
          : 43
          : 10
          : 5667-5675
          Affiliations
          Department of Radiation Oncology, University of California Los Angeles , Los Angeles, California 90024
          Author notes
          [a)]

          Author to whom correspondence should be addressed. Electronic mail: ksheng@ 123456mednet.ucla.edu

          Article
          PMC5045447 PMC5045447 5045447 044610MPH 1.4963212 16-569R2
          10.1118/1.4963212
          5045447
          27782726
          © 2016 American Association of Physicists in Medicine.

          0094-2405/2016/43(10)/5667/9/ $30.00

          Page count
          Pages: 9
          Funding
          Funded by: National Institutes of Health (NIH) http://dx.doi.org/10.13039/100000002
          Award ID: R01CA188300
          Award ID: R43CA183390
          Categories
          THERAPEUTIC INTERVENTIONS
          Research Articles
          Custom metadata

          MRI guided radiation therapy, optimization, Co-60

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