title: Computerized triplet beam orientation optimization for MRI-guided Co-60 radiotherapy.
abstract: OBJECTIVE
  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&NBoost)-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 Dmaxand Dmeanof 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.
authors: Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
authors: Dan
authors: D
authors: Nguyen
authors: Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
authors: David
authors: D
authors: Thomas
authors: Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
authors: Minsong
authors: M
authors: Cao
authors: Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
authors: Daniel
authors: D
authors: O'Connor
authors: Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
authors: James
authors: J
authors: Lamb
authors: Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
authors: Ke
authors: K
authors: Sheng
mesh_terms: D003037:Cobalt Radioisotopes / Q000627:therapeutic use*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009010:Monte Carlo Method; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*; D061089:Radiotherapy, Image-Guided / Q000379:methods*
chemical_list: D003037:Cobalt Radioisotopes