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1.  Effect of Breathing Motion on Radiotherapy Dose Accumulation in the Abdomen Using Deformable Registration 
To investigate the effect of breathing motion and dose accumulation on the planned radiotherapy dose to liver tumors and normal tissues using deformable image registration.
Method and Materials
Twenty one free-breathing stereotactic liver cancer radiotherapy patients, planned on static exhale CT for 27 – 60 Gy in 6 fractions, were included. A biomechanical model-based deformable image registration algorithm, retrospectively deformed each exhale CT to inhale CT. This deformation map was combined with exhale and inhale dose grids from the treatment planning system to accumulate dose over the breathing cycle. Accumulation was also investigated using a simple rigid liver-to-liver registration. Changes to tumor and normal tissue dose were quantified.
Relative to static plans, mean dose change (range) after deformable dose accumulation (as % of prescription dose) was −1 (−14, 8) to minimum tumor, −4 (−15, 0) to max bowel, −4 (−25, 1) to max duodenum, 2 (−1, 9) to max esophagus, −2 (−13, 4) to max stomach, 0 (−3, 4) to mean liver, and −1 (−5, 1) and −2 (−7, 1) to mean left and right kidneys. Compared to deformable registration, rigid modeling had changes up to 8% to minimum tumor and 7% to maximum normal tissues.
Deformable registration and dose accumulation revealed potentially significant dose changes to either a tumor or normal tissue in the majority of cases due to breathing motion. These changes may not be accurately accounted for with rigid motion.
PMCID: PMC3010501  PMID: 20732755
Deformable image registration; respiratory motion; 4D dose calculations; stereotactic body radiotherapy; liver cancer
2.  Navigator channel adaptation to reconstruct three dimensional heart volumes from two dimensional radiotherapy planning data 
BMC Medical Physics  2012;12:1.
Biologically-based models that utilize 3D radiation dosimetry data to estimate the risk of late cardiac effects could have significant utility for planning radiotherapy in young patients. A major challenge arises from having only 2D treatment planning data for patients with long-term follow-up. In this study, we evaluate the accuracy of an advanced deformable image registration (DIR) and navigator channels (NC) adaptation technique to reconstruct 3D heart volumes from 2D radiotherapy planning images for Hodgkin's Lymphoma (HL) patients.
Planning CT images were obtained for 50 HL patients who underwent mediastinal radiotherapy. Twelve image sets (6 male, 6 female) were used to construct a male and a female population heart model, which was registered to 23 HL "Reference" patients' CT images using a DIR algorithm, MORFEUS. This generated a series of population-to-Reference patient specific 3D deformation maps. The technique was independently tested on 15 additional "Test" patients by reconstructing their 3D heart volumes using 2D digitally reconstructed radiographs (DRR). The technique involved: 1) identifying a matching Reference patient for each Test patient using thorax measurements, 2) placement of six NCs on matching Reference and Test patients' DRRs to capture differences in significant heart curvatures, 3) adapting the population-to-Reference patient-specific deformation maps to generate population-to-Test patient-specific deformation maps using linear and bilinear interpolation methods, 4) applying population-to-Test patient specific deformation to the population model to reconstruct Test-patient specific 3D heart models. The percentage volume overlap between the NC-adapted reconstruction and actual Test patient's true heart volume was calculated using the Dice coefficient.
The average Dice coefficient expressed as a percentage between the NC-adapted and actual Test model was 89.4 ± 2.8%. The modified NC adaptation technique made significant improvements to the population deformation heart models (p = 0.01). As standard evaluation, the residual Dice error after adaptation was comparable to the volumetric differences observed in free-breathing heart volumes (p = 0.62).
The reconstruction technique described generates accurate 3D heart models from limited 2D planning data. This development could potentially be used to retrospectively calculate delivered dose to the heart for historically treated patients and thereby provide a better understanding of late radiation-related cardiac effects.
PMCID: PMC3398341  PMID: 22257738
3.  Comparison of simple and complex liver intensity modulated radiotherapy 
Intensity-modulated radiotherapy (IMRT) may allow improvement in plan quality for treatment of liver cancer, however increasing radiation modulation complexity can lead to increased uncertainties and requirements for quality assurance. This study assesses whether target coverage and normal tissue avoidance can be maintained in liver cancer intensity-modulated radiotherapy (IMRT) plans by systematically reducing the complexity of the delivered fluence.
An optimal baseline six fraction individualized IMRT plan for 27 patients with 45 liver cancers was developed which provided a median minimum dose to 0.5 cc of the planning target volume (PTV) of 38.3 Gy (range, 25.9-59.5 Gy), in 6 fractions, while maintaining liver toxicity risk <5% and maximum luminal gastrointestinal structure doses of 30 Gy. The number of segments was systematically reduced until normal tissue constraints were exceeded while maintaining equivalent dose coverage to 95% of PTV (PTVD95). Radiotherapy doses were compared between the plans.
Reduction in the number of segments was achieved for all 27 plans from a median of 48 segments (range 34-52) to 19 segments (range 6-30), without exceeding normal tissue dose objectives and maintaining equivalent PTVD95 and similar PTV Equivalent Uniform Dose (EUD(-20)) IMRT plans with fewer segments had significantly less monitor units (mean, 1892 reduced to 1695, p = 0.012), but also reduced dose conformity (mean, RTOG Conformity Index 1.42 increased to 1.53 p = 0.001).
Tumour coverage and normal tissue objectives were maintained with simplified liver IMRT, at the expense of reduced conformity.
PMCID: PMC3003186  PMID: 21114865
4.  Comparison of Localization Performance with Implanted Fiducial Markers and Cone-Beam Computed Tomography for On-line Image-Guided Radiotherapy of the Prostate 
To assess the accuracy of kV cone-beam CT (CBCT) based setup corrections as compared to orthogonal MV portal image-based corrections for patients undergoing external-beam radiotherapy of the prostate.
Method and Materials
Daily cone-beam CT volumetric images were acquired after setup for patients with three intra-prostatic fiducial markers. The estimated couch shifts were compared retrospectively to patient adjustments based on two orthogonal MV portal images (the current clinical standard of care in our institution). The CBCT soft-tissue based shifts were also estimated by digitally removing the gold markers in each projection to suppress the artifacts in the reconstructed volumes. A total of 256 volumetric images for 15 patients were analyzed.
The Pearson coefficient of correlation for the patient position shifts using fiducial markers in MV vs kV was (R2 = 0.95, 0.84, 0.81) in the L/R, A/P and S/I directions respectively. The correlation using soft-tissue matching was ((R2 = 0.90, 0.49, 0.51) in the L/R, A/P and S/I directions. A Bland-Altman analysis showed no significant trends in the data. The percentage of shifts within a +/−3mm tolerance (the clinical action level) was (99.7, 95.5, 91.3) for fiducial marker matching and (99.5, 70.3, 78.4) for soft-tissue matching.
Cone-beam CT is an accurate and precise tool for image-guidance. It provides an equivalent means of patient setup correction for prostate patients with implanted gold fiducial markers. Use of the additional information provided by the visualization of soft-tissue structures is an active area of research.
PMCID: PMC1906849  PMID: 17293243
cone-beam CT; image-guided; prostate radiotherapy; fiducial markers; surrogates

Results 1-4 (4)