title: Effect of deformable registration on the dose calculated in radiation therapy planning CT scans of lung cancer patients.
abstract: OBJECTIVE
  To characterize the effects of deformable image registration of serial computed tomography (CT) scans on the radiation dose calculated from a treatment planning scan.
  
  
  METHODS
  Eighteen patients who received curative doses (≥ 60 Gy, 2 Gy/fraction) of photon radiation therapy for lung cancer treatment were retrospectively identified. For each patient, a diagnostic-quality pretherapy (4-75 days) CT scan and a treatment planning scan with an associated dose map were collected. To establish correspondence between scan pairs, a researcher manually identified anatomically corresponding landmark point pairs between the two scans. Pretherapy scans then were coregistered with planning scans (and associated dose maps) using the demons deformable registration algorithm and two variants of the Fraunhofer MEVIS algorithm ("Fast" and "EMPIRE10"). Landmark points in each pretherapy scan were automatically mapped to the planning scan using the displacement vector field output from each of the three algorithms. The Euclidean distance between manually and automatically mapped landmark points (dE) and the absolute difference in planned dose (|ΔD|) were calculated. Using regression modeling, |ΔD| was modeled as a function of dE, dose (D), dose standard deviation (SD(dose)) in an eight-pixel neighborhood, and the registration algorithm used.
  
  
  RESULTS
  Over 1400 landmark point pairs were identified, with 58-93 (median: 84) points identified per patient. Average |ΔD| across patients was 3.5 Gy (range: 0.9-10.6 Gy). Registration accuracy was highest using the Fraunhofer MEVIS EMPIRE10 algorithm, with an average dE across patients of 5.2 mm (compared with >7 mm for the other two algorithms). Consequently, average |ΔD| was also lowest using the Fraunhofer MEVIS EMPIRE10 algorithm. |ΔD| increased significantly as a function of dE (0.42 Gy/mm), D (0.05 Gy/Gy), SD(dose) (1.4 Gy/Gy), and the algorithm used (≤ 1 Gy).
  
  
  CONCLUSIONS
  An average error of <4 Gy in radiation dose was introduced when points were mapped between CT scan pairs using deformable registration, with the majority of points yielding dose-mapping error <2 Gy (approximately 3% of the total prescribed dose). Registration accuracy was highest using the Fraunhofer MEVIS EMPIRE10 algorithm, resulting in the smallest errors in mapped dose. Dose differences following registration increased significantly with increasing spatial registration errors, dose, and dose gradient (i.e., SDdose). This model provides a measurement of the uncertainty in the radiation dose when points are mapped between serial CT scans through deformable registration.
authors.affiliation: Department of Radiology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Alexandra R
authors.initials: AR
authors.lastname: Cunliffe
authors.affiliation: Department of Radiation and Cellular Oncology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Clay
authors.initials: C
authors.lastname: Contee
authors.affiliation: Department of Radiology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Samuel G
authors.initials: SG
authors.lastname: Armato
authors.affiliation: Department of Radiology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Bradley
authors.initials: B
authors.lastname: White
authors.affiliation: Department of Radiology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Julia
authors.initials: J
authors.lastname: Justusson
authors.affiliation: Department of Radiation and Cellular Oncology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Renuka
authors.initials: R
authors.lastname: Malik
authors.affiliation: Department of Radiation and Cellular Oncology, The University of Chicago, 5841 South Maryland Avenue, Chicago, Illinois 60637.
authors.forename: Hania A
authors.initials: HA
authors.lastname: Al-Hallaq
mesh_terms: D000368:Aged; D000465:Algorithms; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D003131:Combined Modality Therapy; D005260:Female; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000528:radiation effects; D008297:Male; D008875:Middle Aged; D010363:Pattern Recognition, Automated; D017785:Photons / Q000627:therapeutic use; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods; D012044:Regression Analysis; D012189:Retrospective Studies; D055752:Small Cell Lung Carcinoma / Q000000981:diagnostic imaging* / Q000532:radiotherapy*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D014584:User-Computer Interface