title: Geographic miss of lung tumours due to respiratory motion: a comparison of 3D vs 4D PET/CT defined target volumes.
abstract: BACKGROUND
  PET/CT scans acquired in the radiotherapy treatment position are typically performed without compensating for respiratory motion. The purpose of this study was to investigate geographic miss of lung tumours due to respiratory motion for target volumes defined on a standard 3D-PET/CT.
  
  
  METHODS
  29 patients staged for pulmonary malignancy who completed both a 3D-PET/CT and 4D-PET/CT were included. A 3D-Gross Tumour Volume (GTV) was defined on the standard whole body PET/CT scan. Subsequently a 4D-GTV was defined on a 4D-PET/CT MIP. A 5 mm, 10 mm, 15 mm symmetrical and 15×10 mm asymmetrical Planning Target Volume (PTV) was created by expanding the 3D-GTV and 4D-GTV's. A 3D conformal plan was generated and calculated to cover the 3D-PTV. The 3D plan was transferred to the 4D-PTV and analysed for geographic miss. Three types of miss were measured. Type 1: any part of the 4D-GTV outside the 3D-PTV. Type 2: any part of the 4D-PTV outside the 3D-PTV. Type 3: any part of the 4D-PTV receiving less than 95% of the prescribed dose. The lesion motion was measured to look at the association between lesion motion and geographic miss.
  
  
  RESULTS
  When a standard 15 mm or asymmetrical PTV margin was used there were 1/29 (3%) Type 1 misses. This increased 7/29 (24%) for the 10 mm margin and 23/29 (79%) for a 5 mm margin. All patients for all margins had a Type 2 geographic miss. There was a Type 3 miss in 25 out of 29 cases in the 5, 10, and 15 mm PTV margin groups. The asymmetrical margin had one additional Type 3 miss. Pearson analysis showed a correlation (p < 0.01) between lesion motion and the severity of the different types of geographic miss.
  
  
  CONCLUSIONS
  Without any form of motion suppression, the current standard of a 3D- PET/CT and 15 mm PTV margin employed for lung lesions has an increasing risk of significant geographic miss when tumour motion increases. Use of smaller asymmetric margins in the cranio-caudal direction does not comprise tumour coverage. Reducing PTV margins for volumes defined on 3D-PET/CT will greatly increase the chance and severity of a geometric miss due to respiratory motion. 4D-imaging reduces the risk of geographic miss across the population of tumour sizes and magnitude of motion investigated in the study.
authors.affiliation: Department of Medical Imaging and Radiation Science, Monash University, East Melbourne, Victoria, Australia. Jason.Callahan@petermac.org.|Peter MacCallum Cancer Centre, Centre for Molecular Imaging, St Andrews Place, East Melbourne, Victoria, Australia. Jason.Callahan@petermac.org.
authors.forename: Jason
authors.initials: J
authors.lastname: Callahan
authors.affiliation: Department of Medical Imaging and Radiation Science, Monash University, East Melbourne, Victoria, Australia. tomas.kron@petermac.org.|Sir Peter MacCallum Department of Oncology, The University of Melbourne, East Melbourne, Victoria, Australia. tomas.kron@petermac.org.
authors.forename: Tomas
authors.initials: T
authors.lastname: Kron
authors.affiliation: Sir Peter MacCallum Department of Oncology, The University of Melbourne, East Melbourne, Victoria, Australia. shankar.siva@petermac.org.|Division of Radiation Oncology, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia. shankar.siva@petermac.org.
authors.forename: Shankar
authors.initials: S
authors.lastname: Siva
authors.affiliation: Peter MacCallum Cancer Centre, Centre for Molecular Imaging, St Andrews Place, East Melbourne, Victoria, Australia. n.c.simoens@student.ru.nl.|University of Nijmegen, Netherlands, Nijmegen, Netherlands. n.c.simoens@student.ru.nl.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Simoens
authors.affiliation: Division of Radiation Oncology, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia. amanda.edger@petermac.org.
authors.forename: Amanda
authors.initials: A
authors.lastname: Edgar
authors.affiliation: Department of Medical Imaging and Radiation Science, Monash University, East Melbourne, Victoria, Australia. sarah.everitt@petermac.org.|Sir Peter MacCallum Department of Oncology, The University of Melbourne, East Melbourne, Victoria, Australia. sarah.everitt@petermac.org.|Division of Radiation Oncology, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia. sarah.everitt@petermac.org.
authors.forename: Sarah
authors.initials: S
authors.lastname: Everitt
authors.affiliation: Department of Medical Imaging and Radiation Science, Monash University, East Melbourne, Victoria, Australia. michal.schneider@monash.edu.
authors.forename: Michal E
authors.initials: ME
authors.lastname: Schneider
authors.affiliation: Peter MacCallum Cancer Centre, Centre for Molecular Imaging, St Andrews Place, East Melbourne, Victoria, Australia. rod.hicks@petermac.org.|Sir Peter MacCallum Department of Oncology, The University of Melbourne, East Melbourne, Victoria, Australia. rod.hicks@petermac.org.
authors.forename: Rodney J
authors.initials: RJ
authors.lastname: Hicks
mesh_terms: D016477:Artifacts; D056973:Four-Dimensional Computed Tomography; D006801:Humans; D021621:Imaging, Three-Dimensional; D008175:Lung Neoplasms / Q000532:radiotherapy*; D009038:Motion; D064847:Multimodal Imaging; D049268:Positron-Emission Tomography; D011880:Radiotherapy Planning, Computer-Assisted; D012119:Respiration; D014057:Tomography, X-Ray Computed