title: Defining and assessing an anisotropic delineation margin for modern radiotherapy.
abstract: OBJECTIVE
  Uncertainty in target volume delineation for modern radiotherapy impacts dosimetry and patient outcomes. Delineation uncertainty is generally overlooked in practice as a source of error, potentially since historically, other uncertainties have been the main focus. This work defined and assessed an anisotropic delineation margin in both polar and spherical coordinate systems in order to account for the spatially varying nature of this uncertainty using a whole breast radiotherapy cohort as a proof of concept.
  
  
  METHODS
  A cohort of 21 whole breast radiotherapy patient datasets with clinical target volumes delineated by eight independent observers was utilized. Patients were divided into categories based on target volume and laterality. An anisotropic delineation margin for each category was determined by multiplying the average standard deviation in observer contours in each category by a factor of two. Standard deviation was determined in both polar and spherical coordinates at angular increments. This anisotropic approach was compared to a conventional clinical approach, where the delineation margin was applied in the cardinal directions only. The assessment of the delineation margin was undertaken by comparing the encompassment of the observer volumes by the target volume with added margin. The extra, presumed healthy tissue included in the margin and the malignant tissue missed by the margin were determined.
  
  
  RESULTS
  The proposed delineation margin is effective at accounting for inter-observer variation, producing >95% coverage of all CTVs for polar, spherical, and Cartesian margins in 82%, 79%, and 92% of cases, respectively. Additionally, <1% malignant tissue was missed for 65%, 70%, and 91% of cases and <37% healthy tissue was included in 95%, 89%, and 97% of cases. A conventional delineation margin approach is most appropriate for small and gold standard target volumes. However, for large target volumes, an anisotropic margin is necessary, producing significantly greater coverage of CTVs, including significantly less presumed healthy tissue and missing significantly less malignant tissue.
  
  
  CONCLUSIONS
  All delineation margin methods that account for target volume and laterality proved to be adequate, with appropriate encompassment of interobserver variation and minimal inclusion of extra excess healthy tissue and exclusion of possible malignant tissue. The anisotropic approach was found to be superior to a conventional approach for target volumes >1400 cm3only with significantly greater encompassment of interobserver variation, less missed malignant tissue and less included healthy tissue. This methodology has been validated for a whole breast radiotherapy cohort as a proof of concept, however could be applied to other anatomical sites.
authors.affiliation: Centre for Medical Radiation Physics, University of Wollongong, Wollongong 2522, Australia and Liverpool and Macarthur Cancer Therapy Centres, Ingham Institute, Liverpool 2170, Australia.
authors.forename: L R
authors.initials: LR
authors.lastname: Bell
authors.affiliation: Centre for Medical Radiation Physics, University of Wollongong, Wollongong 2522, Australia; Liverpool and Macarthur Cancer Therapy Centres, Ingham Institute, Liverpool 2170, Australia; and Institute of Medical Physics, University of Sydney, Sydney 2006, Australia.
authors.forename: E M
authors.initials: EM
authors.lastname: Pogson
authors.affiliation: Centre for Medical Radiation Physics, University of Wollongong, Wollongong 2522, Australia and Liverpool and Macarthur Cancer Therapy Centres, Ingham Institute, Liverpool 2170, Australia.
authors.forename: P E
authors.initials: PE
authors.lastname: Metcalfe
authors.affiliation: Centre for Medical Radiation Physics, University of Wollongong, Wollongong 2522, Australia; Liverpool and Macarthur Cancer Therapy Centres, Ingham Institute, Liverpool 2170, Australia; South Western Sydney Clinical School, University of New South Wales, Liverpool 2170, Australia; and Institute of Medical Physics, University of Sydney, Sydney 2006, Australia.
authors.forename: L C
authors.initials: LC
authors.lastname: Holloway
mesh_terms: D016880:Anisotropy; D001943:Breast Neoplasms / Q000532:radiotherapy; D006801:Humans; D011874:Radiometry; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D035501:Uncertainty