title: Dosimetric impact of inter-observer variability for 3D conformal radiotherapy and volumetric modulated arc therapy: the rectal tumor target definition case.
abstract: BACKGROUND
  To assess the dosimetric effect induced by inter-observer variability in target definition for 3D-conformal RT (3DCRT) and volumetric modulated arc therapy by RapidArc (RA) techniques for rectal cancer treatment.
  
  
  METHODS
  Ten patients with rectal cancer subjected to neo-adjuvant RT were randomly selected from the internal database. Four radiation oncologists independently contoured the clinical target volume (CTV) in blind mode. Planning target volume (PTV) was defined as CTV + 7 mm in the three directions. Afterwards, shared guidelines between radiation oncologists were introduced to give general criteria for the contouring of rectal target and the four radiation oncologists defined new CTV following the guidelines. For each patient, six intersections (I) and unions (U) volumes were calculated coupling the contours of the various oncologists. This was repeated for the contours drawn after the guidelines. Agreement Index (AI = I/U) was calculated pre and post guidelines. Two RT plans (one with 3DCRT technique using 3-4 fields and one with RA using a single modulated arc) were optimized on each radiation oncologist's PTV. For each plan the PTV volume receiving at least 95% of the prescribed dose (PTV V95%) was calculated for both target and non-target PTVs.
  
  
  RESULTS
  The inter-operator AI pre-guidelines was 0.57 and was increased up to 0.69 post-guidelines. The maximum volume difference between the various CTV couples, drawn for each patient, passed from 380 ± 147 cm³ to 137 ± 83 cm³ after the introduction of guidelines. The mean percentage for the non-target PTV V95% was 93.7 ± 9.2% before and 96.6 ± 4.9%after the introduction of guidelines for the 3DCRT, for RA the increase was more relevant, passing from 86.5 ± 13.8% (pre) to 94.5 ± 7.5% (post). The OARs were maximally spared with VMAT technique while the variability between pre and post guidelines was not relevant in both techniques.
  
  
  CONCLUSIONS
  The contouring inter-observer variability has dosimetric effects in the PTV coverage. The introduction of guidelines increases the dosimetric consistency for both techniques, with greater improvements for RA technique.
authors.affiliation: Radiation Oncology Department, Humanitas Clinical and Research Center, Rozzano, Milan, Italy.
authors.forename: Francesca
authors.initials: F
authors.lastname: Lobefalo
authors.forename: Mario
authors.initials: M
authors.lastname: Bignardi
authors.forename: Giacomo
authors.initials: G
authors.lastname: Reggiori
authors.forename: Angelo
authors.initials: A
authors.lastname: Tozzi
authors.forename: Stefano
authors.initials: S
authors.lastname: Tomatis
authors.forename: Filippo
authors.initials: F
authors.lastname: Alongi
authors.forename: Antonella
authors.initials: A
authors.lastname: Fogliata
authors.forename: Anna
authors.initials: A
authors.lastname: Gaudino
authors.forename: Piera
authors.initials: P
authors.lastname: Navarria
authors.forename: Luca
authors.initials: L
authors.lastname: Cozzi
authors.forename: Marta
authors.initials: M
authors.lastname: Scorsetti
authors.forename: Pietro
authors.initials: P
authors.lastname: Mancosu
mesh_terms: D005260:Female; D006801:Humans; D008297:Male; D015588:Observer Variation; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D012004:Rectal Neoplasms / Q000532:radiotherapy*