title: PET CT thresholds for radiotherapy target definition in non-small-cell lung cancer: how close are we to the pathologic findings?
abstract: OBJECTIVE
  Optimal target delineation threshold values for positron emission tomography (PET) and computed tomography (CT) radiotherapy planning is controversial. In this present study, different PET CT threshold values were used for target delineation and then compared pathologically.
  
  
  METHODS
  A total of 31 non-small-cell lung cancer patients underwent PET CT before surgery. The maximal diameter (MD) of the pathologic primary tumor was obtained. The CT-based gross tumor volumes (GTV(CT)) were delineated for CT window-level thresholds at 1,600 and -300 Hounsfield units (HU) (GTV(CT1)); 1,600 and -400 (GTV(CT2)); 1,600 and -450 HU (GTV(CT3)); 1,600 and -600 HU (GTV(CT4)); 1,200 and -700 HU (GTV(CT5)); 900 and -450 HU (GTV(CT6)); and 700 and -450 HU (GTV(CT7)). The PET-based GTVs (GTV(PET)) were autocontoured at 20% (GTV(20)), 30% (GTV(30)), 40% (GTV(40)), 45% (GTV(45)), 50% (GTV(50)), and 55% (GTV(55)) of the maximal intensity level. The MD of each image-based GTV in three-dimensional orientation was determined. The MD of the GTV(PET) and GTV(CT) were compared with the pathologically determined MD.
  
  
  RESULTS
  The median MD of the GTV(CT) changed from 2.89 (GTV(CT2)) to 4.46 (GTV(CT7)) as the CT thresholds were varied. The correlation coefficient of the GTV(CT) compared with the pathologically determined MD ranged from 0.76 to 0.87. The correlation coefficient of the GTV(CT1) was the best (r=0.87). The median MD of GTV(PET) changed from 5.72 cm to 2.67 cm as the PET thresholds increased. The correlation coefficient of the GTV(PET) compared with the pathologic finding ranged from 0.51 to 0.77. The correlation coefficient of GTV(50) was the best (r=0.77).
  
  
  CONCLUSIONS
  Compared with the MD of GTV(PET), the MD of GTV(CT) had better correlation with the pathologic MD. The GTV(CT1) and GTV(50) had the best correlation with the pathologic results.
authors.affiliation: Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada.
authors.forename: Kailiang
authors.initials: K
authors.lastname: Wu
authors.forename: Yee C
authors.initials: YC
authors.lastname: Ung
authors.forename: Jennifer
authors.initials: J
authors.lastname: Hornby
authors.forename: Marc
authors.initials: M
authors.lastname: Freeman
authors.forename: David
authors.initials: D
authors.lastname: Hwang
authors.forename: Ming S
authors.initials: MS
authors.lastname: Tsao
authors.forename: Max
authors.initials: M
authors.lastname: Dahele
authors.forename: Gail
authors.initials: G
authors.lastname: Darling
authors.forename: Donna E
authors.initials: DE
authors.lastname: Maziak
authors.forename: Romeo
authors.initials: R
authors.lastname: Tirona
authors.forename: Kathy
authors.initials: K
authors.lastname: Mah
authors.forename: C Shun
authors.initials: CS
authors.lastname: Wong
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy; D005260:Female; D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy; D008297:Male; D008875:Middle Aged; D049268:Positron-Emission Tomography / Q000379:methods* / Q000592:standards; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods; D014057:Tomography, X-Ray Computed / Q000379:methods* / Q000592:standards; D047368:Tumor Burden