title: Effect of normal lung definition on lung dosimetry and lung toxicity prediction in radiation therapy treatment planning.
abstract: OBJECTIVE
  This study aimed to compare lung dose-volume histogram (DVH) parameters such as mean lung dose (MLD) and the lung volume receiving ≥20 Gy (V20) of commonly used definitions of normal lung in terms of tumor/target subtraction and to determine to what extent they differ in predicting radiation pneumonitis (RP).
  
  
  METHODS
  One hundred lung cancer patients treated with definitive radiation therapy were assessed. The gross tumor volume (GTV) and clinical planning target volume (PTVc) were defined by the treating physician and dosimetrist. For this study, the clinical target volume (CTV) was defined as GTV with 8-mm uniform expansion, and the PTV was defined as CTV with an 8-mm uniform expansion. Lung DVHs were generated with exclusion of targets: (1) GTV (DVHG); (2) CTV (DVHC); (3) PTV (DVHP); and (4) PTVc (DVHPc). The lung DVHs, V20s, and MLDs from each of the 4 methods were compared, as was their significance in predicting radiation pneumonitis of grade 2 or greater (RP2).
  
  
  RESULTS
  There are significant differences in dosimetric parameters among the various definition methods (all Ps<.05). The mean and maximum differences in V20 are 4.4% and 12.6% (95% confidence interval 3.6%-5.1%), respectively. The mean and maximum differences in MLD are 3.3 Gy and 7.5 Gy (95% confidence interval, 1.7-4.8 Gy), respectively. MLDs of all methods are highly correlated with each other and significantly correlated with clinical RP2, although V20s are not. For RP2 prediction, on the receiver operating characteristic curve, MLD from DVHG (MLDG) has a greater area under curve of than MLD from DVHC (MLDC) or DVHP (MLDP). Limiting RP2 to 30%, the threshold is 22.4, 20.6, and 18.8 Gy, for MLDG, MLDC, and MLDP, respectively.
  
  
  CONCLUSIONS
  The differences in MLD and V20 from various lung definitions are significant. MLD from the GTV exclusion method may be more accurate in predicting clinical significant radiation pneumonitis.
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, Michigan, USA.
authors.forename: Weili
authors.initials: W
authors.lastname: Wang
authors.forename: Yaping
authors.initials: Y
authors.lastname: Xu
authors.forename: Matthew
authors.initials: M
authors.lastname: Schipper
authors.forename: Martha M
authors.initials: MM
authors.lastname: Matuszak
authors.forename: Timothy
authors.initials: T
authors.lastname: Ritter
authors.forename: Yue
authors.initials: Y
authors.lastname: Cao
authors.forename: Randall K
authors.initials: RK
authors.lastname: Ten Haken
authors.forename: Feng-Ming Spring
authors.initials: FM
authors.lastname: Kong
mesh_terms: D000368:Aged; D002289:Carcinoma, Non-Small-Cell Lung / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D005260:Female; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging / Q000528:radiation effects*; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D008297:Male; D017564:Radiation Pneumonitis / Q000000981:diagnostic imaging / Q000209:etiology*; D011859:Radiography; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted; D020266:Radiotherapy, Conformal; D018709:Statistics, Nonparametric; D047368:Tumor Burden