title: The use of isodose levels to interpret radiation induced lung injury: a quantitative analysis of computed tomography changes.
abstract: BACKGROUND
  Patients treated with stereotactic body radiation therapy (SBRT) for lung cancer are often found to have radiation-induced lung injury (RILI) surrounding the treated tumor. We investigated whether treatment isodose levels could predict RILI.
  
  
  METHODS
  Thirty-seven lung lesions in 32 patients were treated with SBRT and received post-treatment follow up (FU) computed tomography (CT). Each CT was fused with the original simulation CT and treatment isodose levels were overlaid. The RILI surrounding the treated lesion was contoured. The RILI extension index [fibrosis extension index (FEI)] was defined as the volume of RILI extending outside a given isodose level relative to the total volume of RILI and was expressed as a percentage.
  
  
  RESULTS
  Univariate analysis revealed that the planning target volume (PTV) was positively correlated with RILI volume at FU: correlation coefficient (CC) =0.628 and P<0.0001 at 1(st) FU; CE =0.401 and P=0.021 at 2(nd) FU; CE =0.265 and P=0.306 at 3(rd) FU. FEI -40 Gy at 1(st) FU was significantly positively correlated with FEI -40 Gy at subsequent FU's (CC =0.689 and P=6.5×10(-5) comparing 1(st) and 2(nd) FU; 0.901 and P=0.020 comparing 2(nd) and 3(rd) FU. Ninety-six percent of the RILI was found within the 20 Gy isodose line. Sixty-five percent of patients were found to have a decrease in RILI on the second 2(nd) CT.
  
  
  CONCLUSIONS
  We have shown that RILI evolves over time and 1(st) CT correlates well with subsequent CTs. Ninety-six percent of the RILI can be found to occur within the 20 Gy isodose lines, which may prove beneficial to radiologists attempting to distinguish recurrence vs. RILI.
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Miriam A
authors.initials: MA
authors.lastname: Knoll
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Mary
authors.initials: M
authors.lastname: Salvatore
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Ren Dih
authors.initials: RD
authors.lastname: Sheu
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Abraham D
authors.initials: AD
authors.lastname: Knoll
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Kerns
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Yeh-Chi
authors.initials: YC
authors.lastname: Lo
authors.affiliation: 1 Department of Radiation Oncology, 2 Department of Radiology, Mount Sinai School of Medicine, 1 Gustave Levy Place, New York, NY 10029, USA.
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Rosenzweig
keywords: Stereotactic body radiation therapy (SBRT); lung cancer; lung fibrosis; quantitative analysis; radiation side effects; radiation-induced lung injury (RILI)