title: Assessing the impact of radiation-induced changes in soft tissue density ∕ thickness on the study of radiation-induced perfusion changes in the lung and heart.
abstract: OBJECTIVE
  Abnormalities in single photon emission computed tomography (SPECT) perfusion within the lung and heart are often detected following radiation for tumors in∕around the thorax (e.g., lung cancer or left-sided breast cancer). The presence of SPECT perfusion defects is determined by comparing pre- and post-RT SPECT images. However, RT may increase the density of the soft tissue surrounding the lung∕heart (e.g., chest wall∕breast) that could possibly lead to an "apparent" SPECT perfusion defect due to increased attenuation of emitted photons. Further, increases in tissue effective depth will also increase SPECT photon attenuation and may lead to "apparent" SPECT perfusion defects. The authors herein quantitatively assess the degree of density changes and effective depth in soft tissues following radiation in a series of patients on a prospective clinical study.
  
  
  METHODS
  Patients receiving thoracic RT were enrolled on a prospective clinical study including pre- and post-RT thoracic computed tomography (CT) scans. Using image registration, changes in tissue density and effective depth within the soft tissues were quantified (as absolute change in average CT Hounsfield units, HU, or tissue thickness, cm). Changes in HU and tissue effective depth were considered as a continuous variable. The potential impact of these tissue changes on SPECT images was estimated using simulation data from a female SPECT thorax phantom with varying tissue densities.
  
  
  RESULTS
  Pre- and serial post-RT CT images were quantitatively studied in 23 patients (4 breast cancer, 19 lung cancer). Data were generated from soft tissue regions receiving doses of 20-50 Gy. The average increase in density of the chest was 5 HU (range 46 to -69). The average change in breast density was a decrease of -1 HU (range 13 to -13). There was no apparent dose response in neither the dichotomous nor the continuous analysis. Seventy seven soft tissue contours were created for 19 lung cancer patients. The average change in tissue effective depth was +0.2 cm (range -1.9 to 2.2 cm). The changes in HU represent a <2% average change in tissue density. Based on simulation, the small degree of density and tissue effective depth change is unlikely to yield meaningful changes in either SPECT lung or heart perfusion.
  
  
  CONCLUSIONS
  RT doses of 20-50 Gy can cause up to a 46 HU increase in soft tissue density 6 months post-RT. Post-RT soft tissue effective depth may increase by 2.0 cm. These modest increases in soft tissue density and effective depth are unlikely to be responsible for the perfusion changes seen on post-RT SPECT lung or heart scans. Further, there was no clear dose response of the soft tissue density changes. Ultimately, the authors findings suggest that prior perfusion reports do reflect changes in the physiology of the lungs and heart.
authors.affiliation: Department of Radiation Oncology, University of North Carolina, Chapel Hill, NC, USA. michael_lawrence@med.unc.edu
authors.forename: Michael V
authors.initials: MV
authors.lastname: Lawrence
authors.forename: Mert
authors.initials: M
authors.lastname: Saynak
authors.forename: David V
authors.initials: DV
authors.lastname: Fried
authors.forename: Ted A
authors.initials: TA
authors.lastname: Bateman
authors.forename: Rebecca L
authors.initials: RL
authors.lastname: Green
authors.forename: Jessica L
authors.initials: JL
authors.lastname: Hubbs
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Jaszczak
authors.forename: Terence Z
authors.initials: TZ
authors.lastname: Wong
authors.forename: Sumin
authors.initials: S
authors.lastname: Zhou
authors.forename: Shiva K
authors.initials: SK
authors.lastname: Das
authors.forename: Lawrence B
authors.initials: LB
authors.lastname: Marks
mesh_terms: D001783:Blood Flow Velocity / Q000528:radiation effects; D003198:Computer Simulation; D003720:Densitometry / Q000379:methods*; D006321:Heart / Q000503:physiopathology* / Q000528:radiation effects*; D006801:Humans; D008168:Lung / Q000503:physiopathology* / Q000528:radiation effects*; D008954:Models, Biological