Can Increases in Soft Tissue Thickness Lead to Attenuation Artifacts Causing “Apparent” Cardiopulmonary Perfusion Defects Post-RT?

Abstract

Purpose/Objective(s): Abnormalities in 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 are determined by comparing preand post-RT SPECT images. However, the thickness of the soft tissue surrounding the lung/heart (e.g. chest wall/breast) may increase, possibly leading to an “apparent” SPECT perfusion defect due to increased attenuation of emitted photons. We herein quantitatively assess the degree of thickness changes in soft tissues following radiation in a series of patients on a prospective clinical study. Materials/Methods: Patients receiving thoracic RT were enrolled on a prospective clinical study including preand post-RT thoracic CT scans. Using image registration, changes in the thickness of soft tissues of the anterior, posterior, right and left lateral chestwalls (scored as percent change from baseline) were quantified. Previous simulation data revealed that a 20 30% increase in attenuation is needed to cause apparent perfusion defects. A prior study of 23 patients with preand 6 months post-RT CT scans showed that soft tissue density values increase by an average of 2% at 6 months post-RT. Therefore regions with soft tissue thickness increases greater than 20% were scored as having the potential to cause perfusion defects. Results: Preand serial post-RT CT images were quantitatively studied in 20 lung cancer patients. Data was generated from 76 soft tissue contours that were within the RT beams. 34% (26/76) of the chestwall contours had an increase in soft tissue thickness. The overall average change in chest wall thickness was -3%. For the different walls, the average changes were: anterior -2.5% (range -32 to 54), posterior -4% (range -28 to 22%), left 2% (range -18 to 48%), and right -6% (range -42 to 14%). Of the 26 chestwall contours with increases in soft tissue thickness, only 6 were deemed large enough to possibly produce a perfusion defect (i.e. > 20%). Simulations using an increase in tissue attenuation from 10 50% demonstrate that these tissue thickness changes are not sufficient to cause a SPECT perfusion defect. Conclusions: The average changes in soft tissue thickness are unlikely to cause apparent perfusion defects. The modest increases in soft tissue thickness typically seen are unlikely to be responsible for the perfusion changes seen on post-RT SPECT lung or heart scans. Acknowledgement: Supported in part by grants from the Lance Armstrong Foundation, and NIH (R01-CA69579)