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., lungcancer or left-sided breast cancer). The presence of SPECT perfusion defects is determined by comparing pre- and post-RT SPECTimages. 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 SPECTphoton 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 SPECTimages was estimated using simulation data from a female SPECT thorax phantom with varying tissue densities.Results:
Pre- and serial post-RT CTimages were quantitatively studied in 23 patients (4 breast cancer, 19 lungcancer). 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 lungcancer 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 SPECTlung 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 SPECTlung or heart scans. Further, there was no clear dose response of thesoft tissue density changes. Ultimately, the authors findings suggest that prior perfusion reports do reflect changes in the physiology of the lungs and heart.
This work is supported in part by grants from the Department of Defense (17-98-1-8071 and BC010663), the Lance Armstrong Foundation, and the National Institutes of Health (NIH) (R01-CA69579).
II. METHODS AND MATERIALS
- Computed tomography
- Single photon emission computed tomography
- Medical imaging
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