Regional Normal Liver Tissue Density Changes in Patients Treated with Stereotactic Body Radiation Therapy for Liver Metastases

A quantitative approach to evaluate stereo tactic body radiation therapy (SBRT)-induced normal liver tissue changes in patients with liver metastases was performed. 104 non-contrast treatment follow-up computed topography (CT) scans of 35 patients who received SBRT between 2004 and 2011 were retrospectively analyzed (range, 0.7-36 months, median, 8.1 months). The dose distributions from planning CTs were mapped to follow-up CTs using rigid registration. SBRT-induced normal liver density changes on post-SBRT follow-up CT scans were evaluated at approximately 4, 8, 12, 18, and 36 months. Dose-response curves (DRCs) were generated over the entire patient population by computing the mean Hounsfield unit (HU) in liver regions corresponding to dose bins ranging from 0-55 Gy in 5 Gy intervals. A hypo dense radio logic change in irradiated liver linearly related to dose (slope, -0.13 ÄHU/Gy) was observed, with significant mean CT changes of-9.3 ± 0.64 ÄHU and-9.8 ± 0.75 ÄHU at 45-50 Gy and 50-55 Gy, respectively. Furthermore, the data revealed that SBRT induces this hypo dense radiation reaction with demarcation set by the 30 to 35 Gy iso dose volume.