Dosimetric Impact of Image-Guided Radiotherapy in Liver Stereotactic Radiotherapy

Introduction: In the absence of volumetric image-guided radiotherapy (IGRT) with or without intravenous contrast, IGRT with two-dimensional (2D) imaging can improve the accuracy and precision of radiation delivery by correcting the largest sources of geometric uncertainty, facilitating the delivery of higher doses to the tumor and/or reduced doses to normal tissues. The purpose of this work was to estimate dosimetric impact of 2D IGRT for patients undergoing breath hold liver stereotactic body radiotherapy (SBRT). Materials/Methods: Offline residual offsets were determined using orthogonal image pairs acquired with patients positioned with external setup marks (non-IGRT) and following IGRT and repositioning (IGRT) for 30 patients treated with 6-fraction liver SBRT. The diaphragm was used as a surrogate for the liver for craniocaudal positioning, and the vertebral bodies for anterioposterior and right-left positioning, with a 3-mm threshold. The planned dose distributions were shifted by the measured IGRT and non-IGRT offsets. Total doses to target volumes and organs at risk (OAR) were calculated and compared to the prescribed plans. Results: A total of 643 images (416-MV electronic portal images; 227 kV cone beam computed tomography projection images) were evaluated. Residual non-IGRT offsets frequently exceeded 3 mm (72%), resulting in clinically significant variations from the prescribed minimum planning target volume dose (mean change -6.5 Gy; P =.0150). The population mean reductions in minimum gross tumor volume doses (standard deviation (sigma) to 0.5 mL with were 7.2 Gy (6.3) and 4.7 Gy (6.1) for non-IGRT and IGRT, respectively. The mean population increase in maximum OAR dose (to 0.5 mL) was largest for bowel (2.7 Gy, sigma = 5.5 Gy) for non-IGRT. Conclusions: IGRT significantly improves concordance of delivered doses with planned doses for liver target volumes and OARs.