Image-guided radiotherapy (IGRT), adaptive radiotherapy (ART), and online reoptimization rely on accurate mapping of the radiation beam isocenter(s) from planning to treatment space. This mapping involves rigid and/or nonrigid registration of planning (pCT) and intratreatment (tCT) CT images. The purpose of this study was to retrospectively compare a fully automatic approach, including a non-rigid step, against a user-directed rigid method implemented in a clinical IGRT protocol for prostate cancer. Isocenters resulting from automatic and clinical mappings were compared to reference isocenters carefully determined in each tCT. Comparison was based on displacements from the reference isocenters and prostate dose-volume histograms (DVHs). Ten patients with a total of 243？tCTs were investigated. Fully automatic registration was found to be as accurate as the clinical protocol but more precise for all patients. The average of the unsigned and offsets and the standard deviations (σ) of the signed offsets computed over all images were (avg. ±？？σ？(mm)): 1.1 ± 1.4, 1.8 ± 2.3, 2.5 ± 3.5 for the clinical protocol and 0.6 ± 0.8, 1.1 ± 1.5 and 1.1 ± 1.4 for the automatic method. No failures or outliers from automatic mapping were observed, while 8 outliers occurred for the clinical protocol. 1. Introduction Image-guided radiotherapy (IGRT) [1], off-line adaptive radiotherapy (ART) [2], and online reoptimization [3] involve pretreatment imaging, taken here to be CT imaging. A procedure held in common by all three methods is registration of the planning (pCT) and treatment (tCT) images to map the planned isocenter to treatment space. Accuracy and precision of this step are important for delivering an accumulated dose distribution that closely matches the treatment plan. Mapping methods involve at least rigid registration. Ideally a nonrigid step would be included to account for differences in organ shape between planning and treatment times (Figure 1). The composite of the rigid, and possibly nonrigid, matrices is then used to map the planned isocenter to the tCT. Figure 1: (a) Axial slice from the pCT showing planning prostate (white) and isocenter (black). (b) Corresponding slice from the tCT showing the prostate (black) segmented by automatic nonrigid model deformation. The rigidly mapped isocenter comes from translating the planning prostate (dim white) to the tCT. The nonrigidly mapped isocenter comes from applying the deformation matrix resulting from autosegmentation to the planned isocenter. Quantitative evaluation of isocenter mapping methods is muddled by
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