Purpose: The prostate can be localized automatically on daily cone‐beam CT (CBCT) images by means of intensity‐based 3D/3D image registrations. The goal of this study is to evaluate nine different similarity metrics for the best localization accuracy. Method and Materials: Seven prostate patients were retrospectively selected with natural prostate calcifications, which resulted in a dataset of 220 CBCT and 7 simulation CT (SimCT) images. On each SimCT image, a region of interest (ROI) was defined by expanding the treatment planning prostate structure by 6 mm and subtracting the rectum with 10 mm expansion. The expansions were chosen empirically. Then, all CBCT images were registered to the corresponding SimCT images by maximizing the similarity within the defined ROI. Downhill simplex method was employed for optimization with full 6D rigid‐body transformation. The pixel values corresponding to bone and air were also excluded from similarity calculations. After each registration, the calcification mismatch was recorded as a measure of prostate localization error. The registrations were repeated for each similarity metric. The nine similarity metrics investigated were: normalized cross‐correlation (NCC) , entropy of difference (EOD) , mutual information (MI) , correlation ratio (CR) , sum of absolute difference (SAD) , sum of squared difference (SSD) , gradient correlation (GC) , gradient difference (GD) , and pattern intensity (PI) . Results: The absolute error means (±standard deviations) were 2.1(±1.6) mm for GD and GC , 2.6(±2.8) mm for PI , 3.9(±4.3) mm for NCC , and 6.4 (±8.0) mm for MI. The others showed over 10 mm mean errors. The directional errors of GD were −0.1(±0.5) mm, 0.1(±1.4) mm , and −0.2(±2.1) mm in the left‐right, anterior‐posterior, and superior/inferior directions respectively. Conclusion: Among the investigated similarity metrics, registrations using GD and GC showed the most accurate prostate localizations on daily CBCT images.

Purpose: A recent study by Korreman et al (IJROBP, vol 65, 1375–1380, 2006) demonstrates possible >85% relative reduction of cardiac-pulmonary complications for tangential breast irradiation by using deep inspiration breath-hold or gating techniques. The aim of this work is to minimize the risks by correcting the breast displacements using real-time stereovision guidance. Materials/Methods: Twenty breast-cancer patients were accrued over the last seventeen months on an IRB-approved study. Planning target volume (PTV) was encompassed by the prescribed isodose in tangential beams with intensity modulation. The surgical bed was boosted using an electron field or conformal photon beams. The planning information (include CT images) were transferred to an in-house stereovision-guided program which automatically created the reference surface images from the CT-based plans. The reference surfaces were matched with the real-time surface images, captured with 3D cameras mounted in the CT simulation room and treatment vault, to determine the displacements of CT setup markers, daily initial setup isocenter, final target position, and target motion during treatment. Large and significant position error were corrected based the image-guidance. Images and on-line adjustments were automatically stored for the analysis. Portal images were taken weekly for the position verification. Results: The 3D isocenter displacements improved from ∼10 mm to ∼ 4 mm after the application of IGRT. Isocenter setup error and simulation-marking error were detected and corrected. All major changes were confirmed with portal images. The intrafractional motion caused only ∼2.0 mm target displacement. The entire procedure (including the table shifts) took < 5 minutes per day. Conclusions: The clinical results demonstrate improvement for breast target positioning using stereovision guidance. This in-house IGRT for BC is clinically feasible, efficient, and accurate. Research initially sponsored by NIH SBIR-1R43CA91690-01 and CA-88843 grants.