Calcifications are potential surrogates for prostate localization in image-guided radiotherapy

Purpose To investigate the feasibility of using calcifications as surrogates for the prostate position during cone-beam computed tomography (CBCT) image-guided radiotherapy. Methods and Materials The twice-weekly CBCT images taken during the treatment course of 4 patients were retrospectively studied for the stability of the calcifications. The geometric center of three fiducial markers was used as the reference. The planning CT images of 131 prostate patients recently treated with external beam radiotherapy at our center were reviewed to estimate the calcification occurrence rate. Analysis was conducted using the Varian Eclipse treatment planning system. Two patients were treated using prostate calcifications as the landmark in on-line registration. Both the Varian standard and the low-dose CBCT modes were used for imaging. Results The calcifications were found to be stable during the treatment course. At the 95% confidence interval, the difference between the distance from an identified calcification to the fiducial markers on CBCT and the distance on the planning CT scans was 0.2 ± 2.0 mm, 0.8 ± 2.2 mm, and 0.4 ± 2.4 mm in the left–right, anteroposterior, and superoinferior direction, respectively. Of the 131 patients, 46 (35%) had well-defined calcifications either inside the prostate or near the borders. Our experience in treating the first 2 patients demonstrated that the calcifications are easily distinguished on low-dose scans and that calcification registration can be precisely performed. Conclusion The results of our study have shown that calcifications can be reliable markers of prostate position and allow for precise image guidance with a low-imaging dose. With this approach, potentially about one-third of prostate patients could benefit from precise image guidance without the invasive use of markers.