An analysis of intraoperative versus post-operative dosimetry with CT, CT-MRI fusion and XMR for the evaluation of permanent prostate brachytherapy implants.

BACKGROUND AND PURPOSE: To assess the agreement between intraoperative and post-operative dosimetry and to identify factors that influence dose calculations of prostate brachytherapy implants. MATERIALS AND METHODS: Patients treated with prostate brachytherapy implants underwent post-operative CT and XMR (combined X-ray and MR) imaging. Dose-volume histograms were calculated from CT, XMR and CT-MR fusion data and compared with intraoperative values for two observers. Multiple linear regression models assessed the influences of intraoperative D90, gland oedema, gland volume, source loss and migration, and implanted activity/volume prostate on post-operative D90. RESULTS: Forty-nine patients were studied. The mean D90 differences (95% confidence limits) between intraoperative and post-operative CT, XMR and CT-MR fusion assessments were: 11 Gy (-22, 45), 18 Gy (-13, 49) and 20 Gy (-17, 58) for Observer 1; and 15 Gy (-34, 63), 13 Gy (-29, 55) and 14 Gy (-27, 54) for Observer 2. Multiple linear regression modelling showed that the observed oedema and intraoperative D90 were significant independent variables for the prediction of post-operative D90 values for both observers using all modalities. CONCLUSION: This is the first study to report Bland-Altman agreement analysis between intraoperative and post-operative dosimetry. Agreement is poor. Post-operative dosimetry is dependent on the intraoperative D90 and the subjectively outlined gland volume.

Comparison of combined x-ray radiography and magnetic resonance (XMR) imaging-versus computed tomography-based dosimetry for the evaluation of permanent prostate brachytherapy implants.

PURPOSE: To present a method for the dosimetric analysis of permanent prostate brachytherapy implants using a combination of stereoscopic X-ray radiography and magnetic resonance (MR) imaging (XMR) in an XMR facility, and to compare the clinical results between XMR- and computed tomography (CT)-based dosimetry. METHODS AND MATERIALS: Patients who had received nonstranded iodine-125 permanent prostate brachytherapy implants underwent XMR and CT imaging 4 weeks later. Four observers outlined the prostate gland on both sets of images. Dose-volume histograms (DVHs) were derived, and agreement was compared among the observers and between the modalities. RESULTS: A total of 30 patients were evaluated. Inherent XMR registration based on prior calibration and optical tracking required a further automatic seed registration step that revealed a median root mean square registration error of 4.2 mm (range, 1.6-11.4). The observers agreed significantly more closely on prostate base and apex positions as well as outlining contours on the MR images than on those from CT. Coefficients of variation were significantly higher for observed prostate volumes, D90, and V100 parameters on CT-based dosimetry as opposed to XMR. The XMR-based dosimetry showed little agreement with that from CT for all observers, with D90 95% limits of agreement ranges of 65, 118, 79, and 73 Gy for Observers 1, 2, 3, and 4, respectively. CONCLUSIONS: The study results showed that XMR-based dosimetry offers an alternative to other imaging modalities and registration methods with the advantages of MR-based prostate delineation and confident three-dimensional reconstruction of the implant. The XMR-derived dose-volume histograms differ from the CT-derived values and demonstrate less interobserver variability.