Background and purpose: To investigate the accuracy of dwell position detection with a combined electromagnetic tracking (EMT) brachytherapy (BT) system for treatment verification, by quantifying positional errors due to EM field interference in typical pelvic BT clinical settings. Materials and methods: Dedicated prostate and cervix BT phantoms were imaged with CT. For the cervix phantom, the Utrecht applicator + interstitial catheters were used. The implants were reconstructed and treatment plans were created with 270/65 dwell positions for the prostate/cervix phantom. Next, EMT experiments were performed in clinical BT settings using a prototype of a combined EMT/BT system. We quantified positional errors due to EM field interference from surrounding equipment by comparing planned and EMT-measured dwell positions. The mean residual error between planned and EMTmeasured dwell positions was calculated in the prostate interstitial catheters and in the whole cervix implant including the applicator. For the cervix phantom, the analysis was repeated for only the interstitial catheters. Results: Mean residual errors of less than 0.5/0.4 mm in the prostate/cervix catheters were found. For the whole cervix implant including the applicator, large deviations up to 2.4 mm were found. Compared to the interference free set-up, the CT and patient bed environments showed larger residual errors in the interstitial catheters, but residual errors remained <1 mm in all cases. Conclusion: Dwell position detection with the combined system in interstitial catheters is sufficiently accurate to perform EMT-based treatment verification. The effect of EM interference from the surrounding equipment was limited.

Purpose: MRI is increasingly used as a single imaging modality for brachytherapy treatment planning. The presence of a brachytherapy applicator may cause distortions in the images, especially at higher field strengths. Our aim is to develop a procedure to quantify these distortions theoretically for any MR‐sequence and to verify the estimated deformations for clinical sequences. Methods: Image distortions due to perturbation of the B0‐field are proportional to the ratio of the induced frequency shift and the read‐out bandwidth of the applied sequence. By reconstructing a frequency‐shift map from the phase data from a multi‐echo sequence, distortions can be calculated for any MR‐sequence. Verification of this method for estimating distortions was performed by acquiring images with opposing read‐out directions and consequently opposing distortions. The applicator shift can be determined by rigidly matching these images. Clinically, T2W‐TSE‐images are used for this purpose. For pre‐clinical tests, EPI‐sequences with narrow read‐out bandwidth (19.5–47.5Hz), consequently large distortions, were added to the set of clinical MRsequences. To quantify deformations of the Utrecht Interstitial CT/MR applicator (Elekta Brachytherapy) on a Philips Ingenia 3T MRI, pre‐clinical tests were performed in a phantom with the applicator in water, followed by clinical validation. Results: Deformations observed in the narrow bandwidth EPI‐images were well predicted using the frequency‐shift, the latter giving an overestimation up to 30%/up to 1 voxel. For clinically applied MR‐sequences distortions were well below the voxel size. In patient setup distortions determined from the frequency‐shift map were at sub‐voxel level (<0.7mm). Using T2W‐images larger distortions were found (1–2mm). This discrepancy was caused by patient movement between/during acquisition of the T2W‐images with opposing read‐out directions. Conclusion: Phantom experiments demonstrated the feasibility of a clinical procedure for quantification of MR‐image distortions for any MR‐sequence. In a clinical set‐up the distortions from a Utrecht interstitial CT/MR applicator are sub‐voxel level. This work was partially funded by Elekta Brachytherpy

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