Technical Note: A comparison of point set registration methods for electromagnetic tracking

Accuracy of dwell position detection with a combined electromagnetic tracking brachytherapy system for treatment verification in pelvic brachytherapy

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 EMT-measured 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.

Estimation of inter-fractional variations in interstitial multi-catheter breast brachytherapy using a hybrid treatment delivery system

PURPOSE

Irradiation of the tumor bed using interstitial multi-catheter brachytherapy is one of the treatment options for breast cancer patients. In order to ensure the planned dose delivery an advanced quality intervention method using an electromagnetic tracking (EMT) system is presented. The system is used to assess inter-fractional variations within the framework of a patient study.

METHODS AND MATERIALS

Until now 41 patients were included in the study for the evaluation and overall 355 EMT measurements were performed. The catheter traces are measured automatically and sequentially using an afterloader prototype (Flexitron, Elekta, Veenendaal, The Netherlands) equipped with an EMT sensor. The implant geometry is tracked directly after implantation, after CT imaging and after each irradiation fraction. The acquired data is rigidly registered to the catheter traces defined in the treatment plan and the dwell positions (DP) are reconstructed. DPs defined in treatment planning serve as reference. Breathing motion was corrected and recorded using three reference 6DoF sensors placed on the patients' skin. The Euclidean distance between the planned and reconstructed DPs provides information about possible inter-fractional deviations. Further, the influence of various factors on the occurrence of large deviations was investigated, like the patients' age, the length of the catheter, the breast volume, etc. RESULTS: Over all patient measurements a median Euclidean distance of 2.19 mm was determined between the reconstructed DPs and the reference DPs. The median deviation combining all datasets was minimal (1.67 mm) at the measurement directly after CT imaging. The deviations between the different fractions have a median distance of 2.31 mm which could be improved to 2.05 mm by adapting the treatment plan according to the follow-up CT. No correlation between the distance to the skin, ribs, mammilla or the breast volume and the occurrences of large deviations was found. The largest deviations were determined in the upper inner quadrant of the breast.

CONCLUSION

The afterloader prototype could be well integrated into the clinical routine and is beneficial for ensuring the quality of the brachytherapy. Overall, a small median DP deviation, lower than the used step size of 2.5 mm, was detected.

Technical Note: A comparison of point set registration methods for electromagnetic tracking

PURPOSE

High dose rate brachytherapy applies intense and destructive radiation. A treatment plan defines radiation source dwell positions to avoid irradiating healthy tissue. The study discusses methods to quantify any positional changes of source locations along the various treatment sessions.

METHODS

Electromagnetic tracking (EMT) localizes the radiation source during the treatment sessions. But in each session the relative position of the patient relative to the filed generator is changed. Hence, the measured dwell point sets need to be registered onto each other to render them comparable. Two point set registration techniques are compared: a probabilistic method called coherent point drift (CPD) and a multidimensional scaling (MDS) technique.

RESULTS

Both enable using EMT without external registration and achieve very similar results with respect to dwell position determination of the radiation source. Still MDS achieves smaller grand average deviations (CPD-rPSR: MD = 2.55 mm, MDS-PSR: MD = 2.15 mm) between subsequent dwell position determinations, which also show less variance (CPD-rPSR: IQR = 4 mm, MDS-PSR: IQR = 3 mm). Furthermore, MDS is not based on approximations and does not need an iterative procedure to track sensor positions inside the implanted catheters.

CONCLUSION

Although both methods achieve similar results, MDS is to be preferred over rigid CPD while nonrigid CPD is unsuitable as it does not preserve topology.