First clinical experience with a novel, mobile cone-beam CT system for treatment quality assurance in brachytherapy

Assessment of integrated electromagnetic tracking for dwell position monitoring in a clinical HDR brachytherapy setting for prostate cancer

BACKGROUND

Electromagnetic Tracking (EMT) technology has been integrated in a prototype high-dose-rate brachytherapy (HDR-BT) afterloading device. Its potential for dwell position (DP) monitoring has earlier been demonstrated in prostate phantoms. However, its performance for prostate BT in the clinical setting remains to be assessed.

AIM

Assess the reliability and value of EMT measurements in transrectal ultrasound-based (TRUS-based) and computed tomography-based (CT-based) prostate HDR-BT.

METHODS

EMT measurements were conducted on 20 patients undergoing dual-fraction prostate HDR-BT monotherapy. In each treatment fraction an individual TRUS-based or CT-based treatment plan was generated. The measurements were compared to DPs of manually reconstructed needles in those TRUS-based or CT-based treatment plans. An internal reference sensor was also placed in one needle to assess internal movement levels and its potential for movement correction.

RESULTS

For TRUS-based treatments, median Euclidean distances (ED) of 1.00 mm were observed between EMT measurements and manual DP determination. Reference sensor movement was minimal at a median of 0.18 mm. For DPs measured in the CT-room and treatment room, median EDs of 1.60 mm and 2.24 mm compared to CT-based DP determination respectively were observed, indicating the system's ability to detect changes in implant geometry over time and after patient repositioning. Median reference sensor movement of 0.97 mm was observed. Implementing reference sensor-based movement correction led to a significant but small decrease in ED for CT-based treatments.

CONCLUSION

EMT is suitable for TRUS-based prostate HDR-BT quality assurance and error detection. While EMT can identify changes in implant geometry in CT-based prostate HDR-BT treatments, it showed lower accuracy in this setting.

Comprehensive commissioning of a cone beam CT imaging ring for treatment of HDR GYN patients

PURPOSE

A new mobile cone beam computed tomography (CBCT) imaging ring (IRm, Elekta, v2.10.6, Veenendaal, Netherlands) has recently been proposed for brachytherapy to improve procedure efficiency. We describe the commissioning process and end-to-end tests for GYN HDR brachytherapy employing IRm CBCT imaging.

MATERIALS AND METHODS

Commissioning included imaging isocenter test, 3D image quality, 2D imaging quality, image dose, and tube characteristics. CIRS HDR GYN phantom and Venezia CT/MR gynecological applicator were used to perform end-to-end (E2E) tests and optimize workflow. Venezia applicator and four interstitial needles were inserted into the phantom and IRm CBCT images were acquired. Phantom and applicator were scanned with CT scanner (Siemens SOMATOM go.Open Pro) using department's pelvis imaging protocol. MR imaging was performed using 0.35T MR Linac TRUFI pulse sequence. CBCT images were registered to CT and MR using rigid registration to assess image quality and applicator geometry fidelity.

RESULTS

All physics tests passed within acceptance tolerances. Registration of CBCT images to MR and CT scans was acceptable for applicator placement. Applicator registration of CBCT images to CT demonstrated excellent agreement of most distal source dwell position (<1 mm). Slice thickness was also measured to be 1.25 mm, within 0.5 mm of its nominal value.

CONCLUSION

Based on E2E and commissioning results, IRm is an appropriate tool for brachytherapy treatment planning. This study demonstrated good image quality in GYN phantom and Venezia applicator using the IRm. Distal source dwell position agreement between CBCT and CT was acceptable for clinical use.

Initial needle tracking with the first standalone combined infrared camera - CT system for brachytherapy-analysis of tracking accuracy and uncertainties

PURPOSE

A prototype infrared camera - cone-beam computed tomography (CBCT) system for tracking in brachytherapy has recently been developed. We evaluated for the first time the corresponding tracking accuracy and uncertainties, and implemented a tracking-based prediction of needles on CBCT scans.

METHODS

A marker tool rigidly attached to needles was 3D printed. The precision and accuracy of tool tracking was then evaluated for both static and dynamic scenarios. Euclidean distances between the tracked and CBCT-derived markers were assessed as well. To implement needle tracking, ground truth models of the tool attached to 200 mm and 160 mm needles were matched to the tracked positions in order to project the needles into CBCT scans. Deviations between projected and actual needle tips were measured. Finally, we put our results into perspective with simulations of the system's tracking uncertainties.

RESULTS

For the stationary scenario and dynamic movements, we achieved tool-tracking precision and accuracy of 0.04 ± 0.06 mm and 0.16 ± 0.18 mm, respectively. The tracked marker positions differed by 0.52 ± 0.18 mm from the positions determined via CBCT. In addition, the predicted needle tips in air deviated from the actual tip positions by only 1.62 ± 0.68 mm (200 mm needle) and 1.49 ± 0.62 mm (160 mm needle). The simulated tracking uncertainties resulted in tip variations of 1.58 ± 0.91 mm and 1.31 ± 0.69 mm for the 200 mm and 160 mm needles, respectively.

CONCLUSION

With the innovative system it was possible to achieve a high tracking and prediction accuracy of marker tool and needles. The system shows high potential for applicator tracking in brachytherapy.

Therapeutic prostate cancer interventions: a systematic review on pubic arch interference and needle positioning errors

INTRODUCTION

This study focuses on the quantification of and current guidelines on the hazards related to needle positioning in prostate cancer treatment: (1) access restrictions to the prostate gland by the pubic arch, so-called Pubic Arch Interference (PAI) and (2) needle positioning errors. Next, we propose solution strategies to mitigate these hazards.

METHODS

The literature search was executed in the Embase, Medline ALL, Web of Science Core Collection*, and Cochrane Central Register of Controlled Trials databases.

RESULTS

The literature search resulted in 50 included articles. PAI was reported in patients with various prostate volumes. The level of reported PAI varied between 0 and 22.3 mm, depending on the patient's position and the measuring method. Low-Dose-Rate Brachytherapy induced the largest reported misplacement errors, especially in the cranio-caudal direction (up to 10 mm) and the largest displacement errors were reported for High-Dose-Rate Brachytherapy in the cranio-caudal direction (up to 47 mm), generally increasing over time.

CONCLUSIONS

Current clinical guidelines related to prostate volume, needle positioning accuracy, and maximum allowable PAI are ambiguous, and compliance in the clinical setting differs between institutions. Solutions, such as steerable needles, assist in mitigating the hazards and potentially allow the physician to proceed with the procedure.This systematic review was performed in accordance with the PRISMA guidelines. The review was registered at Protocols.io (DOI: dx.doi.org/10.17504/protocols.io.6qpvr89eplmk/v1).

Inter-observer effects in needle reconstruction for temporary prostate brachytherapy: Dosimetric implications and adaptive CBCT-TRUS registration solutions

PURPOSE

To investigate geometric and dosimetric inter-observer variability in needle reconstruction for temporary prostate brachytherapy. To assess the potential of registrations between transrectal ultrasound (TRUS) and cone-beam computed tomography (CBCT) to support implant reconstructions.

METHODS AND MATERIALS

The needles implanted in 28 patients were reconstructed on TRUS by three physicists. Corresponding geometric deviations and associated dosimetric variations to prostate and organs at risk (urethra, bladder, rectum) were analyzed. To account for the found inter-observer variability, various approaches (template-based, probe-based, marker-based) for registrations of CBCT to TRUS were investigated regarding the respective needle transfer accuracy in a phantom study. Three patient cases were examined to assess registration accuracy in-vivo.

RESULTS

Geometric inter-observer deviations >1 mm and >3 mm were found for 34.9% and 3.5% of all needles, respectively. Prostate dose coverage (changes up to 7.2%) and urethra dose (partly exceeding given dose constraints) were most affected by associated dosimetric changes. Marker-based and probe-based registrations resulted in the phantom study in high mean needle transfer accuracies of 0.73 mm and 0.12 mm, respectively. In the patient cases, the marker-based approach was the superior technique for CBCT-TRUS fusions.

CONCLUSION

Inter-observer variability in needle reconstruction can substantially affect dosimetry for individual patients. Especially marker-based CBCT-TRUS registrations can help to ensure accurate reconstructions for improved treatment planning.

Reduction of cone-beam CT artifacts in a robotic CBCT device using saddle trajectories with integrated infrared tracking

BACKGROUND

Cone beam computed tomography (CBCT) is widely used in many medical fields. However, conventional CBCT circular scans suffer from cone beam (CB) artifacts that limit the quality and reliability of the reconstructed images due to incomplete data.

PURPOSE

Saddle trajectories in theory might be able to improve the CBCT image quality by providing a larger region with complete data. Therefore, we investigated the feasibility and performance of saddle trajectory CBCT scans and compared them to circular trajectory scans.

METHODS

We performed circular and saddle trajectory scans using a novel robotic CBCT scanner (Mobile ImagingRing (IRm); medPhoton, Salzburg, Austria). For the saddle trajectory, the gantry executed yaw motion up to± 10 ∘ $\pm 10^{\circ }$ using motorized wheels driving on the floor. An infrared (IR) tracking device with reflective markers was used for online geometric calibration correction (mainly floor unevenness). All images were reconstructed using penalized least-squares minimization with the conjugate gradient algorithm from RTK with0.5 × 0.5 × 0.5 mm 3 $0.5 \times 0.5\times 0.5 \text{ mm}^3$ voxel size. A disk phantom and an Alderson phantom were scanned to assess the image quality. Results were correlated with the local incompleteness value represented bytan ( ψ ) $\tan (\psi)$ , which was calculated at each voxel as a function of the source trajectory and the voxel's 3D coordinates. We assessed the magnitude of CB artifacts using the full width half maximum (FWHM) of each disk profile in the axial center of the reconstructed images. Spatial resolution was also quantified by the modulation transfer function at 10% (MTF10).

RESULTS

When using the saddle trajectory, the region without CB artifacts was increased from 43 to 190 mm in the SI direction compared to the circular trajectory. This region coincided with low values fortan ( ψ ) $\tan (\psi)$ . Whentan ( ψ ) $\tan (\psi)$ was larger than 0.02, we found there was a linear relationship between the FWHM andtan ( ψ ) $\tan (\psi)$ . For the saddle, IR tracking allowed the increase of MTF10 from 0.37 to 0.98 lp/mm.

CONCLUSIONS

We achieved saddle trajectory CBCT scans with a novel CBCT system combined with IR tracking. The results show that the saddle trajectory provides a larger region with reliable reconstruction compared to the circular trajectory. The proposed method can be used to evaluate other non-circular trajectories.

Investigating the impact of breast positioning control on physical treatment parameters in multi-catheter breast brachytherapy

PURPOSE

To assess the effects of a workflow for reproducible patient and breast positioning on implant stability during high-dose-rate multi-catheter breast brachytherapy.

METHODS

Thirty patients were treated with our new positioning control workflow. Implant stability was evaluated based on a comparison of planning-CTs to control-CTs acquired halfway through the treatment. To assess geometric stability, button-button distance variations as well as Euclidean dwell position deviations were evaluated. The latter were also quantified within various separated regions within the breast to investigate the location-dependency of implant alterations. Furthermore, dosimetric variations to target volume and organs at risk (ribs, skin) as well as isodose volume changes were analyzed. Results were compared to a previously treated cohort of 100 patients.

RESULTS

With the introduced workflow, the patient fraction affected by button-button distance variations > 5 mm and by dwell position deviations > 7 mm were reduced from 37% to 10% and from 30% to 6.6%, respectively. Implant stability improved the most in the lateral to medial breast regions. Only small stability enhancements were observed regarding target volume dosimetry, but the stability of organ at risk exposure became substantially higher. D0.2ccm skin dose variations > 12.4% and D0.1ccm rib dose variations > 6.7% were reduced from 11% to 0% and from 16% to 3.3% of all patients, respectively.

CONCLUSION

Breast positioning control improved geometric and dosimetric implant stability for individual patients, and thus enhanced physical plan validity in these cases.

Toward a deep learning-based magnetic resonance imaging only workflow for postimplant dosimetry in I-125 seed brachytherapy for prostate cancer

BACKGROUND AND PURPOSE

The current standard imaging-technique for creating postplans in seed prostate brachytherapy is computed tomography (CT), that is associated with additional radiation exposure and poor soft tissue contrast. To establish a magnetic resonance imaging (MRI) only workflow combining improved tissue contrast and high seed detectability, a deep learning-approach for automatic seed segmentation on MRI-scans was developed.

MATERIAL AND METHODS

Patients treated with I-125 seed brachytherapy received a postplan-CT and a 1.5 T MRI-scan on nominal day 30 after implantation. For MRI-based seed visualization, DIXON-sequences were acquired and deep learning-based quantitative susceptibility maps (QSM) were generated from 3D-gradient-echo-sequences from 20 patients. Seed segmentations created on CT served as ground truth. For automatic seed segmentation on MRI, a 3D nnU-net model was trained using QSM and DIXON, both solely and combined.

RESULTS

Of the implanted seeds 94.8 ± 2.4% were detected with deep learning automatic segmentation entrained on both QSM and DIXON data. Models trained on the individual sequence data-sets performed worse with detection rates of 87.5 ± 2.6% or 88.6 ± 7.5% for QSM and DIXON respectively. The seed centers identified on CT versus QSM and DIXON were on average 1.8 ± 1.3 mm apart. Postimplant dosimetry for evaluation of positioning inaccuracies revealed only small variations of up to 0.4 ± 4.26 Gy in D90 (dose 90% of the prostate receives) between the standard CT-approach and our MRI-only workflow.

CONCLUSION

The proposed deep learning-based MRI-only workflow provided a promisingly accurate and robust seed localization and thus has the potential to compete with current state-of-the-art CT-based postimplant dosimetry in the future.

A new way to visualize prostate brachytherapy needles using ultrasound color Doppler and needle surface modifications

PURPOSE

Suboptimal ultrasound conspicuity of the brachytherapy applicator can lead to inaccurate image reconstructions of the applicator resulting in decreased tumor control or increased normal tissue dose. This feasibility study aims to improve ultrasound conspicuity of high-dose rate (HDR) brachytherapy needles by modifying the surface of the needles to produce a color Doppler twinkling signature.

MATERIALS AND METHODS

Surface modifications of standard 17-gauge titanium HDR brachytherapy needles included laser-scribing, application of polymethyl methacrylate (PMMA), and coating with a commercially available echogenic coating. Laser-scribing was performed with variable widths (0.1-1 mm) and depths (10-100 μm). The echogenic coating was applied with 3 different thicknesses (27, 40, and 64 μm). Unmodified and modified needles were imaged under B-mode and color Doppler ultrasound in phantom and cadaver, and the signal strength was recorded.

RESULTS

Laser-scribed, PMMA-coated, and echogenic-coated brachytherapy needles produced a twinkling signature along the needle shaft on color Doppler ultrasound. Twinkling was observed with laser-scribe depths >20 μm and widths >0.1 mm and from echogenic coatings 40 μm and 64 μm thick. Twinkling was not observed with unmodified needles. The twinkling signature had a spectral composition with a uniform magnitude between the velocities of 2 to 16 cm/s.

CONCLUSIONS

Color Doppler ultrasound of surface-modified brachytherapy applicators may improve applicator conspicuity aiding applicator placement and digitization. HDR brachytherapy needles may be modified to produce the twinkling signature via laser-scribing, PMMA rings, or applying an echogenic coating.

Estimating follow-up CTs from geometric deformations of catheter implants in interstitial breast brachytherapy: A feasibility study using electromagnetic tracking

BACKGROUND

Electromagnetic tracking (EMT) systems have been shown to provide valuable information on the geometry of catheter implants in breast cancer patients undergoing interstitial brachytherapy (iBT). In the context of an extended patient-specific, pre-treatment verification, EMT can play a key role in determining the potential need and, if applicable, the appropriate time for treatment adaptation. To detect dosimetric shortcomings the relative position between catheters, and target volume and critical structures must be known. Since EMT cannot provide the anatomical context and standard imaging techniques such as cone-beam CT are not yet available in most brachytherapy suites, it is not possible to detect anatomic changes on a daily or fraction basis, so the need for adaptive planning cannot be identified.

PURPOSE

The aim of this feasibility study is to develop and evaluate a technique capable of estimating follow-up CTs at any time based on the initial treatment planning CT (PCT) and surrogate information about changes of the implant geometry from an EMT system.

METHODS

A deformation vector field is calculated from two different implant reconstructions acquired in treatment position through EMT, the first immediately after the PCT and the second at another time point during the course of treatment. The calculation is based on discrete displacement vectors of pairs of control and target points. These are extrapolated by means of different radial basis functions in order to cover the entire CT volume. The adequate parameters for the calculation of the deformation field were identified. By warping the PCT according to the deformation field, one obtains an estimated CT (ECT) that reflects the geometric changes. For the proof of concept, ECTs were computed for the time point of the clinical follow-up CT (FCT) that is embedded in the treatment workflow after the fourth fraction.

RESULTS

ECT and clinical FCTs of 20 patients were compared to each other quantitatively in terms of absolute Hounsfield unit differences in the planning target volume (PTV) and in a convex hull (CH) enclosing the catheters. The median differences were 31.2  and 29.5 HU for the CH and the PTV, respectively.

CONCLUSION

The proposed ECT approach was able to approximate the "anatomy of the day" and therefore, in principle, allows a dosimetric appraisal of the treatment plan quality before each fraction. In this way, it can contribute to a more detailed patient-specific quality assurance in iBT of the breast and help to identify the timing for a potential treatment adaptation.

On the implant stability in adaptive multi-catheter breast brachytherapy: Establishment of a decision-tree for treatment re-planning

BACKGROUND AND PURPOSE

To assess implant stability and identify causes of implant variations during high-dose-rate multi-catheter breast brachytherapy.

MATERIALS AND METHODS

Planning-CTs were compared to control-CTs acquired halfway through the treatment for 100 patients. For assessing geometric stability, Fréchet-distance and button-to-button distance changes of all catheters as well as variations of Euclidean distances and convex hulls of all dwell positions were determined. The CTs were inspected to identify the causes of geometric changes. Dosimetric effects were evaluated by target volume transfers and re-contouring of organs at risk. The dose non-uniformity ratio (DNR), 100% and 150% isodose volumes (V₁₀₀ and V₁₅₀), coverage index (CI), and organ doses were calculated. Correlations between the examined geometric and dosimetric parameters were assessed.

RESULTS

Fréchet-distance and dwell position deviations >2.5 mm as well as button-to-button distance changes >5 mm were detected for 5%, 2%, and 6.3% of catheters, but for 32, 17, and 37 patients, respectively. Variations occurred enhanced in the lateral breast and close to the ribs, e.g. due to different arm positions. Only small dosimetric effects with median DNR, V₁₀₀, and CI variations of -0.01 ± 0.02, (-0.5 ± 1.3)ccm, and (-1.4 ± 1.8)% were observed in general. Skin dose exceeded recommended levels for 12 of 100 patients. Various correlations between geometric and dosimetric implant stability were found, based on which decision-tree regarding treatment re-planning was established.

CONCLUSION

Multi-catheter breast brachytherapy shows a high implant stability in general, but considering skin dose changes is important. To increase implant stability for individual patients, we plan to investigate patient immobilization aids during treatments.

Seed-displacements in the immediate post-implant phase in permanent prostate brachytherapy

PURPOSE

To investigate differences in seed-displacements between the immediate post-implant phase (day 0-1) and the time to post-plan computed tomography (CT) (day 1-30) in seed prostate brachytherapy.

MATERIALS AND METHODS

Seed positions were identified on the intra-operatively created ultrasound-based treatment plan (day 0) and CT scans of day 1 and 30 for 33 patients. The day 1 (30) seed arrangement was registered onto the day 0 (1) arrangement using a seed-only approach. Based on a 1:1 assignment of seeds via the Kuhn-Munkres algorithm, seed-displacements were analyzed. Displacements were evaluated depending on strand-length and anatomical implant location. Resulting dosimetric effects were calculated.

RESULTS

Seed-displacements in the immediate post-implant phase (median displacements: 3.8 ± 3.6 mm) were stronger than in the time to post-plan CT (2.1 ± 2.6 mm) and enhanced along the superior-inferior direction. From day 0 to 1, strands containing one (7.3 ± 5.4 mm) or two (8.1 ± 5.8 mm) seeds showed larger displacements than strands of higher lengths (up to 4.2 ± 7.0 mm), whereas no length-dependency was found to day 30. Seeds implanted in base and apex tended to move towards the prostate midzone during both time periods. D₉₀ (dose that 90% of prostate receives) was with variations of 2 ± 15 Gy more stable from day 1 to 30 than in the immediate post-implant phase (-18 ± 11 Gy).

CONCLUSION

Seed-displacements in the immediate post-implant phase was enhanced compared to day 1-30. This may result from uncertainties in the gold-standard ultrasound-based treatment planning and implantation. Adaptive implantation workflows appear useful for ensuring high implant stability from the beginning.

Development and validation of a scatter-corrected CBCT image-guided method for cervical cancer brachytherapy

Background and purpose

Multiple patient transfers have a nonnegligible impact on the accuracy of dose delivery for cervical cancer brachytherapy. We consider using on-site cone-beam CT (CBCT) to resolve this problem. However, CBCT clinical applications are limited due to inadequate image quality. This paper implements a scatter correction method using planning CT (pCT) prior to obtaining high-quality CBCT images and evaluates the dose calculation accuracy of CBCT-guided brachytherapy for cervical cancer.

Materials and methods

The CBCT of a self-developed female pelvis phantom and five patients was first corrected using empirical uniform scatter correction in the projection domain and further corrected in the image domain. In both phantom and patient studies, the CBCT image quality before and after scatter correction was evaluated with registered pCT (rCT). Model-based dose calculation was performed using the commercial package Acuros®BV. The dose distributions of rCT-based plans and corrected CBCT-based plans in the phantom and patients were compared using 3D local gamma analysis. A statistical analysis of the differences in dosimetric parameters of five patients was also performed.

Results

In both phantom and patient studies, the HU error of selected ROIs was reduced to less than 15 HU. Using the dose distribution of the rCT-based plan as the baseline, the γ pass rate (2%, 2 mm) of the corrected CBCT-based plan in phantom and patients all exceeded 98% and 93%, respectively, with the threshold dose set to 3, 6, 9, and 12 Gy. The average percentage deviation (APD) of D90 of HRCTV and D2cc of OARs was less than 1% between rCT-based and corrected CBCT-based plans.

Conclusion

Scatter correction using a pCT prior can effectively improve the CBCT image quality and CBCT-based cervical brachytherapy dose calculation accuracy, indicating promising prospects in both simplified brachytherapy processes and accurate brachytherapy dose delivery.