Is adaptive treatment planning in multi-catheter interstitial breast brachytherapy necessary?

PURPOSE

For 55 patients treated with interstitial multi-catheter breast brachytherapy the need for adaptive treatment planning was assessed.

METHODS AND MATERIALS

For all patients a treatment planning computed tomography (CT) and a follow-up CT were acquired and used for the retrospective evaluation. Keeping dwell time and dwell positions constant, the treatment plan assessed directly after catheter implantation was compared to the situation 48 h after implantation. Both manual catheter reconstructions, based on the planning and follow-up CT, were rigid registered to each other and the resulting deviations analyzed, like the difference between corresponding dwell positions (ΔDP) or the discrete Fréchet distance. Further, the dosimetric changes, e.g., coverage index (ΔCI), conformal index (ΔCOIN) and dose non-uniformity ratio (ΔDNR) were considered for a deformed planning target volume (PTV) and the rigid warped PTV structure. The PTV was deformed according to the vector field estimated between the two acquired CTs.

RESULTS

Over all patients with rigid aligned CTs a mean ΔDP, ΔCI, ΔCOIN and ΔDNR were determined to 2.41 ± 1.73 mm, 3.10 ± 3.17%, 0.009 ± 0.007 and 0.036 ± 0.040, respectively. Considering the deformed PTV ΔCI was estimated to 5.05 ± 4.14%.

CONCLUSION

In conclusion, in 4% of the cases re-planning would have been beneficial to ensure the planned dose delivery. Large PTV changes or large DP deviations were found to be the main reasons for dosimetric variations.

Development of a 3D ultrasound guidance system for permanent breast seed implantation

PURPOSE

Permanent breast seed implantation (PBSI) is a promising radiotherapy technique for early-stage breast cancer, completed in a single visit by permanently implanting 103 Pd seeds using needles inserted through a template and guided by two-dimensional (2D) ultrasound (US). However, operator dependence has been highlighted as a limitation of this procedure. Consequently, we propose and have developed an intraoperative guidance system using three-dimensional (3D) US and an instrumented mechanical arm to provide intraoperative 3D imaging and needle template tracking.

METHODS

A mechatronic 3D US scanner reconstructs a 3D image from 150 2D images. A tracked mechanical arm mounted to the scanner locates four fiducial points on the template, registering the template to the 3D image. 3D reconstruction was validated for linear and volumetric measurement accuracy using phantoms of known geometry. In vivo breast US image quality was evaluated in a healthy volunteer. The encoded arm was calibrated and validated using a jig with divots at known locations relative to the scanner and the scanner registered to the 3D US image using intersecting strings in a fluid-filled test jig. Template registration accuracy was assessed using a machined test jig. Tracking accuracy was assessed in a liquid medium by comparing tracked and imaged needle tip positions. Finally, the system was used to guide a mock procedure in a patient-specific phantom and micro-CT imaging used to evaluate its accuracy.

RESULTS

Geometric validation showed median distances within ±1.1% of expected values and volumetric validation showed differences of ≤4.1%. Tracking arm point measurements showed an average error of 0.43 mm and 3D US volume registration showed target registration error ≤0.9 mm. Mean template registration accuracy in each axis of translation/rotation was ≤1.3 mm/1.0°. Mean needle-targeting error was 2.5 mm and 1.6° for needle tips and trajectories, respectively. Mean needle tip and angular errors of the phantom procedure were 2.1 mm and 2.6°. Modeled seed displacement of the phantom procedure showed mean error of 2.6 mm and a maximum of 3.8 mm.

CONCLUSIONS

A 3D US guidance system for PBSI has been developed. Benchtop performance and image quality in volunteer scans are satisfactory. A phantom PBSI procedure was successfully delivered using the system with maximum seed error within dosimetric benchmarks (<5 mm). Translation of the device into the clinic is forthcoming.

Use of ultrasound in image-guided high-dose-rate brachytherapy: enumerations and arguments

Inherently, brachytherapy is the most conformal radiotherapeutic technique. As an aid to brachytherapy, ultrasonography (USG) serves as a portable, inexpensive, and simple to use method allowing for accurate, reproducible, and adaptive treatments. Some newer brachytherapy planning systems have incorporated USG as the sole imaging modality. Ultrasonography has been successfully used to place applicator and dose planning for prostate, cervix, and anal canal cancers. It can guide placement of brachytherapy catheters for all other sites like breast, skin, and head and neck cancers. Traditional USG has a few limitations, but recent advances such as 3-dimensional (3D) USG and contrast USG have enhanced its potential as a dependable guide in high-dose-rate image-guided brachytherapy (HDR-IGBT). The authors in this review have attempted to enumerate various aspects of USG in brachytherapy, highlighting its use across various sites.

Recommendations from GEC ESTRO Breast Cancer Working Group (II): Target definition and target delineation for accelerated or boost partial breast irradiation using multicatheter interstitial brachytherapy after breast conserving open cavity surgery

OBJECTIVE

To prepare guidelines for target definition and delineations after open cavity breast conserving surgery in accelerated partial breast irradiations or boost treatments using multicatheter interstitial brachytherapy based on the consensus of the Breast Working Group of GEC-ESTRO.

METHOD

Following a study on interobserver variations of target volume delineation in multicatheter breast brachytherapy after open cavity surgery and a number of discussions in consensus meetings these guidelines were worked out by experts on the field.

PROPOSED RECOMMENDATIONS

(1) Consistent windowing has to be used for proper cavity visualization. (2) The cavity visualization score has to be at least 3 in order to minimize the interobserver variations of target definition. (3) At delineation of surgical cavity only the homogeneous part of the postoperative seroma has to be included in the contours and protrusions or sharp irregularities have to be excluded. When surgical clips are present, they have to be surrounded by the contour with close contact. (4) CTV is created from the outlined surgical cavity with a nonisotropic geometrical extension. In each direction the safety margin is calculated by taking into account the size of the free resection margin. The total size of safety margin is always 20mm which is the sum of the surgical and added safety margins. CTV is limited to chest wall/pectoral muscles and 5mm below the skin surface.

CONCLUSION

Following these guidelines the target volume definition in breast brachytherapy after open cavity surgery is expected to be accomplished in more consistent way with low interobserver variations.

Validation of a novel robot-assisted 3DUS system for real-time planning and guidance of breast interstitial HDR brachytherapy

PURPOSE

In current clinical practice, there is no integrated 3D ultrasound (3DUS) guidance system clinically available for breast brachytherapy. In this study, the authors present a novel robot-assisted 3DUS system for real-time planning and guidance of breast interstitial high dose rate (HDR) brachytherapy treatment.

METHODS

For this work, a new computer controlled robotic 3DUS system was built to perform a hybrid motion scan, which is a combination of a 6 cm linear translation with a 30° rotation at both ends. The new 3DUS scanner was designed to fit on a modified Kuske assembly, keeping the current template grid configuration but modifying the frame to allow the mounting of the 3DUS system at several positions. A finer grid was also tested. A user interface was developed to perform image reconstruction, semiautomatic segmentation of the surgical bed as well as catheter reconstruction and tracking. A 3D string phantom was used to validate the geometric accuracy of the reconstruction. The volumetric accuracy of the system was validated with phantoms using magnetic resonance imaging (MRI) and computed tomography (CT) images. In order to accurately determine whether 3DUS can effectively replace CT for treatment planning, the authors have compared the 3DUS catheter reconstruction to the one obtained from CT images. In addition, in agarose-based phantoms, an end-to-end procedure was performed by executing six independent complete procedures with both 14 and 16 catheters, and for both standard and finer Kuske grids. Finally, in phantoms, five end-to-end procedures were performed with the final CT planning for the validation of 3DUS preplanning.

RESULTS

The 3DUS acquisition time is approximately 10 s. A paired Student t-test showed that there was no statistical significant difference between known and measured values of string separations in each direction. Both MRI and CT volume measurements were not statistically different from 3DUS volume (Student t-test: p > 0.05) and they were significantly correlated to 3DUS measurement (Pearson test: MRI p < 0.05 and CT p < 0.001). The mean angular separation distance between catheter trajectories segmented from 3DUS and CT images was 0.42° ± 0.24°, while the maximum and mean trajectory separations were 0.51 ± 0.19 and 0.37 ± 0.17 mm, respectively. Overall, the new finer grid has performed significantly better in terms of dosimetric indices. The planning target volume dosimetric indices were not found statistically different between 3DUS and CT planning (Student t-test, p > 0.05). Both the skin and the pectoral muscle dosimetric indices were within ABS guidelines.

CONCLUSIONS

A novel robot-assisted 3DUS system was designed and validated. To their knowledge, this is the first system capable of performing real-time guidance and planning of breast multicatheter HDR brachytherapy treatments. Future investigation will test the feasibility of using the system in the clinic and for permanent breast brachytherapy.

Adaptation of the CVT algorithm for catheter optimization in high dose rate brachytherapy

PURPOSE

An innovative, simple, and fast method to optimize the number and position of catheters is presented for prostate and breast high dose rate (HDR) brachytherapy, both for arbitrary templates or template-free implants (such as robotic templates).

METHODS

Eight clinical cases were chosen randomly from a bank of patients, previously treated in our clinic to test our method. The 2D Centroidal Voronoi Tessellations (CVT) algorithm was adapted to distribute catheters uniformly in space, within the maximum external contour of the planning target volume. The catheters optimization procedure includes the inverse planning simulated annealing algorithm (IPSA). Complete treatment plans can then be generated from the algorithm for different number of catheters. The best plan is chosen from different dosimetry criteria and will automatically provide the number of catheters and their positions. After the CVT algorithm parameters were optimized for speed and dosimetric results, it was validated against prostate clinical cases, using clinically relevant dose parameters. The robustness to implantation error was also evaluated. Finally, the efficiency of the method was tested in breast interstitial HDR brachytherapy cases.

RESULTS

The effect of the number and locations of the catheters on prostate cancer patients was studied. Treatment plans with a better or equivalent dose distributions could be obtained with fewer catheters. A better or equal prostate V100 was obtained down to 12 catheters. Plans with nine or less catheters would not be clinically acceptable in terms of prostate V100 and D90. Implantation errors up to 3 mm were acceptable since no statistical difference was found when compared to 0 mm error (p > 0.05). No significant difference in dosimetric indices was observed for the different combination of parameters within the CVT algorithm. A linear relation was found between the number of random points and the optimization time of the CVT algorithm. Because the computation time decrease with the number of points and that no effects were observed on the dosimetric indices when varying the number of sampling points and the number of iterations, they were respectively fixed to 2500 and to 100. The computation time to obtain ten complete treatments plans ranging from 9 to 18 catheters, with the corresponding dosimetric indices, was 90 s. However, 93% of the computation time is used by a research version of IPSA. For the breast, on average, the Radiation Therapy Oncology Group recommendations would be satisfied down to 12 catheters. Plans with nine or less catheters would not be clinically acceptable in terms of V100, dose homogeneity index, and D90.

CONCLUSIONS

The authors have devised a simple, fast and efficient method to optimize the number and position of catheters in interstitial HDR brachytherapy. The method was shown to be robust for both prostate and breast HDR brachytherapy. More importantly, the computation time of the algorithm is acceptable for clinical use. Ultimately, this catheter optimization algorithm could be coupled with a 3D ultrasound system to allow real-time guidance and planning in HDR brachytherapy.