Applicator reconstruction in MRI 3D image-based dose planning of brachytherapy for cervical cancer

Background and purpose

To elaborate a method for applicator reconstruction for MRI-based brachytherapy for cervical cancer.

Materials and methods

Custom-made plastic catheters with a copper sulphate solution were made for insertion in the source channels of MR-CT compatible applicators: plastic and titanium tandem ring applicators, and titanium needles. The applicators were CT and MR scanned in a phantom for accurate 3D assessment of applicator visibility and geometry. A reconstruction method was developed and evaluated in 19 patient MR examinations with ring applicator (plastic: 14, titanium: 5). MR applicator reconstruction uncertainties related to inter-observer variation were evaluated.

Results

The catheters were visible in the plastic applicator on T1-weighted images in phantom and in 14/14 clinical applications. On T2-weighted images, the catheters appeared weaker but still visible in phantom and in 13/14 MR clinical applications. In the titanium applicator, the catheters could not be separated from the artifacts from the applicator itself. However, these artifacts could be used to localize both titanium ring applicator (5/5 clinical applications) and needles (6/6 clinical applications). Standard deviations of inter-observer differences were below 2 mm in all directions.

Conclusion

3D applicator reconstruction based on MR imaging could be performed for plastic and titanium applicators. Plastic applicators proved well to be suited for MRI-based reconstruction. For improved practicability of titanium applicator reconstruction, development of MR applicator markers is essential. Reconstruction of titanium applicator and needles at 1.5 T MR requires geometric evaluations in phantoms before using the applicator in patients.     

Applicator reconstruction and applicator shifts in 3D MR-based PDR brachytherapy of cervical cancer

Purpose

To evaluate the methods of applicator reconstruction in 3D MR-based planning for brachytherapy of cervical cancer, and to investigate applicator shifts and changes in DVH parameters during PDR treatment.

Methods

For each application MR scans with applicator in situ were made: three T2-weighted (4.5 mm slices) Turbo Spin Echo (TSE) scans and a balanced Steady State Free Precession scan (1.5 mm). Three observers tested two applicator reconstruction methods: (A) directly on the bSSFP scan and (B) on a resampled combination of the three T2-weighted scans. For 10 patients MR imaging was repeated on the second day of each PDR fraction to determine applicator shifts and changes in DVH parameters.

Results

For both applicator reconstruction methods the interobserver variation for the DVH parameters was comparable (average <1.5% in dose). Differences between the two methods were larger (up to 6.4% for target) and were related to position differences during MR scanning.The average applicator shift relative to the pelvic structures was 5-6 mm into the ventral direction and 3-4 mm cranially. For a single PDR fraction, the average D90 (HR-CTV) on ‘day 2’ was 0.2 (SD 2.0) Gy lower than that for day 1. The average increase in D_2cc (bladder) was 1.0 (SD 3.0) Gy_αβ3 for a single PDR fraction. If the effect of both fractions was combined, for 1 patient a total decrease of D90 of 7 Gy_αβ10 was found, whereas for another patient the total increase in bladder dose was 12 Gy_αβ3.

Conclusions

Applicator reconstruction on MR data is feasible. In the overall accuracy during PDR brachytherapy the reconstruction uncertainty is of minor importance. Applicator and/or organ movement during the course of the PDR fraction produce larger uncertainties.     

Direct reconstruction of the Vienna applicator on MR images

Purpose

To introduce and test a direct reconstruction concept for intracavitary tandem ring applicators in MR image-based brachytherapy treatment planning.

Materials and methods

Optical measurements of transparent ring-phantoms provided the geometric relation between source path and the Vienna ring applicator as visible on MRI. For the manual direct reconstruction method (PLATO), the geometry plotted on a transparency was placed on the screen and rotated to fit with visible ring holes. With the software-integrated reconstruction method (OncentraGYN), the applicator geometry was directly used when placing the visible parts of the applicator in the 3D dataset. Clinical feasibility was tested in 10 clinical insertions. Reconstruction and dose calculation were performed independently on two treatment planning systems (PLATO and OncentraGYN) using MRI alone. DVH parameters for targets and organs at risk were analysed and compared to the clinically used radiograph/MRI registration-based method.

Results

The direct reconstruction concept for both methods was feasible and reduced treatment planning time. Evaluated DVH parameters for plans using direct reconstruction methods differed from clinically used plans (traditional reconstruction) in mean differences ?0.2 Gy for plans with 7 Gy prescribed dose.

Conclusion

If the relation between applicator shape visible on MRI and the source path is defined once, the reconstruction process can be performed by directly placing the applicator in the MRI dataset.     

Exclusive MRI-based tandem and colpostats reconstruction in gynaecological brachytherapy treatment planning

Purpose

According to the GYN GEC-ESTRO Recommendations, MRI is needed with the modality of T2 weighted. CT-MR compatible tandem plus colpostats (T/C) (Nucletron) is widely used in cervical brachytherapy. The major challenge of MRI is the lack of dummy catheters. Therefore, an additional imaging modality is usually required. One disadvantage of sliced-based reconstruction is that resolution in longitudinal directions can be affected by the slice thickness. The aim of this study is to present a modified T/C applicator in which the catheter visibility is significantly improved.

Methods and materials

A modification to the existing T/C applicator has been made available, increasing the catheter entrance diameter. It allows a greater dummy catheter diameter, and consequently it can be filled with more material per unit length. Visibility was studied with different materials.

Results

The catheter filled with saline water plus iodine compound was clearly seen in both axial and longitudinal acquisitions. To avoid uncertainties introduced by the slice thickness, a simple procedure is presented using longitudinal acquisitions.

Conclusions

The modified applicator plus catheter allows a powerful signal in T2 weighted MRI, using a very simple and accessible material. Also, a method is proposed to solve the problem of the uncertainty derived from slice thickness using few longitudinal acquisitions.     

Recommendations from Gynaecological (GYN) GEC-ESTRO Working Group: Considerations and pitfalls in commissioning and applicator reconstruction in 3D image-based treatment planning of cervix cancer brachytherapy

Image-guided brachytherapy in cervical cancer is increasingly replacing X-ray based dose planning. In image-guided brachytherapy the geometry of the applicator is extracted from the patient 3D images and introduced into the treatment planning system; a process referred to as applicator reconstruction. Due to the steep brachytherapy dose gradients, reconstruction errors can lead to major dose deviations in target and organs at risk. Appropriate applicator commissioning and reconstruction methods must be implemented in order to minimise uncertainties and to avoid accidental errors.Applicator commissioning verifies the location of source positions in relation to the applicator by using auto-radiography and imaging. Sectional imaging can be utilised in the process, with CT imaging being the optimal modality. The results from the commissioning process can be stored as library applicators. The importance of proper commissioning is underlined by the fact that errors in library files result in systematic errors for clinical treatment plans.While the source channel is well visualised in CT images, applicator reconstruction is more challenging when using MR images. Availability of commercial dummy sources for MRI is limited, and image artifacts may occur with titanium applicators. The choice of MR sequence is essential for optimal visualisation of the applicator. Para-transverse imaging (oriented according to the applicator) with small slice thickness (?5 mm) is recommended or alternatively 3D MR sequences with isotropic voxel sizes. Preferably, contouring and reconstruction should be performed in the same image series in order to avoid fusion uncertainties.Clear and correct strategies for the applicator reconstruction will ensure that reconstruction uncertainties have limited impact on the delivered dose. Under well-controlled circumstances the reconstruction uncertainties are in general smaller than other brachytherapy uncertainties such as contouring and organ movement.     

Evaluation of thermometric monitoring for intradiscal laser ablation in an open 1.0 T MR scanner

Purpose: The purpose of this study was to evaluate different methods of magnetic resonance thermometry (MRTh) for the monitoring of intradiscal laser ablation therapy in an open 1.0 Tesla magnetic resonance (MR) scanner.

Material and methods: MRTh methods based on the two endogenous MR temperature indicators of spin-lattice relaxation time T1 and water proton resonance frequency (PRF) shift were optimised and compared in vitro. For the latter, we measured the effective spin-spin relaxation times T2* in intervertebral discs of volunteers. Then we compared four gradient echo-based imaging techniques to monitor laser ablations in human disc specimens. Criteria of assessment were outline of anatomic detail, immunity against needle artefacts, signal-to-noise ratio (SNR) and accuracy of the calculated temperature.

Results: T2* decreased in an inverse and almost linear manner with the patients’ age (r = 0.9) from 70 to 30 ms (mean of 49 ms). The optimum image quality (anatomic details, needle artefacts, SNR) and temperature accuracy (±1.09°C for T1-based and ±1.11°C for PRF-based MRTh) was achieved with a non-spoiled gradient-echo sequence with an echo time of TE = 10 ms.

Conclusion: Combination of anatomic and thermometric non-invasive monitoring of laser ablations in the lumbar spine is feasible. The temperature accuracy of the investigated T1- and PRF-based MRTh methods in vitro is high enough and promises to be reliable in vivo as well.     

Three-dimensional high dose rate intracavitary image-guided brachytherapy for the treatment of cervical cancer using a hybrid magnetic resonance imaging/computed tomography approach: feasibility and early results

Aims

To evaluate the feasibility and outcome of image-guided brachytherapy (IGBT) for treating cervical cancer using magnetic resonance imaging (MRI)-based planning for the first fraction followed by computed tomography (CT)-based planning for subsequent fractions.

Materials and methods

Forty-four patients with cervical cancer were treated with three-dimensional high dose rate IGBT. The brachytherapy dose was 5.0–6.0 Gy × five fractions. All but five patients received concurrent weekly cisplatinum at 40 mg/m². All patients received external beam radiotherapy (EBRT) with a median dose of 45 Gy over 25 fractions. Total doses for the high-risk clinical target volume (HRCTV) and organs at risk, including the rectum, bladder and sigmoid, from EBRT and brachytherapy were summated and normalised to a biologically equivalent dose of 2 Gy per fraction (EQD₂). At 3 months after therapy, any early response was assessed with positron emission tomography (PET)/CT imaging.

Results

The mean D₉₀ for the HRCTV was 83.3 (3.0) Gy. The mean 2 cm³ dose to the bladder, rectum and sigmoid colon organs was 79.7 (5.1), 57.5 (4.4) and 66.8 (5.7) Gy, respectively. All but one (2.3%) patient had a complete response. Follow-up PET/CT was carried out in 41 (93.0%) patients, of whom 38 (92.5%) had a complete response. Of the 38 patients with a complete response on PET/CT, two had local recurrences at 6 and 8 months, respectively. Actuarial 2 year local control, disease-specific and overall survival rates were 88, 85 and 86%, respectively.

Conclusion

This is the first report of three-dimensional high dose rate IGBT for the treatment of cervical cancer using a hybrid MRI/CT approach. Early results have shown the feasibility of this approach with excellent local control. Additional studies are needed to assess long-term outcomes of local control and associated morbidities.     

Magnetic resonance imaging at 3.0 T for submandibular gland sparing radiotherapy

Purpose

Besides sparing the parotid gland, sparing the submandibular gland is considered to be important in preventing xerostomia in head-and-neck cancer patients. Delineation of the submandibular gland at CT, and even on T₁- and T₂-weighted MR images, is difficult, due to low contrast with the surrounding tissues. MR sialography might be used for delineation.

Methods and materials

Sixteen oropharyngeal cancer patients received a CT and MRI exam as part of the standard treatment imaging protocol. Patients were scanned in their five-point RT immobilization mask. The MRI exam included T₁- and T₂-weighted MRI scans and an MR sialography scan. Thirty submandibular glands were delineated on only CT, on the combined CT and T₁- and T₂-weighted MRI scans and on all MR images. A Wilcoxon signed-rank test was performed to test if the delineated volumes were significantly different.

Results

The delineated volume of the submandibular gland was 7.3 mL in the CT-delineation, 7.1 mL in the CT/MRI-delineation and 8.1 mL in the MRI-delineation. The MRI-delineation was significantly larger than the other delineations (p < 0.001). The differences were mainly located in the cranial direction.

Conclusion

The delineation of the submandibular gland was improved in the cranial direction by using T₁- and T₂-weighted MRI and MR sialography, compared to the other delineations.