Magnetic Resonance Imaging Protocol Optimization for Delineation of Gross Tumor Volume in Hypopharyngeal and Laryngeal Tumors

doi:10.1016/j.ijrobp.2009.01.014

International Journal of Radiation OncologyBiologyPhysics

Volume 74, Issue 2, 1 June 2009, Pages 630-636

https://doi.org/10.1016/j.ijrobp.2009.01.014 Get rights and content

Purpose

To optimize the use of MRI for delineation of gross tumor volume for radiotherapy treatment planning purposes in hypopharyngeal and laryngeal tumors.

Methods and Materials

Magnetic resonance images (T1 weighted and T2 weighted) of a healthy volunteer were acquired using a 1.5 T and 3.0 T MR scanner. Various receiver coils were investigated that were compatible with the immobilization mask needed for reliable coregistration with computed tomography data. For the optimal receiver coil, the influence of resolution, slice thickness, and strength of magnetic field on the signal-to-noise ratio (SNR) was studied. Feasibility of the definitive protocol was tested on patients with hypopharyngeal (n = 19) and laryngeal (n = 42) carcinoma.

Results

Large differences in SNR were obtained for the various coils. The SNR values obtained using surface coils that were compatible with the immobilization mask were three times higher than those obtained using a standard head-and-neck coil and five times higher than those obtained using a body coil. High-resolution images (0.4 × 0.4 × 4 mm³) showed superior anatomic detail and resulted in a 4-min scan time. Image quality at 3.0 T was not significantly better compared with 1.5 T. In 3 patients the MR study could not be performed; for 5 patients images were severely deteriorated by motion artefacts. High-quality MR images were obtained in 53 patients.

Conclusions

High-resolution MR images of the hypopharynx and larynx can be obtained in the majority of patients using surface receiver coils in combination with the radiotherapy mask. These MR images can be successfully used for tumor delineation in radiotherapy.

Introduction

Delineation of the gross tumor volume (GTV) is the first and probably most important step in the radiotherapy treatment planning process. The capability of three-dimensional conformal therapy and especially intensity-modulated radiotherapy to conform the high-dose volume to the target volumes has further increased the importance of optimal imaging modalities. The introduction of computed tomography (CT) had a large impact on the accuracy of radiotherapy treatment, and CT is currently the standard imaging modality used for radiotherapy treatment planning of head-and-neck tumors. However, MRI has several benefits compared with CT, the most important being superior soft-tissue contrast. Other advantages of MRI are fewer dental-inlay artefacts compared with CT images and, in the case of laryngeal carcinoma, a higher sensitivity to detect tumor invasion of cartilage 1, 2. Other recent developments in MR imaging of laryngeal carcinoma include the potential to predict and monitor local tumor control 3, 4. A disadvantage of MRI, particularly in the region of the larynx, is the occurrence of motion artefacts caused by swallowing and tongue movements due to a longer scan time compared with CT (5).

In the literature there is no agreement on the value of MRI for the determination of the GTV in head-and-neck oncology. By 1988 Toonkel et al.(6) had already recommended MRI for patients needing complex radiotherapy planning for head-and-neck carcinoma. Rasch et al.(7) compared CT- and MR-based tumor delineations for advanced head-and-neck carcinoma and concluded that the delineated GTV and the interobserver variation were smaller for MR than for CT. Gordon et al.(8) investigated the interobserver variability of tumor volume determination for pharyngeal tumors and concluded that MRI-based tumor volumes are sufficiently reproducible. Geets et al.(9) compared MRI and CT for the delineation of pharyngeal–laryngeal tumors and concluded that MRI did not show a clinical advantage over CT in terms of volume determination and interobserver variability. The MR images in the latter study were, however, acquired using a multipurpose body coil in contrast to the other studies, which used dedicated head-and-neck coils generally used for diagnostic MRI. Unfortunately, the radiotherapy immobilization mask does not fit into the dedicated head-and-neck coils. Such a mask is required during MRI to guarantee the millimeter registration accuracy with CT images demanded for radiotherapy purposes.

We believe that these conflicting results can be explained by the relatively little attention paid to MRI protocol optimization, especially the choice of the optimal receiver coils. Currently various flexible surface coils are available, which allow optimal image quality in a dedicated region and are compatible with the use of an immobilization mask. Giron et al.(10) described the gain in signal-to-noise ratio (SNR) of these surface coils used in diagnosing laryngeal malignancies.

The aim of this study was to optimize and clinically test an MRI protocol for the delineation of hypopharyngeal and laryngeal GTVs for treatment planning purposes with the use of the radiotherapy positioning mask. Special attention was paid to the choice of the receiver coils. Furthermore, the influence of the magnetic field strength (1.5 T vs 3.0 T) on image quality was investigated.

Section snippets

Patient fixation

Magnetic resonance protocol optimization was done using a healthy volunteer with no pathology of the larynx. The final MRI protocol was tested on patients with hypopharyngeal and laryngeal carcinoma who were referred to our department for primary radiotherapy. The volunteer and the patients were fixed to a flat tabletop during MR scanning with the same immobilization mask that is used during radiotherapy treatment (five-point head-and-shoulder mask, Posicast PR5; Sinmed, Reeuwijk, The

Receiver coils

The image quality in terms of SNR at 1.5 T was greatly influenced by the type of receiver coil (Fig. 1). High-quality image values were obtained for the dedicated head-and-neck coil and surface coils. Images acquired with the body coil and the phased-array coil were not acceptable, mainly because the body coil senses a much larger volume than the surface coils. Images acquired using the surface coils yielded the highest SNR and CNR values, which were six to seven times higher than those for the

Discussion

The results of this study demonstrate that to utilize the full potential of MRI for radiotherapy purposes, the choice of receiver coils is crucial. The presence of the immobilization mask did not allow the use of dedicated head-and-neck coils that are used in diagnostic imaging. We found that the flexible surface coils offer a good alternative. They allow optimal image quality with sufficient coverage of the head-and-neck region. They can be easily mounted on the mask because of their flexible

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Common guidelines and procedures create a common minimum standard and ground for evaluation and development. In Denmark, the treatment of head and neck cancer is organized within the multidisciplinary Danish Head and Neck Cancer Group (DAHANCA). The radiotherapy quality assurance group of DAHANCA organized a workshop in January 2020 with participants from oncology, radiology, and nuclear medicine from all centers in Denmark, treating patients with head and neck cancer. The participants agreed on a national guideline on imaging for target delineation in head and neck cancer radiotherapy, which has been approved by the DAHANCA group. The guidelines are available in the Supplementary.
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The CT scans were helically acquired, on the same day of the MRI, on a Brilliance Big bore scanner (Philips Medical Systems, Best, The Netherlands) with fixed tube potential of 120 kV, 1 mm slice thickness with in-plane resolution varying between 0.7 and 1.0 mm2. MRI acquisition was performed on a 1.5 T Philips Ingenia scanner (Philips Medical Systems, Best, The Netherlands) using either surface coils (Flex M coils) for the mask MR [16] or a multi-channel Head coil (no-mask MR). MRI scan parameters can be found in Supplementary Table 1.
When using an immobilization mask, a magnetic resonance imaging (MRI) head receive coil cannot be used and patients may experience discomfort during the examination. We therefore wish to assess the added value of an immobilization mask during all MRI scans intended for cranial stereotactic radiotherapy (SRT) planning.
An MRI was acquired with and without a thermoplastic immobilization mask in ten patients eligible for SRT. A planning computed tomography (CT) scan was also made, to which the two MRIs were independently registered. Additionally, the MRI without immobilization was registered to the MRI in mask. On each sequence, gross tumour volume (GTV), the right eye, brain stem and chiasm were delineated. The absolute differences in centre-of-gravity coordinates and Dice coefficients of the volumes of the delineated structures between the two MRIs were compared.
Differences in GTV volume between the two MRIs were low, with median Dice coefficients between 0.88 and 0.91. Similarly, the median absolute differences in centre-of-gravity coordinates between the GTVs, organs at risk and landmarks delineated on the two MRIs were within 0.5 mm. The 95% confidence intervals of the median absolute differences in the three GTV coordinates was within 1 mm, which corresponds to the target volume safety margin used to account for possible errors during the SRT treatment chain.
The effect of scanning a patient without the immobilization mask falls within acceptable bounds of error for the geometrical accuracy of the SRT treatment chain. Consequently, placing the head in treatment position during all MRI scans for patients undergoing radiotherapy of brain metastasis is deemed unnecessary.
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Physics ContributionMagnetic Resonance Imaging Protocol Optimization for Delineation of Gross Tumor Volume in Hypopharyngeal and Laryngeal Tumors

Purpose

Methods and Materials

Results

Conclusions

Introduction

Section snippets

Patient fixation

Receiver coils

Discussion

Semin Roentgenol

Magn Reson Imaging

Int J Radiat Oncol Biol Phys

Radiother Oncol

Int J Radiat Oncol Biol Phys

Physics Contribution
Magnetic Resonance Imaging Protocol Optimization for Delineation of Gross Tumor Volume in Hypopharyngeal and Laryngeal Tumors