Does the presence of Magtrace preclude adaptive breast radiotherapy on an MR-Linac?

INTRODUCTION: This work reports on an unusual finding observed during image quality assessment in the preparation for the clinical implementation of breast magnetic resonance image-guided radiotherapy (MRIgRT) on a 1.5 Tesla (T) magnetic resonance linear accelerator (MR-Linac) (Elekta AB, Stockholm, Sweden). CASE AND OUTCOMES: A patient with T2 N0 M0 right breast invasive ductal carcinoma, receiving adjuvant radiotherapy, underwent two imaging sessions on the MR-Linac. The imaging protocol included T1- and T2-weighted (W) turbo spin echo (TSE) sequences, a T1W mDixon, and a T2W TSE navigated sequence acquired on end-expiration. All images were reconstructed in the axial plane. Images were assessed for image quality and appropriateness for use within the treatment pathway using visual grading analysis (VGA). An artefact in the right breast was noted independently by all observers. The patient's skin and medical notes were reviewed for possible explanation. The findings were discussed with the patient's responsible clinician, and subsequent referral to the local multi-disciplinary team (MDT) for radiologist review was made. On further investigation, the patient's images demonstrated a signal void in the subareolar region of the right breast coinciding with the surgical site. This was distal from the tumour bed and deemed unlikely to be related to a Magseed marker or intraoperative clips. The patient reported no history of nipple tattoo or piercing. There was nothing on clothing that this could be attributed to. DISCUSSION: Following MDT review, where all potential sources of signal void were considered, it was concluded that the cause was Magtrace, a superparamagnetic iron oxide tracer, recommended for sentinel lymph node localisation in patients with breast cancer in the United Kingdom. The artefact was characteristic of a magnetic susceptibility artefact. These can arise from local magnetic field inhomogeneities caused by the presence of the metal compounds in MagTrace. For breast MRIgRT on the MR-Linac, treatment verification and the possibility of real-time replanning is a critical aspect. The magnetic susceptibility artefact significantly inhibited plan adaption and confidence in the online image registration process making the patient ineligible for treatment on the MR-Linac. CONCLUSION: As part of ongoing work-up for breast MRIgRT, the screening of patients for Magtrace is now included. Optimisation of MR imaging sequences for radiotherapy planning and image review to minimise distortion are being developed.

Paediatric CBCT protocols for image-guided radiotherapy; outcome of a survey across SIOP Europe affiliated countries and literature review.

BACKGROUND: Implementation of daily cone-beam CT (CBCT) into clinical practice in paediatric image-guided radiotherapy (IGRT) lags behind compared to adults. Surveys report wide variation in practice for paediatric IGRT and technical information remains unreported. In this study we report on technical settings from applied paediatric CBCT protocols and review the literature for paediatric CBCT protocols. METHODS: From September to October 2022, a survey was conducted among 246 SIOPE-affiliated centres across 35 countries. The survey consisted of 3 parts: 1) baseline information; technical CBCT exposure settings and patient set-up procedure for 2) brain/head, and 3) abdomen. Descriptive statistics was used to summarise current practice. The literature was reviewed systematically with two reviewers obtaining consensus RESULTS: The literature search revealed 22 papers concerning paediatric CBCT protocols. Seven papers focused on dose-optimisation. Responses from 50/246 centres in 25/35 countries were collected: 44/50 treated with photons and 10/50 with protons. In total, 48 brain/head and 53 abdominal protocols were reported. 42/50 centres used kV-CBCT for brain/head and 35/50 for abdomen; daily CBCT was used for brain/head = 28/48 (58%) and abdomen = 33/53 62%. Greater consistency was seen in brain/head protocols (dose range 0.32 - 67.7 mGy) compared to abdominal (dose range 0.27 - 119.7 mGy). CONCLUSION: Although daily CBCT is now widely used in paediatric IGRT, our survey demonstrates a wide range of technical settings, suggesting an unmet need to optimise paediatric IGRT protocols. This is in accordance with the literature. However, there are only few paediatric optimisation studies suggesting that dose reduction is possible while maintaining image quality.

The need for consensus on delineation and dose constraints of dentofacial structures in paediatric radiotherapy: Outcomes of a SIOP Europe survey.

Background and purpose: Children receiving radiotherapy for head-and-neck tumours often experience severe dentofacial side effects. Despite this, recommendations for contouring and dose constraints to dentofacial structures are lacking in clinical practice. We report on a survey aiming to understand current practice in contouring and dose assessment to dentofacial structures. Methods: A digital survey was distributed to European Society for Paediatric Oncology members of the Radiation Oncology Working Group, and member-affiliated centres in Europe, Australia, and New Zealand. The questions focused on clinical practice and aimed to establish areas for future development. Results: Results from 52 paediatric radiotherapy centres across 27 countries are reported. Only 29/52 centres routinely delineated some dentofacial structures, with the most common being the mandible (25 centres), temporo-mandibular joint (22), dentition (13), orbit (10) and maxillary bone (eight). For most bones contoured, an 'As Low As Reasonably Achievable' dose objective was implemented. Only four centres reported age-adapted dose constraints.The largest barrier to clinical implementation of dose constraints was firstly, the lack of contouring guidance (49/52, 94%) and secondly, that delineation is time-consuming (33/52, 63%). Most respondents who routinely contour dentofacial structures (25/27, 90%) agreed a contouring atlas would aid delineation. Conclusion: Routine delineation of dentofacial structures is infrequent in paediatric radiotherapy. Based on survey findings, we aim to 1) define a consensus-contouring atlas for dentofacial structures, 2) develop auto-contouring solutions for dentofacial structures to aid clinical implementation, and 3) carry out treatment planning studies to investigate the importance of delineation of these structures for planning optimisation.

Streamlining the image-guided radiotherapy process for proton beam therapy.

OBJECTIVES: This work evaluated the on-treatment imaging workflow in the UK's first proton beam therapy (PBT) centre, with a view to reducing times and unnecessary imaging doses to patients. METHODS: Imaging dose and timing data from the first 20 patients (70% paediatrics, 30% TYA/adult) treated with PBT using the initial image-guided PBT (IGPBT) workflow of a 2-dimensional kilo-voltage (2DkV), followed by cone-beam computed-tomography (CBCT) and repeat 2DkV was included. Pearson correlations and Bland-Altman analysis were used to describe correlations between 2DkV and CBCT images to determine if any images were superfluous. RESULTS: 229 treatment sessions were evaluated. Patient repositioning following the initial 2DkV (i2DkV) was required on 19 (8.3%) fractions. This three-step process resulted in an additional mean imaging dose of 3.4 mGy per patient, and 5.1 minutes on the treatment bed for the patient, over a whole course of PBT, compared to a two-step workflow (removing the i2DkV image). Correspondence between the mean displacements from i2DkV and CBCT was high, with R = 0.94, 0.94 and 0.80 in the anteroposterior, superiorinferior and right-left directions, respectively. Bland-Altman analysis showed very little bias and narrow limits of agreement. CONCLUSIONS: Removing the i2DkV, streamlining to a two-step workflow, would reduce treatment times and imaging dose, and has been implemented as standard verification protocol. For challenging cases (e.g. paediatric patients under GA), further investigations are required before the three-step workflow can be modified. ADVANCES IN KNOWLEDGE: This is the first report assessing a preliminary imaging protocol in PBT in the UK and determining a way to reduce dose and time, which ultimately benefits the patient.