Technical note: Cryostat transmission characterization for MR linac - temporal stability, clinical impact and change implementation.

BACKGROUND: In the Unity MR linac (Elekta AB, Stockholm, Sweden), the radiation beam traverses the cryostat and the coil support structure. The resulting beam attenuation must be considered for output calibration and its variation with gantry angle must be characterized in the treatment planning system (TPS). PURPOSE: The aim of this work was to investigate the impact of a change of the cryostat transmission characterization (CTC) curve, due to the helium level modification, on clinical treatment plan dosimetry and to report on the experience with the CTC curve update. METHODS: Twenty stereotactic body radiotherapy (SBRT) treatment plans: 10 prostate and 10 oligo-metastatic cancer plans, prepared with a beam model incorporating the CTC curve acquired at installation time, were re-calculated using the model implementing CTC curve post helium top-up. To account for the CTC change as well as to align our system to the recent reference conditions recommendations, the new model was commissioned with the emphasis on the specifics associated with the treatment plan adaptation and the existence of the offline and online TPS components. RESULTS: Average CTV mean dose reduction by 0.45% in prostate cases and average GTV mean dose reduction by 0.22% in oligo-metastatic cases was observed. Updated model validation showcased good agreement between measurements and TPS calculations. CONCLUSIONS: The agreement between CTC measurements demonstrates its temporal constancy and robustness of the measurement method employed. A helium fill level change was shown to affect the CTC and led to a small but systematic dose calculation inaccuracy. Finally, model validation and end-to-end testing results presented, underscore the minimal impact of transitioning to the new beam model and new reference conditions.

MR-Linac guided adaptive stereotactic ablative body radiotherapy for recurrent cardiac sarcoma with mitral valve bioprosthesis - a case report.

We present the first case in the literature of a 78-year-old woman with recurrent cardiac sarcoma adjacent to a bioprosthetic mitral valve treated with magnetic resonance linear accelerator (MR-Linac) guided adaptive stereotactic ablative body radiotherapy (SABR). The patient was treated using a 1.5 T Unity MR-Linac system (Elekta AB, Stockholm, Sweden). The mean gross tumour volume (GTV) size was 17.9 cm3 (range 16.6-18.9 cm3 ) based on daily contours and the mean dose received by the GTV was 41.4 Gy (range 40.9-41.6 Gy) in five fractions. All fractions were completed as planned and the patient tolerated the treatment well with no acute toxicity reported. Follow-up appointments at 2 and 5 months after the last treatment showed stable disease and good symptomatic relief. Results of transthoracic echocardiogram after radiotherapy showed that the mitral valve prosthesis was normally seated with regular functionality. This study provides evidence that MR-Linac guided adaptive SABR is a safe and viable option for the treatment of recurrent cardiac sarcoma with mitral valve bioprosthesis.

Old dogs, new tricks: MR-Linac training and credentialing of radiation oncologists, radiation therapists and medical physicists.

The introduction of magnetic resonance (MR) linear accelerators (MR-Linacs) into radiotherapy departments has increased in recent years owing to its unique advantages including the ability to deliver online adaptive radiotherapy. However, most radiation oncology professionals are not accustomed to working with MR technology. The integration of an MR-Linac into routine practice requires many considerations including MR safety, MR image acquisition and optimisation, image interpretation and adaptive radiotherapy strategies. This article provides an overview of training and credentialing requirements for radiation oncology professionals to develop competency and efficiency in delivering treatment safely on an MR-Linac.

Introduction of radiation therapist-led adaptive treatments on a 1.5 T MR-Linac.

The introduction of magnetic resonance (MR) linear accelerators (MR-Linac) marks the beginning of a new era in radiotherapy. MR-Linac systems are currently being operated by teams of radiation therapists (RTs), radiation oncology medical physicists (ROMPs) and radiation oncologists (ROs) due to the diverse and complex tasks required to deliver treatment. This is resource-intensive and logistically challenging. RT-led service delivery at the treatment console is paramount to simplify the process and make the best use of this technology for suitable patients with commonly treated anatomical sites. This article will discuss the experiences of our department in developing and implementing an RT-led workflow on the 1.5 T MR-Linac.

Early experience with MR-guided adaptive radiotherapy using a 1.5 T MR-Linac: First 6 months of operation using adapt to shape workflow.

INTRODUCTION: The magnetic resonance linear accelerator (MRL) offers improved soft tissue visualization to guide daily adaptive radiotherapy treatment. This manuscript aims to report initial experience using a 1.5 T MRL in the first 6 months of operation, including training, workflows, timings and dosimetric accuracy. METHODS: All staff received training in MRI safety and MRL workflows. Initial sites chosen for treatment were stereotactic and hypofractionated prostate, thoraco-abdomino-pelvic metastasis, prostate bed and bladder. The Adapt To Shape (ATS) workflow was chosen to be the focus of treatment as it is the most robust solution for daily adaptive radiotherapy. A workflow was created addressing patient suitability, simulation, planning, treatment and peer review. Treatment times were recorded breaking down into the various stages of treatment. RESULTS: A total of 37 patients were treated and 317 fractions delivered (of which 313 were delivered using an ATS workflow) in our initial 6 months. Average treatment times over the entire period were 50 and 38 min for stereotactic and non-stereotactic treatments respectively. Average treatment times reduced each month. The average difference between reference planned and ionization chamber measured dose was 0.0 ± 1.4%. CONCLUSION: The MRL was successfully established in an Australian setting. A focus on training and creating a detailed workflow from patient selection, review and treatment are paramount to establishing new treatment programmes.

Dosimetric validation of the Theragenics AgX-100® I-125 seed for ROPES eye plaque brachytherapy.

With the discontinued distribution of the I-125 Oncura Onco seed (model 6711), the Theragenics AgX100® I-125 seeds were considered as a suitable alternative for eye plaque brachytherapy as their physical properties matched the requirements for use with the ROPES eye plaques. The purpose of this study aims at validating the dosimetry of the AgX-100 loaded ROPES plaques (11 mm diameter, 15 mm diameter with flange, 15 mm diameter with notch, 18 mm diameter) and assess the differences with the discontinued I-125 6711 model. To independently verify the plaque dosimetry, the brachytherapy module of RADCALC® version 6.2.3.6 was commissioned for the new AgX-100 I-125 seed using the published AAPM TG43 data from the literature. Experimental dosimetry verification was performed using EBT3 Gafchromic™ film and TLD-100 micro-cubes in a specially designed Solid Water® phantom. Both RADCALC® and film confirmed the dosimetry calculated by Plaque Simulator (PS) version 6.4.6 The dose calculated by PS agrees with RADCALC® to within 2% for depths of 1-15 mm for the 4 available ROPES plaques. The dosimetric measurements agreed with the calculations of PS for clinically relevant depths (4 mm to 6 mm) within the evaluated uncertainties of 4.7% and 7.2% for EBT3 film and TLDs respectively. The AgX-100 I-125 seed was a suitable replacement for the 6711 I-125 seed. Due to the introduction of the stainless-steel backscatter factor in PS v6.4.6, the department has decided to report both the homogenous dose and heterogeneity corrected dose for each eye plaque patient.