Quality assurance for on-table adaptive magnetic resonance guided radiation therapy: A software tool to complement secondary dose calculation and failure modes discovered in clinical routine.

Online adaption of treatment plans on a magnetic resonance (MR)-Linac enables the daily creation of new (adapted) treatment plans using current anatomical information of the patient as seen on MR images. Plan quality assurance (QA) relies on a secondary dose calculation (SDC) that is required because a pretreatment measurement is impossible during the adaptive workflow. However, failure mode and effect analysis of the adaptive planning process shows a large number of error sources, and not all of them are covered by SDC. As the complex multidisciplinary adaption process takes place under time pressure, additional software solutions for pretreatment per-fraction QA need to be used. It is essential to double-check SDC input to ensure a safe treatment delivery. Here, we present an automated treatment plan check tool for adaptive radiotherapy (APART) at a 0.35 T MR-Linac. It is designed to complement the manufacturer-provided adaptive QA tool comprising SDC. Checks performed by APART include contour analysis, electron density map examinations, and fluence modulation complexity controls. For nine of 362 adapted fractions (2.5%), irregularities regarding missing slices in target volumes and organs at risks as well as in margin expansion of target volumes have been found. This demonstrates that mistakes occur and can be detected by additional QA measures, especially contour analysis. Therefore, it is recommended to implement further QA tools additional to what the manufacturer provides to facilitate an informed decision about the quality of the treatment plan.

A practical implementation of risk management for the clinical introduction of online adaptive Magnetic Resonance-guided radiotherapy.

BACKGROUND AND PURPOSE: The clinical introduction of on-table adaptive radiotherapy with Magnetic Resonance (MR)-guided linear accelerators (Linacs) yields new challenges and potential risks. Since the adapted plan is created within a highly interdisciplinary workflow with the patient in treatment position, time pressure or erroneous communication may lead to various possibly hazardous situations. To identify risks and implement a safe workflow, a proactive risk analysis has been conducted. MATERIALS AND METHODS: A process failure mode, effects and criticality analysis (P-FMECA) was performed within a group of radiation therapy technologists, physicians and physicists together with an external moderator. The workflow for on-table adaptive MR-guided treatments was defined and for each step potentially hazardous situations were identified. The risks were evaluated within the team in order to homogenize risk assessment. The team elaborated and discussed possible mitigation strategies and carried out their implementation. RESULTS: In total, 89 risks were identified for the entire MR-guided online adaptive workflow. After mitigation, all risks could be minimized to an acceptable level. Overall, the need for a standardized workflow, clear-defined protocols together with the need for checklists to ensure protocol adherence were identified among the most important mitigation measures. Moreover, additional quality assurance processes and automated plan checks were developed. CONCLUSIONS: Despite additional workload and beyond the fulfilment of legal requirements, execution of the P-FMECA within an interdisciplinary team helped all involved occupational groups to develop and foster an open culture of safety and to ensure a consensus for an efficient and safe online adaptive radiotherapy workflow.

Dosimetry in magnetic fields with dedicated MR-compatible ionization chambers.

MR-integrated radiotherapy requires suitable dosimetry detectors to be used in magnetic fields. This study investigates the feasibility of using dedicated MR-compatible ionization chambers at MR-integrated radiotherapy devices. MR-compatible ionization chambers (Exradin A19MR, A1SLMR, A26MR, A28MR) were precisely modeled and their relative response in a 6MV treatment beam in the presence of a magnetic field was simulated using EGSnrc. Monte Carlo simulations were carried out with the magnetic field in three orientations: the magnetic field aligned perpendicular to the chamber and beam axis (transverse orientation), the magnetic field parallel to the chamber as well as parallel to the beam axis. Monte Carlo simulation results were validated with measurements using an electromagnet with magnetic field strength upto 1.1 T with the chambers in transverse orientation. The measurements and simulation results were in good agreement, except for the A26MR ionization chamber in transverse orientation. The maximum increase in response of the ionization chambers observed was 8.6% for the transverse orientation. No appreciable change in chamber response due to the magnetic field was observed for the magnetic field parallel to the ionization chamber and parallel to the photon beam. Polarity and recombination correction factor were experimentally investigated in the transverse orientation. The polarity effect and recombination effect were not altered by a magnetic field. This study further investigates the response of the ionization chambers as a function of the chambers' rotation around their longitudinal axis. A variation in response was observed when the chamber was not rotationally symmetric, which was independent of the magnetic field.

The effect of density overrides on magnetic resonance-guided radiation therapy planning for lung cancer.

BACKGROUND AND PURPOSE: Inverse treatment planning for lung cancer can be challenging since density heterogeneities may appear inside the planning target volume (PTV). One method to improve the quality of intensity modulation is the override of low density tissues inside the PTV during plan optimization. For magnetic resonance-guided radiation therapy (MRgRT), where the influence of the magnetic field on secondary electrons is sensitive to the tissue density, the reliability of density overrides has not yet been proven. This work, therefore, gains a first insight into density override strategies for MRgRT. MATERIAL AND METHODS: Monte Carlo-based treatment plans for five lung cancer patients were generated based on free-breathing CTs and two density override strategies. Different magnetic field configurations were considered with their effect being accounted for during optimization. Optimized plans were forward calculated to 4D-CTs and accumulated for the comparison of planned and expected delivered dose. RESULTS: For MRgRT, density overrides led to a discrepancy between the delivered and planned dose. The tumor volume coverage deteriorated for perpendicular magnetic fields of 1.5 T to 93.6% (D98%). For inline fields a maximal increase of 2.2% was found for the mean dose. In terms of organs at risk, a maximal sparing of 0.6 Gy and 0.9 Gy was observed for lung and heart, respectively. CONCLUSIONS: In this work, first results on the effect of density overrides on treatment planning for MRgRT are presented. It was observed that the underestimation of magnetic field effects in overridden densities during treatment planning resulted in an altered delivered dose, depending on the field strength and orientation.

Effects of magnetic field orientation and strength on the treatment planning of nonsmall cell lung cancer.

PURPOSE: Magnetic resonance image-guided radiotherapy (MRgRT) has the potential to increase the accuracy of radiation treatment delivery. Several research groups have developed hybrid MRgRT devices differing by radiation source used and magnetic field orientation and strength. In this work, we investigate the impact of different magnetic field orientations and strengths on the treatment planning of nonsmall cell lung cancer patients (NSCLC). METHODS: A framework using the in-house developed treatment planning system matRad and the EGSnrc Monte Carlo code system was introduced to perform Monte Carlo-based treatment planning in the presence of a magnetic field. A specialized spectrum-based source model for the beam qualities of 6 MV and cobalt-60 was applied. Optimized plans for stereotactic body radiation therapy (SBRT) and intensity-modulated radiation therapy (IMRT) were generated for four NSCLC patients in the presence of different magnetic field orientations and strengths which are applied in hybrid MRgRT devices currently under development or in clinical use. RESULTS: SBRT and IMRT treatment planning could be performed with consistent plan quality for all magnetic field setups. Only minor effects on the treatment planning outcome were found in the case of magnetic fields orientated perpendicular to the beam direction. Compared to the perpendicular magnetic field orientation, the inline orientation showed the capability to reduce the dose to lung while maintaining equal target coverage. Particularly for tumors with a central position in lung, a distinct dose reduction was obtained which led to a maximum reduction of mean lung dose by 18.5% (0.5 Gy), when applying a 1 T inline magnetic field. CONCLUSION: All plans generated in this work obtained dose metrics within clinical constraints according to RTOG guidelines. When considering conventional dose metrics, no detrimental effects due to the magnetic fields were observed on the dose to the tumor or to organs at risk. An evaluation of the effects on skin dose was not ascertainable due to the simplified specification of the source model used. By accounting for the magnetic field during treatment planning, a dose reduction in lung could be achieved for inline-oriented magnetic fields. An inline orientation of the magnetic field therefore showed a potential benefit when treating NSCLC with MRgRT.