Individual lymph nodes: "See it and Zap it"

Intra-fraction motion of pelvic oligometastases and feasibility of PTV margin reduction using MRI guided adaptive radiotherapy

Purpose

This study assesses the impact of intra-fraction motion and PTV margin size on target coverage for patients undergoing radiation treatment of pelvic oligometastases. Dosimetric sparing of the bowel as a function of the PTV margin is also evaluated.

Materials and methods

Seven patients with pelvic oligometastases previously treated on our MR-linac (35 Gy in 5 fractions) were included in this study. Retrospective adaptive plans were created for each fraction on the daily MRI datasets using PTV margins of 5 mm, 3 mm, and 2 mm. Dosimetric constraint violations and GTV coverage were measured as a function of PTV margin size. The impact of intra-fraction motion on GTV coverage was assessed by tracking the GTV position on the cine MR images acquired during treatment delivery and creating an intra-fraction dose distribution for each IMRT beam. The intra-fraction dose was accumulated for each fraction to determine the total dose delivered to the target for each PTV size.

Results

All OAR constraints were achieved in 85.7%, 94.3%, and 100.0% of fractions when using 5 mm, 3 mm, and 2 mm PTV margins while scaling to 95% PTV coverage. Compared to plans with a 5 mm PTV margin, there was a 27.4 ± 12.3% (4.0 ± 2.2 Gy) and an 18.5 ± 7.3% (2.7 ± 1.4 Gy) reduction in the bowel D_0.5cc dose for 2 mm and 3 mm PTV margins, respectively. The target dose (GTV V_{35 Gy}) was on average 100.0 ± 0.1% (99.6 - 100%), 99.6 ± 1.0% (97.2 - 100%), and 99.0 ± 1.4% (95.0 - 100%), among all fractions for the 5 mm, 3 mm, and 2 mm PTV margins on the adaptive plans when accounting for intra-fraction motion, respectively.

Conclusion

A 2 mm PTV margin achieved a minimum of 95% GTV coverage while reducing the dose to the bowel for all patients.

Proposal and Evaluation of a Physician-Free, Real-Time On-Table Adaptive Radiotherapy (PF-ROAR) Workflow for the MRIdian MR-Guided LINAC

With the implementation of MR-LINACs, real-time adaptive radiotherapy has become a possibility within the clinic. However, the process of adapting a patient’s plan is time consuming and often requires input from the entire clinical team, which translates to decreased throughput and limited patient access. In this study, the authors propose and simulate a workflow to address these inefficiencies in staffing and patient throughput. Two physicians, three radiation therapists (RTT), and a research fellow each adapted bladder and bowel contours for 20 fractions from 10 representative patient plans. Contouring ability was compared via calculation of a Dice Similarity Index (DSI). The DSI for bladder and bowel based on each potential physician–therapist pair, as well as an inter-physician comparison, exhibited good overlap amongst all comparisons (p = 0.868). Plan quality was compared through calculation of the conformity index (CI), as well as an evaluation of the plan’s dose to a ‘gold standard’ set of structures. Overall, non-physician plans passed 91.2% of the time. Of the eight non-physician plans that failed their clinical evaluation, six also failed their evaluation against the ‘gold standard’. Another two plans that passed their clinical evaluation subsequently failed in their evaluation against the ‘gold standard’. Thus, the PF-ROAR process has a success rate of 97.5%, with 78/80 plans correctly adapted to the gold standard or halted at treatment. These findings suggest that a physician-free workflow can be well tolerated provided RTTs continue to develop knowledge of MR anatomy and careful attention is given to understanding the complexity of the plan prior to treatment.

Patient positioning and immobilization procedures for hybrid MR-Linac systems

Hybrid magnetic resonance (MR)-guided linear accelerators represent a new horizon in the field of radiation oncology. By harnessing the favorable combination of on-board MR-imaging with the possibility to daily recalculate the treatment plan based on real-time anatomy, the accuracy in target and organs-at-risk identification is expected to be improved, with the aim to provide the best tailored treatment. To date, two main MR-linac hybrid machines are available, Elekta Unity and Viewray MRIdian. Of note, compared to conventional linacs, these devices raise practical issues due to the positioning phase for the need to include the coil in the immobilization procedure and in order to perform the best reproducible positioning, also in light of the potentially longer treatment time. Given the relative novelty of this technology, there are few literature data regarding the procedures and the workflows for patient positioning and immobilization for MR-guided daily adaptive radiotherapy. In the present narrative review, we resume the currently available literature and provide an overview of the positioning and setup procedures for all the anatomical districts for hybrid MR-linac systems.

Stereotactic body radiotherapy of lymph node metastases under MR-guidance: First clinical results and patient-reported outcomes

Objective

Stereotactic body radiotherapy (SBRT) is a noninvasive treatment option for lymph node metastases (LNM). Magnetic resonance (MR)-guidance offers superior tissue contrast and enables treatment of targets in close vicinity to radiosensitive organs at risk (OAR). However, literature on MR-guided SBRT of LNM is scarce with no report on outcome parameters.

Materials and methods

We report a subgroup analysis of a prospective observational study comprising patients with LNM. Patients received MR-guided SBRT at our MRIdian Linac (ViewRay Inc., Mountain View, CA, USA) between January 2019 and February 2020. Local control (LC), progression-free survival (PFS) and overall survival (OS) analysis were performed using the Kaplan–Meier method with log rank test to test for significance (p < 0.05). Our patient-reported outcome questionnaire was utilized to evaluate patients’ perspective. The CTCAE (Common Terminology Criteria for Adverse Events) v. 5.0 was used to describe toxicity.

Results

Twenty-nine patients (72.4% with prostate cancer; 51.7% with no distant metastases) received MR-guided SBRT for in total 39 LNM. Median dose was 27 Gy in three fractions, prescribed to the 80% isodose. At 1‑year, estimated LC, PFS and OS were 92.6, 67.4 and 100.0%. Compared to baseline, six patients (20.7%) developed new grade I toxicities (mainly fatigue). One grade II toxicity occurred (fatigue), with no adverse event grade ≥III. Overall treatment experience was rated particularly positive, while the technically required low room temperature still represents the greatest obstacle in the pursuit of the ideal patient acceptance.

Conclusion

MR-guided SBRT of LNM was demonstrated to be a well-accepted treatment modality with excellent preliminary results. Future studies should evaluate the clinical superiority to conventional SBRT.

Video online

The online version of this article contains one video. The article and the video are available online (10.1007/s00066-021-01834-w). The video can be found in the article back matter as “Supplementary Information”.

Online Magnetic Resonance-Guided Radiotherapy (oMRgRT) for Gynecological Cancers

Radiation therapy (RT) is increasingly being used in gynecological cancer management. RT delivered with curative or palliative intent can be administered alone or combined with chemotherapy or surgery. Advanced treatment planning and delivery techniques such as intensity-modulated radiation therapy, including volumetric modulated arc therapy, and image-guided adaptive brachytherapy allow for highly conformal radiation dose delivery leading to improved tumor control rates and less treatment toxicity. Quality on-board imaging that provides accurate visualization of target and surrounding organs at risk is a critical feature of these advanced techniques. As soft tissue contrast resolution is superior with magnetic resonance imaging (MRI) compared to other imaging modalities, MRI has been used increasingly to delineate tumor from adjacent soft tissues and organs at risk from initial diagnosis to tumor response evaluation. Gynecological cancers often have poor contrast resolution compared to the surrounding tissues on computed tomography scan, and consequently the benefit of MRI is high. One example is in management of locally advanced cervix cancer where adaptive MRI guidance has been broadly implemented for adaptive brachytherapy. The role of MRI for external beam RT is also steadily increasing. MRI information is being used for treatment planning, predicting, and monitoring position shifts and accounting for tissue deformation and target regression during treatment. The recent clinical introduction of online MRI-guided radiation therapy (oMRgRT) could be the next step in high-precision RT. This technology provides a tool to take full advantage of MRI not only at the time of initial treatment planning but as well as for daily position verification and online plan adaptation. Cervical, endometrial, vaginal, and oligometastatic ovarian cancers are being treated on MRI linear accelerator systems throughout the world. This review summarizes the current state, early experience, ongoing trials, and future directions of oMRgRT in the management of gynecological cancers.

SBRT treatment of abdominal and pelvic oligometastatic lymph nodes using ring-mounted Halcyon Linac

PURPOSE/OBJECTIVES

This work seeks to evaluate the plan quality, treatment delivery efficiency, and accuracy of single-isocenter volumetric modulated arc therapy (VMAT) of abdominal/pelvic oligometastatic lymph nodes (LNs) stereotactic body radiation therapy (SBRT) on Halcyon Linac.

MATERIALS AND METHODS

After completing the in-house multitarget end-to-end phantom testing and independent dose verification using MD Anderson's single-isocenter/multi-target (lung and spine target inserts) thorax phantom, eight patients with two to three abdominal/pelvic oligometastatic LNs underwent highly conformal single-isocenter VMAT-SBRT treatment using the Halcyon Linac 6MV flattening filter free (FFF) beam. Targets were identified using an Axumin PET/CT scan co-registered with planning CT images and a single-isocenter was placed between/among the targets. Doses between 25 and 36.25 Gy in 5 fractions were delivered. Patients were treated every other day. Plans were calculated in Eclipse with advanced AcurosXB algorithm for heterogeneity corrections. For comparison, Halcyon VMAT-SBRT plans were retrospectively generated for SBRT-dedicated TrueBeam with a 6MV-FFF beam using identical planning geometry and objectives. Target coverage, conformity index (CI), dose to 2 cm away from each target (D2cm) and dose to adjacent organs-at-risk (OAR) were evaluated. Additionally, various treatment delivery parameters including beam-on time were recorded.

RESULTS

Phantom measurements showed acceptable spatial accuracy of conebeam CT-guided Halcyon SBRT treatments including compliance with MD Anderson's single-isocenter/multi-targets phantom credentialing results. For patients, the mean isocenter to tumor center distance was 3.4 ± 1.2 cm (range, 1.5-4.8 cm). The mean combined PTV was 18.9 ± 10.9 cc (range, 5.6-39.5 cc). There was no clinically significant difference in dose to LNs, CI, D2cm and maximal doses to OAR between single-isocenter Halcyon and Truebeam VMAT-SBRT plans, although, Halcyon plans provided preferably lower maximal dose to adjacent OAR. Additionally, total monitor units, beam-on time and overall treatment time was lower with Halcyon plans. Halcyon's portal dosimetry demonstrated a high pass rate of 98.1 ± 1.6% for clinical gamma passing criteria of 2%/2 mm.

CONCLUSION

SBRT treatment of abdominal/pelvic oligometastatic LNs with single-isocenter VMAT on Halcyon was dosimetrically equivalent to TrueBeam. Faster treatment delivery to oligometastatic LNs via single-isocenter Halcyon VMAT can improve clinic workflow and patient compliance, potentially reducing intrafraction motion errors for well-suited patients. Clinical follow-up of these patients is ongoing.

Stereotactic body radiotherapy for oligometastatic castration sensitive prostate cancer using 1.5 T MRI-Linac: preliminary data on feasibility and acute patient-reported outcomes

OBJECTIVES

To report preliminary data on feasibility and patient-reported outcomes following PSMA-PET/CT guided SBRT by means of 1.5 T MRI-Linac.

METHODS AND MATERIALS

Between October 2019 and April 2020, twenty consecutive castration sensitive oligorecurrent prostate cancer patients were enrolled in an ethical committee approved prospective observational study (Protocol n. XXXX) and treated with PSMA-PET/CT guided SBRT by means of 1.5 T MRI-Linac (Unity, Elekta AB, Stockholm, Sweden). The mean delivered dose was 35 Gy in 5 fractions. Clinicians reported toxicity was prospectively collected according to Common Terminology Criteria for Adverse Events v5.0. Quality of life (QoL) assessment was performed using EORTC-QLQ C30 questionnaires administered at baseline, end of treatment and at first follow-up.

RESULTS

Twenty-five lesions in 20 castration sensitive oligorecurrent patients were treated: the most commonly treated anatomic sites were nodal (n = 16) and pelvic bone (n = 9). Median PSA-value preMRI guided SBRT was 1.16 ng/mL (range, 0.27-8.9), whereas median PSA value at first follow-up after SBRT was 0.44 ng/mL (range, 0.06-8.15). At first follow-up, for 16 patients showing detectable PSA, PSMA-PET/CT was performed detecting, respectively, in 6 cases partial response and in 10 cases complete response. In the remaining cases, PSA-value was undetectable after SBRT. Radiotherapy treatment was safe and well tolerated according to the PROMs. No acute G2 or higher toxicities were recorded.

CONCLUSIONS

The current series represent the largest one exploring the feasibility and patient-reported outcomes following PSMA-PET/CT guided SBRT by means of 1.5 T MRI-Linac. The preliminary findings here reported are encouraging in terms of effectiveness and tolerability.

Online adaptive planning for prostate stereotactic body radiotherapy using a 1.5 Tesla magnetic resonance imaging-guided linear accelerator

Recent advances in integrating 1.5 Tesla magnetic resonance (MR) imaging with a linear accelerator (MR-Linac) allow MR-guided stereotactic body radiotherapy (SBRT) for prostate cancer. Choosing an optimal strategy for daily online plan adaptation is particularly important for MR-guided radiotherapy. We analyzed deformable dose accumulation on scans from four patients and found that daily anatomy changes had little impact on the delivered dose, with the dose to the prostate within 0.5% and dose to the rectum/bladder mostly less than 0.5 Gy. These findings could help in the choice of an optimal strategy for online plan adaptation for MR-guided prostate SBRT.

Case Report: First in Human Online Adaptive MR Guided SBRT of Peritoneal Carcinomatosis Nodules: A New Therapeutic Approach for the Oligo-Metastatic Patient

Peritoneal carcinosis (PC) is characterized by poor prognosis. PC is currently treated as a locoregional disease and the possibility to perform very precise treatments such as stereotactic body radiation therapy (SBRT) has opened up new therapeutic perspectives. More recently, the introduction of Magnetic Resonance-guided Radiation Therapy (MRgRT) allowed online adaptation (OA) of treatment plan to optimize daily dose distribution based on patient’s anatomy. The aim of this study is the evaluation of the effectiveness of SBRT OA workflow in an oligometastatic patient affected by PC. We report the clinical case of a patient affected by PC originating from colon cancer, previously treated with chemotherapy and surgery, addressed to OA SBRT treatment on a single chemoresistant PC nodule, delivered with a 0.35 T MR Linac. Treatment was delivered using gating approach in deep inspiration breath hold condition in order to reduce intrafraction variability. Prescription dose was 35 Gy in 5 fractions. The PTV V95% of the original plan was 96.6%, while the predicted values for the following fractions were 11.9, 56.4, 0, 0, and 61%. Similarly, the small bowel V19.5 Gy of the original plan was 4.63 cc, while the predicted values for the following fractions were 3.7, 8.6, 10.7, 1.96, 3.7 cc. Thanks to the OA approach, the re-optimized PTV V95% coverage improved to 96.1, 89.0, 85.5, 94.5, and 94%; while the small bowel V19.5 Gy to 3.36; 3.28; 1.84; 2.62; 2.6 cc respectively. After the end of RT, the patient was addressed to follow-up, and the re-evaluation ¹⁸F-FDG PET-CT was performed after 10 months from irradiation showed complete response. No acute or late toxicities were recorded. MRgRT with OA approach in PC patients is technically and clinically feasible with clean toxicity result. Online adaptive SBRT for oligometastases opens up new therapeutic scenarios in the management of this category of patients.

Impact of a vacuum cushion on intrafraction motion during online adaptive MR-guided SBRT for pelvic and para-aortic lymph node oligometastases

BACKGROUND AND PURPOSE

Vacuum cushion immobilization is commonly used during stereotactic body radiotherapy (SBRT) to reduce intrafraction motion. We investigated target and bony anatomy intrafraction motion (translations and rotations) during online adaptive SBRT on an MR-linac for pelvic/para-aortic lymph node metastases with and without vacuum cushion.

MATERIALS AND METHODS

Thirty-nine patients underwent 5x7 Gy SBRT on a 1.5T MR-linac, 19 patients were treated with vacuum cushion, 19 without and 1 patient sequentially with and without. Intrafraction motion was calculated for target lymph nodes (GTVs) and nearby bony anatomy, for three time intervals (pre-position verification (PV), pre-post, PV-post, relating to the online MRI scans) per treatment fraction.

RESULTS

Vacuum cushion immobilization significantly reduced anterior-posterior translations for the pre-PV and pre-post intervals, for bony anatomy and pre-post interval for GTV (p < 0.05). Mean GTV intrafraction motion reduction in posterior direction was 0.7 mm (95% confidence interval 0.3-1.1 mm) for pre-post interval (mean time = 32 min). Shifts in other directions were not significantly reduced. More motion occurred in pre-PV interval than in PV-post interval (mean time = 16 min for both); vacuum cushion immobilization did not reduce intrafraction motion during the beam-on period.

CONCLUSION

A vacuum cushion reduces GTV and bony anatomy intrafraction motion in posterior direction during pelvic/para-aortic lymph node SBRT. This motion reduction was found for the first 16 min per session. For single targets this motion can be corrected for directly with an MR-linac. Intrafraction motion was not reduced during the second half of the session, the period of radiotherapy delivery on an MR-linac. Vacuum cushion immobilization may not be necessary for patients with single lymph node oligometastases undergoing SBRT on an MR-linac.

Dosimetric feasibility of hypofractionation for SBRT treatment of lymph node oligometastases on the 1.5T MR-linac

PURPOSE

At our department, MR-guided stereotactic body radiation therapy (SBRT) using the 1.5T MR-linac system (Unity, Elekta AB, Stockholm, Sweden) has been initiated for patients with lymph node oligometastases. Superior soft tissue contrast and the possibility for online plan adaptation on the Unity may allow for hypofractionated treatment. The purpose of this study was to investigate the dosimetric feasibility and compare the plan quality of different hypofractionated schemes.

METHODS AND MATERIALS

Data was used from 12 patients with single lymph node oligometastases (10 pelvic, 2 para-aortic), which were all treated on the Unity with a prescribed dose of 5x7 Gy to 95% of the PTV. Hypofractionation was investigated for 3x10 Gy and 1x20 Gy schemes (all 60 Gy BED α/β = 10). The pre-treatment plans were evaluated based on dose criteria and plan quality. If all criteria were met, the number of online adapted plans which also met all dose criteria was investigated. For pre-treatment plans meeting the criteria for all three fractionation schemes, the plan quality after online adaptation was compared using the four parameters described in the NRG-BR001 phase 1 trial.

RESULTS

Pre-treatment plans met all clinical criteria for the three different fractionation schemes in 10, 9 and 6 cases. 50/50, 45/45 17/30 of the corresponding online adapted plans met all criteria, respectively. Violations were primarily caused by surrounding organs at risk overlapping or adjacent to the PTV. The 1x20 Gy treatment plans were, in general, of lesser quality than the 5x7 Gy and 3x10 Gy plans.

CONCLUSION

Hypofractionated radiotherapy for lymph node oligometastases on the 1.5T MR-linac is feasible based on dose criteria and plan quality metrics. The location of the target relative to critical structures should be considered in choosing the most suitable fractionation scheme. Especially for single fraction treatment, meeting all dose criteria in the pre-treatment situation does not guarantee that this also applies during online treatment.

Online adaptive radiotherapy compared to plan selection for rectal cancer: quantifying the benefit

Background

To compare online adaptive radiation therapy (ART) to a clinically implemented plan selection strategy (PS) with respect to dose to the organs at risk (OAR) for rectal cancer.

Methods

The first 20 patients treated with PS between May–September 2016 were included. This resulted in 10 short (SCRT) and 10 long (LCRT) course radiotherapy treatment schedules with a total of 300 Conebeam CT scans (CBCT). New dual arc VMAT plans were generated using auto-planning for both the online ART and PS strategy.

For each fraction bowel bag, bladder and mesorectum were delineated on daily Conebeam CTs. The dose distribution planned was used to calculate daily DVHs. Coverage of the CTV was calculated, as defined by the dose received by 99% of the CTV volume (D99%). The volume of normal tissue irradiated with 95% of the prescribed fraction dose was calculated by calculating the volume receiving 95% of the prescribed fraction or more dose minus the volume of the CTV. For each fraction the difference between the plan selection and online adaptive strategy of each DVH parameter was calculated, as well as the average difference per patient.

Results

Target coverage remained the same for online ART. The median volume of the normal tissue irradiated with 95% of the prescribed dose dropped from 642 cm3 (PS) to 237 cm3 (online-ART)(p < 0.001). Online ART reduced dose to the OARs for all tested dose levels for SCRT and LCRT (p < 0.001). For V15Gy of the bowel bag the median difference over all fractions of all patients was − 126 cm³ in LCRT, while the average difference per patient ranged from − 206 cm³ to − 40 cm³. For SCRT the median difference was − 62 cm³, while the range of the average difference per patient was − 105 cm3 to − 51 cm³.

For V15Gy of the bladder the median difference over all fractions of all patients was 26% in LCRT, while the average difference per patient ranged from − 34 to 12%. For SCRT the median difference of V95% was − 8%, while the range of the average difference per patient was − 29 to 0%.

Conclusions

Online ART for rectal cancer reduces dose the OARs significantly compared to a clinically implemented plan selection strategy, without compromising target coverage.

Trial registration

Medical Research Involving Human Subjects Act (WMO) does not apply to this study and was retrospectively approved by the Medical Ethics review Committee of the Academic Medical Center (W19_357 # 19.420; Amsterdam University Medical Centers, Location Academic Medical Center, Amsterdam, The Netherlands).

Image guidance in radiation therapy for better cure of cancer

The key goal and main challenge of radiation therapy is the elimination of tumors without any concurring damages of the surrounding healthy tissues and organs. Radiation doses required to achieve sufficient cancer-cell kill exceed in most clinical situations the dose that can be tolerated by the healthy tissues, especially when large parts of the affected organ are irradiated. High-precision radiation oncology aims at optimizing tumor coverage, while sparing normal tissues. Medical imaging during the preparation phase, as well as in the treatment room for localization of the tumor and directing the beam, referred to as image-guided radiotherapy (IGRT), is the cornerstone of precision radiation oncology. Sophisticated high-resolution real-time IGRT using X-rays, computer tomography, magnetic resonance imaging, or ultrasound, enables delivery of high radiation doses to tumors without significant damage of healthy organs. IGRT is the most convincing success story of radiation oncology over the last decades, and it remains a major driving force of innovation, contributing to the development of personalized oncology, for example, through the use of real-time imaging biomarkers for individualized dose delivery.

Target coverage and dose criteria based evaluation of the first clinical 1.5T MR-linac SBRT treatments of lymph node oligometastases compared with conventional CBCT-linac treatment

BACKGROUND AND PURPOSE

Patients were treated at our institute for single and multiple lymph node oligometastases on the 1.5T MR-linac since August 2018. The superior soft-tissue contrast and additional software features of the MR-linac compared to CBCT-linacs allow for online adaptive treatment planning. The purpose of this study was to perform a target coverage and dose criteria based evaluation of the clinically delivered online adaptive radiotherapy treatment compared with conventional CBCT-linac treatment.

MATERIALS AND METHODS

Patient data was used from 14 patients with single lymph node oligometastases and 6 patients with multiple (2-3) metastases. All patients were treated on the 1.5T MR-linac with a prescribed dose of 5 × 7 Gy to 95% of the PTV and a CBCT-linac plan was created for each patient. The difference in target coverage between these plans was compared and plans were evaluated based on dose criteria for each fraction after calculating the CBCT-plan on the daily anatomy. The GTV coverage was evaluated based on the online planning and the post-delivery MRI.

RESULTS

For both single and multiple lymph node oligometastases the GTV V_35Gy had a median value of 100% for both the MR-linac plans and CBCT-plans pre- and post-delivery and did not significantly differ. The percentage of plans that met all dose constraints was improved from 19% to 84% and 20% to 67% for single and multiple lymph node cases, respectively.

CONCLUSION

Target coverage and dose criteria based evaluation of the first clinical 1.5T MR-linac SBRT treatments of lymph node oligometastases compared with conventional CBCT-linac treatment shows a smaller amount of unplanned violations of high dose criteria. The GTV coverage was comparable. Benefit is primarily gained in patients treated for multiple lymph node oligometastases: geometrical deformations are accounted for, dose can be delivered in one plan and margins can be reduced.

Magnetic resonance-guided radiation therapy: A review

Magnetic resonance-guided radiation therapy (MRgRT) is a promising approach to improving clinical outcomes for patients treated with radiation therapy. The roles of image guidance, adaptive planning and magnetic resonance imaging in radiation therapy have been increasing over the last two decades. Technical advances have led to the feasible combination of magnetic resonance imaging and radiation therapy technologies, leading to improved soft-tissue visualisation, assessment of inter- and intrafraction motion, motion management, online adaptive radiation therapy and the incorporation of functional information into treatment. MRgRT can potentially transform radiation oncology by improving tumour control and quality of life after radiation therapy and increasing convenience of treatment by shortening treatment courses for patients. Multiple groups have developed clinical implementations of MRgRT predominantly in the abdomen and pelvis, with patients having been treated since 2014. While studies of MRgRT have primarily been dosimetric so far, an increasing number of trials are underway examining the potential clinical benefits of MRgRT, with coordinated efforts to rigorously evaluate the benefits of the promising technology. This review discusses the current implementations, studies, potential benefits and challenges of MRgRT.

Adaptive radiotherapy: The Elekta Unity MR-linac concept

BACKGROUND AND PURPOSE

The promise of the MR-linac is that one can visualize all anatomical changes during the course of radiotherapy and hence adapt the treatment plan in order to always have the optimal treatment. Yet, there is a trade-off to be made between the time spent for adapting the treatment plan against the dosimetric gain. In this work, the various daily plan adaptation methods will be presented and applied on a variety of tumour sites. The aim is to provide an insight in the behavior of the state-of-the-art 1.5 T MRI guided on-line adaptive radiotherapy methods.

MATERIALS AND METHODS

To explore the different available plan adaptation workflows and methods, we have simulated online plan adaptation for five cases with varying levels of inter-fraction motion, regions of interest and target sizes: prostate, rectum, esophagus and lymph node oligometastases (single and multiple target). The plans were evaluated based on the clinical dose constraints and the optimization time was measured.

RESULTS

The time needed for plan adaptation ranged between 17 and 485 s. More advanced plan adaptation methods generally resulted in more plans that met the clinical dose criteria. Violations were often caused by insufficient PTV coverage or, for the multiple lymph node case, a too high dose to OAR in the vicinity of the PTV. With full online replanning it was possible to create plans that met all clinical dose constraints for all cases.

CONCLUSION

Daily full online replanning is the most robust adaptive planning method for Unity. It is feasible for specific sites in clinically acceptable times. Faster methods are available, but before applying these, the specific use cases should be explored dosimetrically.