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

Pelvic lymph node motion during cone-beam computed tomography guided stereotactic radiotherapy

Background and purpose

Stereotactic body radiotherapy (SBRT) is increasingly applied for pelvic lymph node recurrence. Thus far, knowledge on pelvic lymph node motion during CBCT-guided SBRT is lacking and the applied margins vary between institutions. This study evaluated pelvic lymph node motion during CBCT-guided SBRT and assessed the currently applied PTV margins of 3 and 5 mm.

Material and methods

In total, 45 pelvic lymph node metastases were included. One observer delineated 45 GTVs on planning CT, 224 GTVs on pre-fraction and 216 on post-fraction CBCT. The GTV centroid coordinates were derived from all images for inter- and intrafraction motion analysis. Additionally, we assessed the influence of treatment time and lesion location on lesion motion. The expected coverage of a 3-mm and 5-mm PTV margin was assessed using the inclusiveness index for GTVs on pre- and post-fraction CBCT.

Results

Lymph node interfraction motion was limited to 5 mm in 96-97 % of fractions for all translational directions and intrafraction lesion motion was limited to 3 mm in 97-100 % of fractions. Para-rectal lesions (11 %) were associated with significantly larger inter- and intrafraction motion compared to other pelvic locations and treatment duration showed no correlation with lesion motion. The mean (sd) lesion inclusiveness index was 99 % (5 %) for the 5-mm PTV margin and 96 % (9 %) for the 3-mm margin.

Conclusion

Pelvic lymph node motion during CBCT-guided stereotactic radiotherapy was within the widely applied PTV margin of 5 mm, providing an opportunity to reduce this margin for pelvic lymph node SBRT.

Super-resolution neural networks improve the spatiotemporal resolution of adaptive MRI-guided radiation therapy

BACKGROUND

Magnetic resonance imaging (MRI) offers superb non-invasive, soft tissue imaging of the human body. However, extensive data sampling requirements severely restrict the spatiotemporal resolution achievable with MRI. This limits the modality's utility in real-time guidance applications, particularly for the rapidly growing MRI-guided radiation therapy approach to cancer treatment. Recent advances in artificial intelligence (AI) could reduce the trade-off between the spatial and the temporal resolution of MRI, thus increasing the clinical utility of the imaging modality.

METHODS

We trained deep learning-based super-resolution neural networks to increase the spatial resolution of real-time MRI. We developed a framework to integrate neural networks directly onto a 1.0 T MRI-linac enabling real-time super-resolution imaging. We integrated this framework with the targeting system of the MRI-linac to demonstrate real-time beam adaptation with super-resolution-based imaging. We tested the integrated system using large publicly available datasets, healthy volunteer imaging, phantom imaging, and beam tracking experiments using bicubic interpolation as a baseline comparison.

RESULTS

Deep learning-based super-resolution increases the spatial resolution of real-time MRI across a variety of experiments, offering measured performance benefits compared to bicubic interpolation. The temporal resolution is not compromised as measured by a real-time adaptation latency experiment. These two effects, an increase in the spatial resolution with a negligible decrease in the temporal resolution, leads to a net increase in the spatiotemporal resolution.

CONCLUSIONS

Deployed super-resolution neural networks can increase the spatiotemporal resolution of real-time MRI. This has applications to domains such as MRI-guided radiation therapy and interventional procedures.

Case Report: MR-LINAC-guided adaptive radiotherapy for gastric cancer

Background

The stomach is one of the most deformable organs. Its shape can be easily affected by breathing movements, and daily diet, and it also varies when the body position is different. The susceptibility of stomach has made it challenging to treat gastric cancer using the conventional image-guided radiotherapy, i.e., the techniques based on kilovoltage X-ray imaging. The magnetic resonance imaging guided radiotherapy (MRgRT) is usually implemented using a hybrid system MR-LINAC. It is feasible to implement adaptive radiotherapy using MR-LINAC for deformable organs such as stomach. In this case report, we present our clinical experience to treat a gastric cancer patient using MR-LINAC.

Case description

The patient is a 58-year-old male who started having black stools with no apparent cause a year ago. Gastroscopy result showed pancreatic cancer, pathology: adenocarcinoma on gastric cancer biopsy, adenocarcinoma on gastric body minor curvature biopsy. The patient was diagnosed with gastric cancer (adenocarcinoma, cTxN+M1, stage IV, HER-2 positive). The patient was treated in 25 fractions with radiotherapy using MR-LINAC with online adaptive treatment plans daily. The target area in daily MR images varied considerably when compared with the target area on the CT simulation images. During the course of treatment, there have even been instances where the planned target area where the patient received radiotherapy did not cover the lesion of the day.

Conclusion

Online adaptive MRgRT can be a meaningful innovation for treating malignancies in the upper abdomen. The results in the current study are promising and are indicative for further optimizing online adaptive MRgRT in patients with inoperable tumors of the upper abdomen.

X-ray and MR Contrast Bearing Nanoparticles Enhance the Therapeutic Response of Image-Guided Radiation Therapy for Oral Cancer

INTRODUCTION

Radiation therapy for head and neck squamous cell carcinoma is constrained by radiotoxicity to normal tissue. We demonstrate 100 nm theranostic nanoparticles for image-guided radiation therapy planning and enhancement in rat head and neck squamous cell carcinoma models.

METHODS

PEG conjugated theranostic nanoparticles comprising of Au nanorods coated with Gadolinium oxide layers were tested for radiation therapy enhancement in 2D cultures of OSC-19-GFP-luc cells, and orthotopic tongue xenografts in male immunocompromised Salt sensitive or SS rats via both intratumoral and intravenous delivery. The radiation therapy enhancement mechanism was investigated.

RESULTS

Theranostic nanoparticles demonstrated both X-ray/magnetic resonance contrast in a dose-dependent manner. Magnetic resonance images depicted optimal tumor-to-background uptake at 4 h post injection. Theranostic nanoparticle + Radiation treated rats experienced reduced tumor growth compared to controls, and reduction in lung metastasis.

CONCLUSIONS

Theranostic nanoparticles enable preprocedure radiotherapy planning, as well as enhance radiation treatment efficacy for head and neck tumors.

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.

1.5 T MR-Guided Daily Adapted SBRT on Lymph Node Oligometastases from Prostate Cancer

Introduction: The aim of our study was to evaluate the efficacy and toxicity of a daily adaptive MR-guided SBRT on 1.5 T MR-linac in patients affected by lymph node oligometastases from PCa. Materials and Methods: The present study is a prospective observational study conducted in a single institution (protocol n°: MRI/LINAC n. 23748). Patients with oligometastatic lymph nodes from PCa treated with daily adaptive MR-guided SBRT on 1.5 T MR-linac were included in the study. There was a minimum required follow-up of 3 months after SBRT. The primary end-point was local progression-free survival (LPFS). The secondary end-points were: nodal progression-free survival (NPFS), progression-free survival (PFS), and toxicity. Results: A total of 118 lymph node oligometastases from PCa were treated with daily adaptive 1.5 T MR-guided SBRT in 63 oligometastatic patients. Of the patients, 63.5% were oligorecurrent and 36.5% were oligoprogressive. The two-year LPFS was 90.7%. The median NPFS was 22.3 months and the 2-year NPFS was 46.5%. Receiving hormone therapy before SBRT was correlated with a lower NPFS at the multivariate analysis (1 y NPFS 87.1% versus 42.8%; p = 0.002-HR 0.199, 95% CI 0.073-0.549). Furthermore, the oligorecurrent state during ADT was correlated with a lower NPFS than was the oligoprogressive state. The median PFS was 10.3 months and the 2-year PFS was 32.4%. Patients treated with hormone therapy before SBRT had a significantly lower 1-year PFS the others (28% versus 70.4%; p = 0.01-HR 0.259, 95% CI 0.117-0.574). No acute and late toxicities occurred during treatment. Conclusions: The present study is the largest prospective study of 1.5 T lymph node SBRT on MR-linac in patients with PCa. Lymph node SBRT by 1.5 T MR-linac provides high local control rates with an excellent toxicity profile.

To see or not to see: Evaluation of magnetic resonance imaging sequences for use in MR Linac-based radiotherapy treatment

BACKGROUND/PURPOSE

This work evaluated the suitability of MR derived sequences for use in online adaptive RT workflows on a 1.5 Tesla (T) MR-Linear Accelerator (MR Linac).

MATERIALS/METHODS

Non-patient volunteers were recruited to an ethics approved MR Linac imaging study. Participants attended 1-3 imaging sessions in which a combination of DIXON, 2D and 3D volumetric T1 and T2 weighted images were acquired axially, with volunteers positioned using immobilisation devices typical for radiotherapy to the anatomical region being scanned. Images from each session were appraised by three independent reviewers to determine optimal sequences over six anatomical regions: head and neck, female and male pelvis, thorax (lung), thorax (breast/chest wall) and abdomen. Site specific anatomical structures were graded by the perceived ability to accurately contour a typical organ at risk. Each structure was independently graded on a 4-point Likert scale as 'Very Clear', 'Clear', 'Unclear' or 'Not visible' by observers, consisting of radiographers (therapeutic and diagnostic) and clinicians.

RESULTS

From July 2019 to September 2019, 18 non-patient volunteers underwent 24 imaging sessions in the following anatomical regions: head and neck (n=3), male pelvis (n=4), female pelvis (n=5), lung/oesophagus (n=5) abdomen (n=4) and chest wall/breast (n=3). T2 sequences were the most preferred for perceived ability to contour anatomy in both male and female pelvis. For all other sites T1 weighted DIXON sequences were most favourable.

CONCLUSION

This study has determined the preferential sequence selection for organ visualisation, as a pre-requisite to our institution adopting MR-guided radiotherapy for a more diverse range of disease sites.

MRI-guided Radiotherapy (MRgRT) for treatment of Oligometastases: Review of clinical applications and challenges

PURPOSE

Early clinical results on the application of magnetic resonance imaging (MRI) coupled with a linear accelerator to deliver MR-guided radiation therapy (MRgRT) have demonstrated feasibility for safe delivery of stereotactic body radiotherapy (SBRT) in treatment of oligometastatic disease. Here we set out to review the clinical evidence and challenges associated with MRgRT in this setting.

METHODS AND MATERIALS

We performed a systematic review of the literature pertaining to clinical experiences and trials on the use of MRgRT primarily for the treatment of oligometastatic cancers. We reviewed the opportunities and challenges associated with the use of MRgRT.

RESULTS

Benefits of MRgRT pertaining to superior soft-tissue contrast, real-time imaging and gating, and online adaptive radiotherapy facilitate safe and effective dose escalation to oligometastatic tumors while simultaneously sparing surrounding healthy tissues. Challenges concerning further need for clinical evidence and technical considerations related to planning, delivery, quality assurance (QA) of hypofractionated doses, and safety in the MRI environment must be considered.

CONCLUSIONS

The promising early indications of safety and effectiveness of MRgRT for SBRT-based treatment of oligometastatic disease in multiple treatment locations should lead to further clinical evidence to demonstrate the benefit of this technology.

Comparing patient acceptability of MR-guided radiotherapy to conventional CBCT on two Elekta systems: a questionnaire-based survey

Background and Purpose:

The magnetic resonance linear accelerator system (MR Linac) is a novel piece of radiotherapy (RT) equipment allowing the routine application of daily MR-guided treatment adaptation. The hardware design required for such technical capabilities and the increased complexity of the treatment workflow entails a notable departure from cone beam computed tomography (CBCT)-based RT. Patient tolerability of treatment is paramount to RT practice where high compliance is required. Presented is a comparative analysis of how such modality specific characteristics may ultimately impact the patient experience of treatment.

Materials and Methods:

Forty patients undergoing RT for prostate cancer (PCa) on either the MR Linac (n = 20) or a CBCT-based linac (n = 20) were provided with a validated patient reported outcomes measures (PROM’s) questionnaire at fraction 1 and fraction 20. The 18-item questionnaire provided patient responses recorded using a 4-point Likert scale, 0 denoting a response of ‘Not at all’, 1 ‘Slightly’, 2 ‘Moderately’ and 3 signifying ‘Very’. The analysis provided insight into both comparisons between modalities at singular time points (fractions 1 and 20), as well as a temporal analysis within a single modality, denoting changing patient experience.

Results:

Patients generally found the MR Linac treatment couch more comfortable, however, found the increase in treatment duration harder to tolerate. Responses for all items remained stable between first and last fraction across both cohorts, indicating minimal temporal variation within a single modality. None of the responses were statistically significant at the 0·01 level.

Conclusion:

Whether radiotherapy for PCa is delivered on a CBCT linac or the MR Linac, there is little difference in patient experience with minimal experiential variation within a single modality.

Integrated MRI-guided radiotherapy - opportunities and challenges

MRI can help to categorize tissues as malignant or non-malignant both anatomically and functionally, with a high level of spatial and temporal resolution. This non-invasive imaging modality has been integrated with radiotherapy in devices that can differentially target the most aggressive and resistant regions of tumours. The past decade has seen the clinical deployment of treatment devices that combine imaging with targeted irradiation, making the aspiration of integrated MRI-guided radiotherapy (MRIgRT) a reality. The two main clinical drivers for the adoption of MRIgRT are the ability to image anatomical changes that occur before and during treatment in order to adapt the treatment approach, and to image and target the biological features of each tumour. Using motion management and biological targeting, the radiation dose delivered to the tumour can be adjusted during treatment to improve the probability of tumour control, while simultaneously reducing the radiation delivered to non-malignant tissues, thereby reducing the risk of treatment-related toxicities. The benefits of this approach are expected to increase survival and quality of life. In this Review, we describe the current state of MRIgRT, and the opportunities and challenges of this new radiotherapy approach.

First multicentre experience of SABR for lymph node and liver oligometastatic disease on the unity MR-Linac

•

Since August 2018 patients are treated on a 1.5 Tesla MR-Linac (MRL) for oligometastatic disease.

•

We present current workflows and practice standards from seven institutions for the initial patients treated for lymph node and liver metastases.

•

In this work a large variation in treatment strategies was found. Since currently there is little evidence preferring one strategy over the other, clinical registries and future research need to focus on the clinical relevance of the variations in institutional treatment strategies.

The treatment of oligometastatic disease using MR guidance is an evolving field. Since August 2018 patients are treated on a 1.5 Tesla MR-Linac (MRL). We present current workflows and practice standards from seven institutions for the initial patients treated for lymph node and liver metastases.

Clinical implementation and feasibility of long-course fractionated MR-guided chemoradiotherapy for patients with esophageal cancer: an R-IDEAL stage 1b/2a evaluation of technical innovation

•

Online MR-guided long-course fractionated chemoradiotherapy for patients with esophageal cancer was feasible in 7 out of 9 patients.

•

Median treatment time was 53 min per fraction.

•

MRgRT resulted in a reduction in mean heart dose (12%) and mean lung dose (26%) compared to CBCT-guided radiotherapy.

•

Limited intrafraction motion was observed during dose delivery.

Purpose

This R-Ideal stage 1b/2a study describes the workflow and feasibility of long-course fractionated online adaptive MR-guided chemoradiotherapy with reduced CTV-to-PTV margins on the 1.5T MR-Linac for patients with esophageal cancer.

Methods

Patients with esophageal cancer scheduled to undergo chemoradiation were treated on a 1.5T MR-Linac. Daily MR-images were acquired for online contour adaptation and replanning. Contours were manually adapted to match the daily anatomy and an isotropic CTV-to-PTV margin of 6 mm was applied. Time was recorded for all individual steps in the workflow. Feasibility and patient tolerability were defined as on-table time of ≤60 min and completion of >95% of the fractions on the MR-Linac, respectively. Positioning verification and post-treatment MRIs were retrospectively analyzed and dosimetric parameters were compared to standard non-adaptive conventional treatment plans.

Results

Nine patients with esophageal cancer were treated with chemoradiation; eight patients received 41.4 Gy in 23 fractions and one received 50.4 Gy in 28 fractions. Four patients received all planned fractions on the MR-Linac, whereas for two patients >5% of fractions were rescheduled to a conventional linac for reasons of discomfort. A total of 183 (86%) of 212 scheduled fractions were successfully delivered on the MR-Linac. Three fractions ended prematurely due to technical issues and 26 fractions were rescheduled on a conventional linac due to MR-Linac downtime (n = 10), logistical reasons (n = 3) or discomfort (n = 13).

The median time per fraction was 53 min (IQR = 3 min). Daily adapted MR-Linac plans had similar target coverage, whereas dose to the organs-at-risk was significantly reduced compared to conventional treatment (26% and 12% reduction in mean lung and heart dose, respectively).

Conclusion

Daily online adaptive fractionated chemoradiotherapy with reduced PTV margins is moderately feasible for esophageal cancer and results in better sparing of heart and lungs. Future studies should focus on further optimization and acceleration of the current workflow.

SMART ablation of lymphatic oligometastases in the pelvis and abdomen: Clinical and dosimetry outcomes

PURPOSE

To demonstrate dosimetry benefits and report clinical outcomes of stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) of abdominopelvic lymphatic oligometastases.

PATIENTS & METHODS

Prospective registry data of 26 patients with 31 oligoprogressive lymphatic metastases (1 - 2 lesions) who received SMART between April 2020 and April 2021 was analyzed. Prostate cancer was the most common histology (69%). Most patients (63%) had received previous abdominopelvic radiotherapy (RT). SMART was delivered in 3 - 7 fractions based on planning target volume (PTV) location and previous dose exposures. For SMART, the baseline plan was recalculated on daily 3D MR-imaging (predicted plan), and plan adaptation was mandatory in case of planning objective violations.

RESULTS

Plan adaptation was mostly performed due to violation of planning objectives in the predicted plan (134/140 fractions, 96%) and significantly improved plan dosimetry: 1) PTV coverage was increased (predicted: median 89%, adapted: median 95%, p < 0.001), 2) organs-at-risk (OAR) overdoses were reduced (predicted: 27/140 (19%), adapted: 1/140 (1%), p < 0.001) and 3) PTV overdoses were reduced (predicted: 21/140 (15%), adapted: 1/140 (1%), p < 0.001). After a median follow-up of 9.8 months, one patient had in-field tumor progression and twelve patients had out-field tumor progression (at 6 months: progression-free survival: 63% [46 - 88%], local control rate: 97% [90 - 100%]). Treatment was tolerated well and no grade ≥ 3 toxicity was reported.

CONCLUSION

SMART improves target volume coverage and yields superior OAR protection compared to non-adaptive radiotherapy, thus representing an innovative approach to challenging cases, such as repeated radiotherapy.

Comparison of Prospectively Generated Glioma Treatment Plans Clinically Delivered on Magnetic Resonance Imaging (MRI)-Linear Accelerator (MR-Linac) Versus Conventional Linac: Predicted and Measured Skin Dose

Introduction: Magnetic resonance imaging-linear accelerator radiotherapy is an innovative technology that requires special consideration for secondary electron interactions within the magnetic field, which can alter dose deposition at air–tissue interfaces. As part of ongoing quality assurance and quality improvement of new radiotherapy technologies, the purpose of this study was to evaluate skin dose modelled from the treatment planning systems of a magnetic resonance imaging-linear accelerator and a conventional linear accelerator, and then correlate with in vivo measurements of delivered skin dose from each linear accelerator. Methods: In this prospective cohort study, 37 consecutive glioma patients had treatment planning completed and approved prior to radiotherapy initiation using commercial treatment planning systems: a Monte Carlo-based algorithm for magnetic resonance imaging-linear accelerator or a convolution-based algorithm for conventional linear accelerator. In vivo skin dose was measured using an optically stimulated luminescent dosimeter. Results: Monte Carlo-based magnetic resonance imaging-linear accelerator plans and convolution-based conventional linear accelerator plans had similar dosimetric parameters for target volumes and organs-at-risk. However, magnetic resonance imaging-linear accelerator plans had 1.52 Gy higher mean dose to air cavities (P < .0001) and 1.10 Gy higher mean dose to skin (P < .0001). In vivo skin dose was 14.5% greater for magnetic resonance imaging-linear accelerator treatments (P = .0027), and was more accurately predicted by Monte Carlo-based calculation (ρ = 0.95, P < .0001) versus convolution-based (ρ = 0.80, P = .0096). Conclusion: This is the first prospective dosimetric comparison of glioma patients clinically treated on both magnetic resonance imaging-linear accelerator and conventional linear accelerator. Our findings suggest that skin doses were significantly greater with magnetic resonance imaging-linear accelerator plans but correlated better with in vivo measurements of actual skin dose from delivered treatments. Future magnetic resonance imaging-linear accelerator planning processes are being designed to account for skin dosimetry and treatment delivery.

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.

Mitigation on bowel loops daily variations by 1.5-T MR-guided daily-adaptive SBRT for abdomino-pelvic lymph-nodal oligometastases

PURPOSE

We report preliminary dosimetric data concerning the use of 1.5-T MR-guided daily-adaptive radiotherapy for abdomino-pelvic lymph-nodal oligometastases. We aimed to assess the impact of this technology on mitigating daily variations for both target coverage and organs-at-risk (OARs) sparing.

METHODS

A total of 150 sessions for 30 oligometastases in 23 patients were analyzed. All patients were treated with MR-guided stereotactic body radiotherapy (SBRT) for a total dose of 35 Gy in five fractions. For each fraction, a quantitative analysis was performed for PTV volume, V35Gy and D_mean. Similarly, for OARs, we assessed daily variations of volume, D_mean, D_max. Any potential statistically significant change between baseline planning and daily-adaptive sessions was assessed using the Wilcoxon signed-rank test, assuming a p value < 0.05 as significant.

RESULTS

Average baseline PTV, bowel, bladder, and single intestinal loop volumes were respectively 8.9 cc (range 0.7-41.2 cc), 1176 cc (119-3654 cc), 95 cc (39.7-202.9 cc), 18.3 cc (9.1-37.7 cc). No significant volume variations were detected for PTV (p = 0.21) bowel (p = 0.36), bladder (p = 0.47), except for single intestinal loops, which resulted smaller (p = 0.026). Average baseline V35Gy and D_mean for PTV were respectively 85.6% (72-98.8%) and 35.6 Gy (34.6-36.1 Gy). We recorded a slightly positive trend in favor of daily-adaptive strategy vs baseline planning for improved target coverage, although not reaching statistical significance (p = 0.11 and p = 0.18 for PTV-V35Gy and PTV-D_mean). Concerning OARs, a significant difference was observed in favor of daily-adapted treatments in terms of single intestinal loop D_max [23.05 Gy (13.2-26.9 Gy) at baseline vs 20.5 Gy (12.1-24 Gy); p value = 0.0377] and D_mean [14.4 Gy (6.5-18 Gy) at baseline vs 13.0 Gy (6.7-17.6 Gy); p value = 0.0003]. Specifically for bladder, the average D_max was 18.6 Gy (0.4-34.3 Gy) at baseline vs 18.3 Gy (0.7-34.3 Gy) for a p value = 0.28; the average D_mean was 7.0 Gy (0.2-16.6 Gy) at baseline vs 6.98 Gy (0.2-16.4 Gy) for a p value = 0.66. Concerning the bowel, no differences in terms of D_mean [4.78 Gy (1.3-10.9 Gy) vs 5.6 Gy (1.4-10.5 Gy); p value = 0.23] were observed between after daily-adapted sessions. A statistically significant difference was observed for bowel D_max [26.4 Gy (7.7-34 Gy) vs 25.8 Gy (7.8-33.1 Gy); p value = 0.0086].

CONCLUSIONS

Daily-adaptive MR-guided SBRT reported a significantly improved single intestinal loop sparing for lymph-nodal oligometastases. Also, bowel D_max was significantly reduced with daily-adaptive strategy. A minor advantage was also reported in terms of PTV coverage, although not statistically significant.

The potential role of MR-guided adaptive radiotherapy in pediatric oncology: Results from a SIOPE-COG survey

BACKGROUND AND PURPOSE

Magnetic resonance guided radiotherapy (MRgRT) has been successfully implemented for several routine clinical applications in adult patients. The purpose of this study is to map the potential benefit of MRgRT on toxicity reduction and outcome in pediatric patients treated with curative intent for primary and metastatic sites.

MATERIALS AND METHODS

Between May and August 2020, a survey was distributed among SIOPE- and COG-affiliated radiotherapy departments, treating at least 25 pediatrics patients annually and being (candidate) users of a MRgRT system. The survey consisted of a table with 45 rows (clinical scenarios for primary (n = 28) and metastatic (n = 17) tumors) and 7 columns (toxicity reduction, outcome improvement, PTV margin reduction, target volume daily adaptation, online re-planning, intrafraction motion compensation and on-board functional imaging) and the option to answer by 'yes/no' . Afterwards, the Dutch national radiotherapy cohort was used to estimate the percentage of pediatric treatments that may benefit from MRgRT.

RESULTS

The survey was completed by 12/17 (71% response rate) institutions meeting the survey inclusion criteria. Responders indicated an 'expected benefit' from MRgRT for toxicity/outcome in 7% (for thoracic lymphomas and abdominal rhabdomyosarcomas)/0% and 18% (for mediastinal lymph nodes, lymph nodes located in the liver/splenic hilum, and liver metastases)/0% of the considered scenarios for the primary and metastatic tumor sites, respectively, and a 'possible benefit' was estimated in 64%/46% and 47%/59% of the scenarios. When translating the survey outcome into a clinical perspective a toxicity/outcome benefit, either expected or possible, was anticipated for 55%/24% of primary sites and 62%/38% of the metastatic sites.

CONCLUSION

Although the benefit of MRgRT in pediatric radiation oncology is estimated to be modest, the potential role for reducing toxicity and improving clinical outcomes warrants further investigation. This fits best within the context of prospective studies or registration trials.

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.

Technical Challenges of Real-Time Adaptive MR-Guided Radiotherapy

In the past few years, radiotherapy (RT) has experienced a major technological innovation with the development of hybrid machines combining magnetic resonance (MR) imaging and linear accelerators. This new technology for MR-guided cancer treatment has the potential to revolutionize the field of adaptive RT due to the opportunity to provide high-resolution, real-time MR imaging before and during treatment application. However, from a technical point of view, several challenges remain which need to be tackled to ensure safe and robust real-time adaptive MR-guided RT delivery. In this manuscript, several technical challenges to MR-guided RT are discussed. Starting with magnetic field strength tradeoffs, the potential and limitations for purely MR-based RT workflows are discussed. Furthermore, the current status of real-time 3D MR imaging and its potential for real-time RT are summarized. Finally, the potential of quantitative MR imaging for future biological RT adaptation is highlighted.

An unusual case of oedematous prostate volumetric changes observed over the course of radiotherapy on the MR linear accelerator

INTRODUCTION

The integration of magnetic resonance (MR) imaging into radiotherapy through new technology, including the MR -linear accelerator (MRL), has allowed further advancements into image guided radiotherapy (IGRT). Better soft tissue visualisation has led to some unusual findings.

CASE AND OUTCOMES

A patient with T1c N0 M0 prostate adenocarcinoma received 60Gy in 20# radiotherapy on the MRL. Radiotherapy planning (RTP) scans were completed on both CT and MR (using T2 and T1 weighted three-dimensional turbo spin echo sequences, reconstructed transaxially (TRA). The MR scans revealed atypical oedema in the right peripheral zone, visualised on T2-weighted (T2w) MR Images as an accumulation of high signal intensity fluid. Daily MRL treatment includes a (T2w 3D Tra) sequence with which oedematous changes could be monitored. The images demonstrated an increase in oedematous volume over fractions 1-10 causing the prostate contour variations from the initial planning scans. Despite the prostate volume variations PTV coverage was never breached and dose constraints were always met for both PTV and surrounding organs at risk (OAR's), excluding the need for oncologist input. A single Therapeutic Radiographer (RTT) experienced in MRL delivery, contoured the prostate and oedematous volumes on the radiotherapy plan (RTP) MR and all on-treatment MR images to assess change over the radiotherapy course. The initial volumes were 53.4 cm3 and 8.3 cm3 for the prostate plus oedema and oedema alone respectively. The most significant change was seen for both the prostate and oedema on fraction nine (68.0 cm3 and 10.1 cm3, respectively). Reductions were noted after this with final (fraction 20) volumes of 55.2 cm3 and 0.58 cm3 respectively.

DISCUSSION

The ability to visualise prostatic oedema was new to the radiotherapy treatment team due to better soft tissue visualisation than standard radiotherapy. The results from contouring the prostate and oedema volumes confirmed radiographer observations and demonstrated how oedema impacted the overall prostate volume by quantifying the oedematous variations over time. The changes in oedema volume are presumed to be in response to radiotherapy.

CONCLUSION

Further adaptive radiotherapy work-flow developments, utilising an "Adapt to Shape" model will allow real-time re-contouring of the prostate to ensure tumour control is not compromised. Further work investigating the frequency and impact of oedemotous changes to external beam prostate patients will help to inform practice.

Impact of field number and beam angle on ERE for lung stereotactic body radiotherapy with 1.5T MR-Linac

PURPOSE

To investigate the impact of field number and beam angle on ERE during IMRT planning with 1.5T MR-Linac for lung stereotactic body radiotherapy (SBRT).

MATERIAL AND METHODS

IMRT plans for three representative lung cases who treated with 1.5T MR-Linac were generated using TPS Monaco 5.40.01. In each case, five types of plans were generated: (1) the "original plan" generated using 5-fields and beam angles manually optimized in ipsilateral tumor side, with no transverse magnetic field (TMF); (2) the plans generated by reoptimizing and recalculating the original plan, with a 1.5T TMF; (3) the plans generated with 5-fields, with beam angles manually optimized in anterior-posterior (AP) direction; (4) the plans generated with equidistant 5-fields; and (5) the plans generated with equidistant 9-fields. These plans were compared using a variety of dose-volume-parameters (DVPs).

RESULTS

The DVPs under TMF showed the presence of 1.5T TMF can slightly change for all lung cases studied. When the plans generated with beam angles manually optimized in anterior-posterior direction, results showed that the plans improved conformity and homogeneity for PTV and reduced the dose to tissue surface. Furthermore, the results showed that the equidistant/non-equidistant comparison of the previous target anatomy result in a more favourable dose distribution for the equidistant beam setting. At last, the results showed the dose distributions for PTV with 9 and 5 equidistant beams were not significantly different.

CONCLUSION

The plan quality can be significantly changed by the presence of a 1.5T TMF for lung SBRT with MR-Linac. The selection of optimal fields number and beam angle can substantially reduce or even eliminate these changes, without deteriorating overall plan quality.

[Magnetic-resonance-guided radiotherapy : The beginning of a new era in radiation oncology?]

CLINICAL ISSUE

Image-guided radiotherapy (IGRT) using X‑rays and cone-beam computed tomography (CT) has fostered precision radiotherapy. However, inter- and intrafractional variations of target volume position and organs at risk still limit target volume dose and sparing of radiosensitive organs at risk.

METHODOLOGICAL INNOVATIONS

Hybrid machines directly combining linear accelerators and magnetic resonance (MR) imaging allow for live imaging during radiotherapy.

PERFORMANCE

Besides highly improved soft tissue contrast, MR-linacs enable online, on-table adaptive radiotherapy. Thus, adaptation of the treatment plan to the anatomy of the day, dose escalation and superior sparing of organs at risk become possible.

ACHIEVEMENTS

This article summarizes the underlying intention for the development of MR-guided radiotherapy, technical innovations and challenges as well as the current state-of-the-art. Potential clinical benefits and future developments are discussed.

PRACTICAL RECOMMENDATIONS

Increasing availability of MR imaging at linear accelerators calls for the ability to review and interpret MR images. Therefore, close collaborations of diagnostic radiologists and radiation oncologists are mandatory to foster this fascinating technique.

Stereotactic Body Radiotherapy for Lymph Node Oligometastases: Real-World Evidence From 90 Consecutive Patients

AIMS

To evaluate the efficacy and toxicity of extracranial stereotactic body radiotherapy (SBRT) in the treatment of oligometastatic lymph node involvement in the mediastinum, retroperitoneum, or pelvis, in a consecutive group of patients from real clinical practice outside clinical trials.

METHODS

A retrospective analysis of 90 patients with a maximum of four oligometastases and various primary tumors (the most common being colorectal cancers). The endpoints were local control of treated metastases (LC), freedom from widespread dissemination (FFWD), progression-free survival (PFS), overall survival (OS), and freedom from systemic treatment (FFST). Acute and delayed toxicities were also evaluated.

RESULTS

The median follow-up after SBRT was 34.9 months. The LC rate at three and five years was 68.4 and 56.3%, respectively. The observed median FFWD was 14.6 months, with a five-year FFWD rate of 33.7%. The median PFS was 9.4 months; the three-year PFS rate was 19.8%. The median FFST was 14.0 months; the five-year FFST rate was 23.5%. The OS rate at three and five years was 61.8 and 39.3%, respectively. Median OS was 53.1 months. The initial dissemination significantly shortened the time to relapse, death, or activation of systemic treatment-LC (HR 4.8, p < 0.001), FFWD (HR 2.8, p = 0.001), PFS (HR 2.1, p = 0.011), FFST (HR 2.4, p = 0.005), OS (HR 2.2, p = 0.034). Patients classified as having radioresistant tumors noticed significantly higher risk in terms of LC (HR 13.8, p = 0.010), FFWD (HR 3.1, p = 0.006), PFS (HR 3.5, p < 0.001), FFST (HR 3.2, p = 0.003). The multivariable analysis detected statistically significantly worse survival outcomes for initially disseminated patients as well as separately in groups divided according to radiosensitivity. No grade III or IV toxicity was reported.

CONCLUSION

Our study shows that targeted SBRT is a very effective and low toxic treatment for oligometastatic lymph node involvement. It can delay the indication of cytotoxic chemotherapy and thus improve and maintain patient quality of life. The aim of further studies should focus on identifying patients who benefit most from SBRT, as well as the correct timing and dosage of SBRT in treatment strategy.

MR-Guided Radiotherapy: The Perfect Partner for Immunotherapy?

During the last years, preclinical and clinical studies have emerged supporting the rationale to integrate radiotherapy and immunotherapy. Radiotherapy may enhance the effects of immunotherapy by improving tumor antigen release, antigen presentation, and T-cell infiltration. Recently, magnetic resonance guided radiotherapy (MRgRT) has become clinically available. Compared to conventional radiotherapy techniques, MRgRT firstly allows for daily on-table treatment adaptation, which enables both dose escalation for increasing tumor response and superior sparing of radiosensitive organs-at-risk for reducing toxicity. The current review focuses on the potential of combining MR-guided adaptive radiotherapy with immunotherapy by providing an overview on the current status of MRgRT, latest developments in preclinical and clinical radio-immunotherapy, and the unique opportunities and challenges for MR-guided radio-immunotherapy. MRgRT might especially assist in answering open questions in radio-immunotherapy regarding optimal radiation dose, fractionation, timing of immunotherapy, appropriate irradiation volumes, and response prediction.

In Silico Single-Fraction Stereotactic Ablative Radiation Therapy for the Treatment of Thoracic and Abdominal Oligometastatic Disease With Online Adaptive Magnetic Resonance Guidance

Purpose

Although SABR can improve oncologic outcomes for patients with oligometastatic disease, treatment of metastases near critical organs remains challenging. The purpose of this study is to determine the dosimetric feasibility of delivering magnetic resonance imaging (MRI)-guided adaptive SABR in a single fraction for abdominal and thoracic metastases.

Methods and Materials

Previously delivered MRI-guided radiation therapy plans for 20 patients with oligometastatic disease in the thorax or abdomen, with 70% (14/20) of the lesions within 8 mm from dose-limiting organs at risk (OARs), were used to simulate the delivery of 24 Gy in a single fraction. Planning objectives included planning target volume (PTV) V_95% >90%, optimized PTV (PTVopt) V_95% >90%, and PTVopt D_99% >20 Gy with no OAR dose violations, where PTVopt removed overlap with nearby planning organ at risk volume (PRV). Single-fraction plans were simulated on the first 5 daily setup breath-hold MRI scans, and the plans were reoptimized to consider variations in setup position and anatomy.

Results

The mean PTV V_95% for single-fraction SABR plans was lower compared with multifraction plans (mean 85.4% vs 92.6%, P = .02), but mean PTVopt V_95% was not different (95.3% vs 98.2%, P = .62). After reoptimization of the single-fraction plan to the treatment day MRI, there was an increase in mean PTV V_95% (85.0% vs 88.1%, P = .05), increase in mean PTVopt V_95% (92.7% vs 96.3%, P = .02), increase in mean PTVopt D_99% (19.7 Gy vs 23.8 Gy, P < .01), increase in mean frequency of meeting PTV D_99% >20 Gy (52% vs 87%, P < .01), and increase in mean gross tumor volume minimum dose (17.5 Gy vs 19.3 Gy, P < .01). Reoptimization decreased mean frequency of OAR dose constraint violation (48% vs 0%, P < .01).

Conclusions

Single-fraction MRI-guided SABR is a dosimetrically feasible treatment for oligometastases that allows for on-table adaptation to avoid OAR dose constraint violations, but this method requires clinical validation.

Current Status of Anatomical Magnetic Resonance Imaging in Brachytherapy and External Beam Radiotherapy Planning and Delivery

Radiotherapy planning and delivery have dramatically improved in recent times. Imaging is key to a successful three-dimensional and increasingly four-dimensional based pathway with computed tomography embedded as the backbone modality. Computed tomography has significant limitations for many tumour sites where soft-tissue discrimination is suboptimal, and where magnetic resonance imaging (MRI) has largely superseded in the diagnostic arena. MRI is increasingly used together with computed tomography in the radiotherapy planning pathway and is now established as a prerequisite for several tumours. With the advent of combined MRI and linear accelerator (MR-linac) systems, a transition to MRI-based radiotherapy planning is becoming reality, with increasing experience and research involving these new platforms. In this overview, we aim to highlight how magnetic resonance-guided imaging has improved radiotherapy, using gynaecological malignancies to illustrate, in both external beam radiotherapy and image-guided brachytherapy, and will assess the early evidence for magnetic resonance-guided radiotherapy using combined MR-linac systems.

The role of computational methods for automating and improving clinical target volume definition

Treatment planning in radiotherapy distinguishes three target volume concepts: the gross tumor volume (GTV), the clinical target volume (CTV), and the planning target volume (PTV). Over time, GTV definition and PTV margins have improved through the development of novel imaging techniques and better image guidance, respectively. CTV definition is sometimes considered the weakest element in the planning process. CTV definition is particularly complex since the extension of microscopic disease cannot be seen using currently available in-vivo imaging techniques. Instead, CTV definition has to incorporate knowledge of the patterns of tumor progression. While CTV delineation has largely been considered the domain of radiation oncologists, this paper, arising from a 2019 ESTRO Physics research workshop, discusses the contributions that medical physics and computer science can make by developing computational methods to support CTV definition. First, we overview the role of image segmentation algorithms, which may in part automate CTV delineation through segmentation of lymph node stations or normal tissues representing anatomical boundaries of microscopic tumor progression. The recent success of deep convolutional neural networks has also enabled learning entire CTV delineations from examples. Second, we discuss the use of mathematical models of tumor progression for CTV definition, using as example the application of glioma growth models to facilitate GTV-to-CTV expansion for glioblastoma that is consistent with neuroanatomy. We further consider statistical machine learning models to quantify lymphatic metastatic progression of tumors, which may eventually improve elective CTV definition. Lastly, we discuss approaches to incorporate uncertainty in CTV definition into treatment plan optimization as well as general limitations of the CTV concept in the case of infiltrating tumors without natural boundaries.

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.

Gross tumour volume delineation in anal cancer on T2-weighted and diffusion-weighted MRI - Reproducibility between radiologists and radiation oncologists and impact of reader experience level and DWI image quality

PURPOSE

To assess how gross tumour volume (GTV) delineation in anal cancer is affected by interobserver variations between radiologists and radiation oncologists, expertise level, and use of T2-weighted MRI (T2W-MRI) vs. diffusion-weighted imaging (DWI), and to explore effects of DWI quality.

METHODS AND MATERIALS

We retrospectively analyzed the MRIs (T2W-MRI and b800-DWI) of 25 anal cancer patients. Four readers (Senior and Junior Radiologist; Senior and Junior Radiation Oncologist) independently delineated GTVs, first on T2W-MRI only and then on DWI (with reference to T2W-MRI). Maximum Tumour Diameter (MTD) was calculated from each GTV. Mean GTVs/MTDs were compared between readers and between T2W-MRI vs. DWI. Interobserver agreement was calculated as Intraclass Correlation Coefficient (ICC), Dice Similarity Coefficient (DSC) and Hausdorff Distance (HD). DWI image quality was assessed using a 5-point artefact scale.

RESULTS

Interobserver agreement between radiologists vs. radiation oncologists and between junior vs. senior readers was good-excellent, with similar agreement for T2W-MRI and DWI (e.g. ICCs 0.72-0.94 for T2W-MRI and 0.68-0.89 for DWI). There was a trend towards smaller GTVs on DWI, but only for the radiologists (P = 0.03-0.07). Moderate-severe DWI-artefacts were observed in 11/25 (44%) cases. Agreement tended to be lower in these cases.

CONCLUSION

Overall interobserver agreement for anal cancer GTV delineation on MRI is good for both radiologists and radiation oncologists, regardless of experience level. Use of DWI did not improve agreement. DWI artefacts affecting GTV delineation occurred in almost half of the patients, which may severely limit the use of DWI for radiotherapy planning if no steps are undertaken to avoid them.