Helen Barnes – The Royal Marsden NHS Foundation Trust

In September 2019, staff at the Royal Marsden NHS Foundation trust in partnership with the Institute of Cancer Research (ICR) carried out the first stereotactic ablative body radiotherapy (SABR) treatment on the MR Linac (MRL) in the UK. SABR uses highprecision treatment delivery over hypofractionated regimes of three to eight fractions. The radiobiological rationale for hypofractionation is to achieve a greater therapeutic ratio than with standard treatment with the aim of improved local control [1]. SABR is a well-established treatment, traditionally delivered using intensity modulated radiation therapy (IMRT) on a standard C-arm linear accelerator (linac) with on-board cone-beam CT (CBCT) imaging capabilities for geometric verification. While CBCT produces images in three dimensions and good bone matching capabilities, it has disadvantages with soft tissue visualisation and additional radiation dose to the patient [2]. The MRL (Elekta, Stockholm, Sweden) is the fusion of a linear accelerator with a 1.5T Philips MRI scanner providing excellent soft tissue imaging visualisation and real time adaptive radiotherapy capabilities [3]. This treatment utilised the novel technology of the MRL to visualise the lesion to be treated using a T1 weighted (T1w) MRI sequence before delivering online adapted, highly targeted, radiation treatment.

The first SABR treatment on the MRL at our centre targeted an oligometastatic aortocaval lymph node. Classification of oligometastatic disease was originally coined by Hellman and Weichselbaum in 1995 [4] and has since revolutionised the way in which metastatic cancer is staged and treated. Previously treatment intent for metastatic disease would have been entirely palliative, but the development of this sub-categorisation has allowed for the advance of more personalised treatments with the potential for progression free survival [5]. Developing SABR treatments for oligometastatic disease on the MRL with the potential for increased target accuracy and reduced dose to OARs is an exciting achievement for radiographers and the wider MDT alike.

The patient was recruited to the PERMIT trial (IRAS: 236188), which is an umbrella study allowing for a range of patients to be recruited for treatment on the MRL under current standards of practice to test feasibility of this novel machine. Through the trial the patient underwent a “Day 0” imaging session on the MRL, allowing for the acquisition of different types of MRI sequences to determine which provided the optimal visualisation of the target and organs at risk (OARs), whilst sparing the patient additional radiation. Figure 1.0 shows a T1w MRL scan that demonstrates the clear visualisation of the target, notably the distinct definition from the surrounding blood vessels and bowel. This image can be used alongside the radiotherapy planning CT scan to better inform the planning process.

The MRL daily online adaptive workflow allows for staff to amend contours and re-calculate, if necessary, to correct for any discrepancy of the target and OARs producing optimal treatment plan at each fraction. This could potentially lead to future developments such as dose escalation or tumour response assessment and target margin reduction. Further beneficial uses for this technology will be in the treatment of pancreas cancer, an anatomical site notorious for poor visibility on imaging and difficulty targeting the organ for radiotherapy delivery. Additionally, the MRL has online cine-imaging capabilities, providing the radiographers with real-time information of the patient’s anatomy during treatment delivery. This information along with post treatment MRI sequences can be used to reconstruct the dose delivered to the target and OARs during treatment.

Radiographers have led imaging verification on treatment for SABR on the MRL by using our experience and judgement to determine if the patient is correctly positioned for treatment by comparing multiple images. Radiographer-led SABR treatments have been proven to be effective in other areas of radiotherapy already and we look forward to using the same model to expand this into the MRL workflow [6, 7]. As radiographers we are eager to develop our skills in online contouring and treatment planning to expand the radiographer role and facilitate a radiographer-led service on the MRL.

At the Royal Marsden we are developing a training programme to provide radiographers with the knowledge and skills to permit the expansion of the radiographer role. The aim of the training is to empower radiographers to confidently and precisely perform organ delineation online during the MRL workflow. Our current treatment workflow used for oligometastases and other body sites on the MRL requires a clinician to amend or re-contour the treatment target and/or organs at risk (OARs) at each fraction. This process is staff intensive and the ability to train and utilise radiographers for this role would streamline the service to a clinician-free workflow. Studies have already proven that radiographers can achieve accurate prostate contours offline using MRI imaging, with higher agreement than on CT [8]. By increasing the radiographer role further to include online contouring we can lessen the staffing impact of running the unit and remove additional time pressures on clinicians.

References
1. Distefano G, Baker A, Scott AJD, Webster GJ; UK SABR Consortium Quality Assurance Group. Survey of stereotactic ablative body radiotherapy in the UK by the QA group on behalf of the UK SABR Consortium. Br J Radiol. 2014; 87:20130681.
2. Werensteijn-Honingh AM, et al. Feasibility of stereotactic radiotherapy using a 1.5 T MR-linac: Multi-fraction treatment of pelvic lymph node oligometastases. Radiotherapy and Oncology. 2019; 134: 50-54.
3. Winkel D, et al. Adaptive radiotherapy: The Elekta Unity MR-linac concept. Clinical and Translational Radiation Oncology. 2019; 18: 54–59
4. Hellman S, Weichselbaum RR. Oligometastases. J Clin Oncol. 1995; 13: 8–10.
5. Zeng KL, Tseng CL, Soliman H, Weiss Y, Sahgal A, Myrehaug S. (2019). Stereotactic Body Radiotherapy (SBRT) for Oligometastatic Spine Metastases: An Overview. Frontiers in Oncology 2019; 9: 337.
6. McNair HA, Hafeez S, Taylor H, Lalondrelle S, McDonald F, Hansen VN, Huddart R. Radiographer-led plan selection for bladder cancer radiotherapy: initiating a training programme and maintaining competency. Br J Radiol. 2015; 88: 1048.
7. Hudson J, Doolan C, Locke I, McDonald F, Ahmed M, Gunapala R and McNair HA. Are Therapeutic Radiographers able to achieve clinically acceptable verification for stereotactic lung radiotherapy treatment (SBRT)? Journal of Radiotherapy in practice 2015; 10-17.
8. Pathmanathan AU, McNair HA, Schmidt MA, Brand DH, Delacroix L, Eccles CL, et al. Comparison of prostate delineation on multimodality imaging for MR-guided radiotherapy. Br J Radiol. 2019; 92: 20180948.

Acknowledgements
We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research. The views expressed in this publication are those of the authors and not necessarily those of the NHS Executive. The Institute of Cancer Research is supported by Cancer Research UK Programme Grants (C33589/A19727 and C33589/A19908) and the CRUK ART-NET Network Accelerator Award (A21993); MRC Grant MR/M009068/.

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