ESTRO 2020 Abstract Book

S381 ESTRO 2020

Purpose or Objective Elective radiotherapy (RT) in cN0 head and neck (HN) cancer is a proved strategy for regional control. Nevertheless, large treated volumes are associated with elective RT. CT is used to define targets, lacking sufficient sensitivity to identify individual lymph nodes (iLNs). In contrast, MRI can be used to directly delineate the iLNs for elective RT planning. MRI/RT hybrid modalities, such as the 1.5 T MRI linear accelerator (MRI-linac, Unity, Elekta) could facilitate new RT strategies targeting the iLNS in the neck with on-line MRI capability. Aim is to investigate potential reduction of organ at risk (OAR) dose by comparing standard CT-based elective RT to MRI-linac- based RT of iLNs in the neck. Material and Methods Ten cases of larynx carcinoma T1-4N0 were retrospectively selected. For treatment planning, HN imaging in an immobilization mask was performed on CT (Brilliance, Philips) and on a 3 T MRI with two flexible receive coils (Ingenia, Philips). A T2-weighted turbo spin echo sequence with multiple-Dixon water-fat separation was registered to the planning-CT, and used to delineate LN levels II- IV (CTV), OARs and all iLNs in the LN levels. A 3-mm margin to the CTV was applied to generate the conventional PTV (cPTV). The encompassing volume of all iLNs with 3-mm margin created the so-called selective PTV (sPTV). Numbers of iLNs and volumes of cPTV and sPTV were compared. Three strategies (35x1.55Gy=54.25Gy) were investigated: (A) conventional elective RT on the cPTV, by volumetric-arc RT on a 6-MV linear accelerator (linac); (B) MRI-linac highly selective RT: intensity-modulated RT (IMRT) targeting the sPTV, performed on a 7-MV MRI-linac; (C) Cascaded MRI-linac selective RT: IMRT on cPTV and sPTV, with a lower dose prescribed to cPTV (35x1.03 Gy=36 Gy). Strategy C serves as intermediate technique which targets iLNs but also irradiates the LN levels. In order to show solely the effect of irradiating iLNs vs. entire LN levels, primary tumor dose was not taken into account. Treatment planning software Monaco (Elekta) was used for strategies A (5.10), and for B and C (5.40), which incorporates the magnetic field. V95% was aimed at >99%. Dose plans were compared by assessing the mean OAR dose in two ways: B vs. A and C vs. A. Results An average of 35 (range: 25-58) iLNs were identified per patient. An average 85% target volume reduction of the sPTV (22.5cc) compared to the cPTV (155.7cc) was achieved. Mean OAR dose (table 1, figure 1) was lowered by 44% on average per OAR in strategy B vs. A and by 24% per OAR for strategy C vs. A.

evaluation. The roles and responsibilities of the three professional groups (clinicians, RTTs and physicists) and how they have evolved to achieve a ‘clinician-lite’ workflow are in Figure 1. Responsibility for CT-MR registration has been devolved solely to RTTs, and applying a definitive framework identifying agreed threshold and action levels for anticipated treatment scenarios similar to those in CBCT-based image-guided radiotherapy (IGRT), responsibility for approving the new plan has been transferred to physicists and RTTs. To date, a clinician was required for online approval in <4% of fractions. Early indications are that this methodology has the potential to improve patient throughput and improve efficiency (Table 1). This ‘clinician-lite’ approach will be repeated during the establishment of additional clinical sites for ATP MRgRT.

Conclusion A ‘clinician-lite’ MRgRT ATP prostate treatment workflow has been successfully implemented at our institution. Further devolution of roles and responsibilities such as validation of online contouring, planning and plan review competence is in development to enable RTTs to fully lead in the online workflow including in more complex adaptive scenarios.

OC-0684 Stereotactic MR-Guided Adaptive Radiation Therapy for Inoperable Pancreatic Cancer

Abstract withdrawn from presentation

OC-0685 MRI-linac-based head-and-neck radiotherapy: a new concept for optimal neck sparing T. Van Heijst 1 , J. Mases 1 , S.L. Meyer Viol 1 , E. Brand 1 , C.H.J. Terhaard 1 , P.H. Doornaert 1 , M.E.P. Philippens 1 , C.P.J. Raaijmakers 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands