ESTRO 38 Abstract book

S1146 ESTRO 38

References: [1] https://doi.org/10.1667/RR14018.1 [2] https://doi.org/10.1073/pnas.0603567103 [3] https://doi.org/10.1118/1.4791648 [4] https://doi.org/10.1038/s41598-017-14786-y [5] https://doi.org/10.1016/S0167-8140(18)30879-X [6] https://doi.org/10.1118/1.4930960 [7] https://doi.org/10.1002/mp.12175 [8] https://doi.org/10.1088/0031-9155/49/19/007 EP-2077 De-intensification of radiotherapy dose to the elective neck in oropharyngeal squamous cell cancers R. Valentine 1 , A. Martin 2 , S. Currie 1 , C. Paterson 2 1 Beatson West of Scotland Cancer Centre, Radiotherapy Physics Department, Glasgow, United Kingdom ; 2 Beatson West of Scotland Cancer Centre, Radiation Oncology Department, Glasgow, United Kingdom Purpose or Objective The incidence of oropharyngeal squamous cell cancer (OPSCC) is rising due to Human Papilloma Virus (HPV) infection. HPV-positive (HPV+) OPSCC has a good prognosis and is sensitive to treatment. Standard of care is high dose radiotherapy which is very effective but causes significant and permanent toxicity. There is a consensus that investigation into less toxic treatments for these patients is required. A retrospective planning study was carried out evaluating the effect of delivering a reduced radiotherapy dose to the elective neck, known as Planning Target Volume (PTV_LR) on dose received by the organs at risk (OARs). Material and Methods Ten representative patients (HPV+, non-smokers, OPSCC) were re-planned retrospectively using Eclipse TPS v15.5 [Varian Medical Systems, Palo Alto, CA, USA], RapidPlan ® and multi-criteria optimisation (MCO). All plans consisted of two VMAT fields with two full rotational arcs at 6 MV and 600 MU/min. In our centre, head & neck squamous cancers are typically prescribed 65Gy in 30 fractions to gross disease; High Risk Planning Target Volume (PTV_HR) while the PTV_LR is treated to 54Gy in 30 fractions (50Gy equivalent). The original clinical plans were re-optimised with a locally published RapidPlan ® model and MCO to achieve optimal PTV coverage while maintaining OAR sparing consistent with our centre’s specified dose constraints. Subsequently, all plans were similarly re- optimised only this time PTV_LR was prescribed 42.1Gy in 30 Fractions [40Gy (EQD2Gy)] while retaining 65Gy to PTV_HR. Finally, plan deliverability for all plans was assessed using the modulation factor (MF) and MLC average leaf pair opening (ALPO). Results Table 1 highlights the dose differences found between PTV_HR, PTV_LR and pertinent OARs when comparing between the 54Gy and 42.1Gy prescriptions to PTV_LR. Acceptable and within protocol, PTV_HR coverage was still achieved for 42.1Gy with only a small % reduction found in the D99% and D95% constraints. Comparing between the prescriptions, the dose constraints for PTV_LR were clinically acceptable with each of the appointed OAR doses significantly reduced for 42.1Gy (p<0.05). On average, we found the MF to be 0.40 and 0.46 and the ALPO to be 2.9 and 3.0 for the 54Gy and 42.1Gy plans, respectively, suggesting highly deliverable plans.

Conclusion By reducing the dose from 54Gy to 42.1Gy to PTV_LR, we found a significant reduction in OAR doses while maintaining acceptable doses to the PTV_HR. Due to steep dose response curves seen in OARS, we anticipate that this reduction in radiotherapy dose will translate into potentially important toxicity gains for patients. This approach is likely to be safe due to maintained dose to gross disease yet with potential clinically relevant reductions in dose received by OARS. On the basis of these results a larger dosimetric study is in progress and will form the basis for the radiotherapy protocol and QA work for a clinical trial investigating this strategy. EP-2078 Comprehensive risk assessment for the clinical introduction of an MR-linac J. Geuze 1 , R. Bouwman 1 , U.A. Van der Heide 1 , S. Ten Hoeve 1 , M.E. Nowee 1 , T.M. Janssen 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective Bodies like Joint Commission International and Health & Safety Executive advise hospitals to use risk assessment methodologies like HFMEA (Healthcare Failure Mode Effect Analysis) to enhance patient safety and reduce unwanted incidents 1 . A major event like the clinical introduction of the Unity MR-linac (Elekta AB, Stockholm, Sweden) has impact on the complete treatment chain. However, the scope and complexity of this process make a single HFMEA assessment inadequate for thorough identification of potential risks 2 . Alternatively, the execution of multiple HFMEA based risk assessments would be quite time consuming. The aim of this work is to present a comprehensive risk assessment program, where multiple risk assessment methodologies are combined to ensure patients safety in a complex process like the introduction of an MR-linac. Material and Methods In the MR-linac treatment process 5 different sub processes are identified (fig. 1). Depending on whether a sub process was MR-linac specific or already clinically implemented, different risk assessment methodologies were chosen.

The sub processes of the pre-treatment phase are quite similar to those for a conventional linear accelerator. The