ESTRO 2020 Abstract Book

S1045 ESTRO 2020

depth of PTV showed a strong positive correlation with separation (R=0.618). Conclusion The dose distribution obtained from an anterior beam might be inadequate. The relationship between reduced PTV coverage and separation appears to be more robust when separation is 18 cm or greater. The maximum axillary separation is a simple parameter that can be used to predict the need to utilize other techniques to ensure adequate treatment of regional lymph nodes. PO-1876 A visual grading analysis-based audit for MR simulation sequence development L. McDaid 1 , L. Cooper 2 , T. Edwards 2 , A. McPartlin 2 , S. Bonington 3 , C. Eccles 1 1 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom ; 2 The Christie NHS Foundation Trust, Proton Beam Therapy Centre, Manchester, United Kingdom ; 3 The Christie NHS Foundation Trust, Radiology, Manchester, United Kingdom Purpose or Objective To assess recently implemented MR Simulation protocols used for treatment planning in head and neck patients being treated as part of a newly introduced proton beam radiotherapy (PBT) service. Material and Methods Based on a review of the literature, an imaging protocol comprising four sequences was implemented. Imaging was performed using a 1.5 Tesla (T) magnet, utilising a combination of 32-channel body array, 44-channel spinal array and two large flexible 2-channel transmit receive coils. Table 1 details the sequences used, but in short included 2- and 3 – dimensional acquisitions using turbo spin echo (TSE), driven equilibrium (DRIVE), spectral pre- saturation with inversion (SPIR) and mDixon sequences. All patients were imaged axially from superior orbital margin to sternal notch in the treatment position. To evaluate image quality and suitability, an audit was undertaken, using a visual grading analysis (VGA). The VGA was based on a 4-point scale, ranging from “very clear” to “not visible” for a series of pre- determined structures (table 2) by 3 multidisciplinary team members. Results Sixteen patient datasets were available for review. The preliminary analysis demonstrated a scan time of 31.26 minutes, with a mean ‘in-room’ time of 64.07 minutes (range 24-101 minutes). Using VGA tool, one radiographer, one radiologist and one clinical oncologist reviewed images (Table 1). Overall, an average of 69.4% of the sequences and structures were reported as “very clear” or “clear” (range 57.1-87.3%) (Table 2). The T2W TSE mDixon and T1 SPIR with intravenous contrast were the 2 sequences that scored the highest. The 3D T2W DRIVE sequence came out as unclear or had structures that weren’t visible in nearly 43% of the cases.

Conclusion We conclude that physician-led contoured PTV for cranial boost improved the 95% isodose coverage of the PTV at the ON and CP compared to bony landmarks referenced PTV. Further work is planned to develop autocontouring of structures to streamline this process and to incorporate this in a RTT–led clinical workflow, including methods to optimise the coverage of the upper cervical spinal canal and adjacent vertebrae which is crucial in growing children. PO-1875 Is a single anterior field sufficient to treat regional nodes in breast cancer? H. Almasri 1 , P. Jain 1 , S. Pace 1 , S. Allen 1 , S. Kumar 1 1 Leeds teaching hospitals NHS trust, Radiation Oncology, Leeds, United Kingdom Purpose or Objective The use of axillary radiotherapy in adjuvant treatment of breast cancer has been recently increasing mainly as an alternative to axillary dissection after positive sentinel lymph nodes in early stages. However, as contouring nodal volumes can be time consuming, most centres in the UK still treat with standard fields based on bony landmarks. This is usually done utilizing a single anterior field. The dose distribution resulting from this technique might not be always adequate. The aim of this study is to identify a subset of patients in which nodal irradiation using a single field would produce insufficient dose coverage even when high photon energy is used. Material and Methods CT data sets of thirty patients who were treated with 10 MV single anterior field were retrieved. The nodal target volume was retrospectively contoured according to the ESTRO consensus guideline. A sub-volume consisting of the PTV encompassed within the anterior field was created for the purpose of dose evaluation. DVH statistics were generated to evaluate the dose coverage from existing fields that were used in patients’ treatment where a dose of 40 Gy was prescribed to 95% (approximately 4 cm depth). The maximum axillary separation was measured consistently as the anteroposterior diameter just medial to the humeral head on CT slices. The maximum depth of PTV was measured from the skin to the deepest point of PTV along the beam axis. Coverage was considered adequate if at least 90% of the target volume received at least 90% of the prescribed dose (V36>90%). Results Inadequate PTV coverage (V36<90%) was found in 15/30 patients (50%). In 14 out of these 15 patients (93%), the measured separation was ≥18 cm, (median=19.4, range=18-23). The maximum axillary separation was >18 cm in 17/30 patients, 14 of which (82%) had a V36<90% while the remaining 3 had a V36<95%.Mean V36 was 78% for patients with separation ≥18 cm, whereas in patients with a separation <18 cm mean V36 was 95.9%.The deepest point of the PTV in patients who had V36<90% was located at a median of 9.8 cm (range= 8.8 -12.8). Depth was the reason of low coverage in all the 15 patients. Miss from the medial field border also contributed to poor dosimetry in 8/15 patients. A strong negative correlation was found between V36 and separation (R=0.735). The maximum