ESTRO 2020 Abstract Book

S1049 ESTRO 2020

Purpose or Objective Improved conformality of radiotherapy methods and increasingly adaptive radiotherapy requires optimal on-set verification. Standard verification for radically treated oropharyngeal cancer often involves off-line cone beam CT (CBCT). Many oropharyngeal tumours are influenced by the movement of the hyoid due to direct fixation or infiltration of swallowing muscles. The position of the hyoid can change significantly as tumours respond to treatment; it may move inferiorly over the course of radiotherapy as base of tongue tumours respond to radiotherapy and release extrinsic muscles of the tongue. If the hyoid bone is used as a surrogate for primary gross tumour volume (GTV_P) position, we may well inaccurately verify treatment during the latter fractions of radiotherapy as the hyoid moves more independently of the GTV_P. Alternatively, if we don’t consider the hyoid bone in verification and match to other bony landmarks in patients where the primary moves with the hyoid, we could risk geographical miss, or unplanned increase in dose to organs at risk (OARs). As far as we are aware, there is a paucity of literature addressing this issue and no standard protocols to reduce its prevalence or impact upon verification. In our centre, radically treated head and neck patients who have persistent on set changes to the hyoid position on CBCT at verification, are referred for review and discussion of potential geographical miss. Standardly a 5-6mm and 3-5mm margin is added to the GTV_P inferiorly and superiorly to account for microscopic spread and internal movement/set up error respectively. This is a total of 8-11mm. We studied the magnitude of hyoid displacement in head and neck patients at our centre to identify those whose hyoid moved superiorly or inferiorly by 8mm or more and so were at risk of geographical miss. Material and Methods The 2018 records of patients who were referred for set up review were screened. Patients having radical head and neck radiotherapy referred on account of hyoid movement were identified. The date of referral was the date of the on-set CBCT demonstrating the hyoid displacement. The referral CBCT was fused with the baseline planning CT on Prosoma. Using the CT slice numbers, the position of the GTVp and hyoid on baseline CT, and hyoid on CBCT was recorded. Results 32 radical head and neck patients were referred for hyoid bone displacement during 2018. 29 oropharyngeal primary tumours, 75% staged as T4 primaries. We had complete imaging histories on 27 patients. In 20 patients the GTV_P overlapped with the hyoid bone at baseline. 17 patients had superior or inferior hyoid displacement of 8mm or more.

1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom Purpose or Objective Pelvic insufficiency fractures (PIF) are fractures caused by normal or physiological stress on weakened bone. Radiotherapy (RT) increases the risk of insufficiency fractures in patients as a late effect. 1 Symptoms include debilitating pain, with limited management options. The dose-effect relationship has previously been defined with a risk of fracture at 22% at 35Gy and 45% at 40Gy at sacrum D50%. 2 Aim To determine in patients with PIF, if the sacrum D50% at radiotherapy planning exceeded the dose levels defined by Patients with confirmed radiological PIF were included in analysis. Radiotherapy technique was volumetric modulated arc therapy (VMAT), patients were treated with a library of 2 adaptive plans or with 1 plan. Retrospective dosimetric analysis was done on the planning CT, by delineation of the sacrum in Eclipse V15.5 (Varian Medical Systems, Palo Alto) . A structure was created using the boolean operator to define sacrum overlap with PTV. Dose volume histograms were exported for the sacrum structure and overlap structure and planning characteristics recorded. Results All Ca Cervix patients had 4 Brachytherapy insertions (24Gy) and Ca Endometrium patient had no insertions. 6 patients presenting symptom was lower back pain and 4 patients PIF were radiological incidental findings. All patients had concurrent weekly cisplatin. Dose prescribed 50Gy/25# (n=6) and 45Gy/25# (n=4). 10 gynaecological patients treated with external RT between March 2013-June 2019,had confirmed sacral insufficiency fractures on diagnostic magnetic resonance imaging (MRI),RT completed between March 2013- December 2018. Median age of patients was 57 (IQR 37.5- 64.75) with 7 (70%) patients > 50 years. 2 patients were pre menopausal on hormone replacement therapy (HRT) remaining patients not on HRT. Time to confirmed diagnosis of insufficiency fractures from radiotherapy completion was mean of 12 (IQR 5-20.75) months. The median sacrum volume was 189.1cm 3 (IQR 194.23-157.26). The median sacrum overlap with PTV volume was 42.66cm 3 (IQR 39.26-46.42). Figure 1 shows individual patients sacrum and sacrum-PTV volumes The max dose to the sacrum was 54.05Gy, min 16.22Gy and mean 34.16Gy (IQR 36.13-39.74). The median D50% of the sacrum was 39.03Gy (IQR 37.04-40.61). 9/10 (90%) of patients D50% were above 35Gy and 4/10 (40%) were above 40Gy . For the patients who were symptomatic the mean sacrum dose was 39.05Gy and the median D50% 40.52 Gy. Conclusion Although a sub group of patients may be more at risk of PIF, applying constraints to sacrum for all patients at the planning stage could reduce this. Future work will look at the feasibility of achieving constraints whilst maintaining optimum PTV coverage, and constraints to other OAR. Ramlov et al (2017). Material and Methods

Poster: RTT track: Image guided radiotherapy and verification protocols

PO-1883 Movement of the hyoid during head and neck radiotherapy: Implications for off-line verification S. Hargreaves 1 , S. Jones 1 , T. Rackley 1 , J. Staffurth 1 , M. Evans 1 1 Velindre Cancer Centre, Radiotherapy, Cardiff, United Kingdom