ESTRO 2020 Abstract Book

S1021 ESTRO 2020

after which it was also applied to 12 SRS patients receiving 1–3 fractions. For all patients, 2 CBCT’s were acquired prior to the treatment delivery and the detected translational shifts were corrected with treatment couch. An extra verification CBCT was acquired after the treatment. This CBCT was used to determine the overall setup accuracy of the system. All CBCT’s were automatically registered to planning CT based on bony anatomy. For SRS patients, pitch setup errors were manually adjusted if the first CBCT indicates a deviation of ≥ 1°. The accuracy of the pitch correction were evaluated using match results of the second CBCT’s.

use during CT planning can enhance soft tissue contrast of the scan facilitating Target Volume (TV) and Organ At Risk (OAR) delineation. Despite this known benefit, there are no current guidelines for what sites should receive IVC during the Planning CT scan and because of this variation may occur. There are international diagnostic imaging recommendations on how IVC should be administered. The primary aim of the study is to examine the patterns of practice in relation to the use of IVC in RT Planning scans in Ireland and to determine the level of compliance with recommendations. Radiation Therapists (RTT) IVC training An evidence-based anonymised online survey was distributed to all RT department in Ireland. The IVC questions in the survey were taken or adapted from published literature and based on the UK Royal College Radiologists ‘Imaging for Oncology’ publication (2004). The questions relating to patient safety were informed by the European Society of Urogenital Radiology (ESUR) and the Royal Australian and New Zealand College of Radiologists (RANZCR) IVC guidelines. The survey contained open, closed and Likert scale questions that investigated IVC protocols in each department. Results 75% of departments responded (n=9/12) and all responding departments used IVC to varying degrees (Figure 1). RTTs cannulated patients in 67% of departments and administration contrast in all departments. Variations from guidelines were reported in the disease sites where IVC was used (Figure 2) and in the assessment of renal functioning prior to contrast use. Training varied in duration and number of supervised procedures. Figure 1: The frequency of intravenous contrast (IVC) usage per department in one month was also investigated. Material and Methods

Results For the first 12 palliative cranial patients, the mean-of mean (µ), -systematic (Σ) and -random (σ) of the translational setup uncertainties are calculated. When these values are applied to the margin formula proposed by van Herks [ M = 2.5 Σ + 0.7σ], the resulting setup margins for the lateral, longitudinal and vertical directions are 0.5, 0.5 and 1.0mm, respectively. For the 12 SRS patients, the means, standard deviations and maximum shifts detected with the post treatment CBCT’s are shown in Table 1. In 4 fractions, pitch deviation of >1°were detected in the first CBCT. After the manual correction with the Solstice system, the pitch setup errors were reduced to 0.1–0.5°. This range may be caused by the adjustment step size of 0.25° (half turn of positioning screw) we adopt in our clinic for pitch correction and possible slight difference in location of the correction reference point of the CBCT and the center of the pitch-rotational axis of the head support. Conclusion The Solstice system provides sub-millimeter positioning accuracy which is required for high precision SRS treatment delivery. Its unique design enable manual correction of the pitch-rotational setup error. This correction can be clinically important for SRS treatment as it permits a more frequent use of single isocenter to treat patients with multiple brain metastases. PO-1831 Intravenous contrast media in radiation therapy planning ct scans - current practice and learning S. Minogue 1 , C. Gillham 2 , M. Kearney 1 , L. Mullaney 1 1 Trinity College Dublin, Discipline of Radiation Therapy- School of Medicine, Dublin, Ireland ; 2 St Luke's Radiation Oncology Network, Radiation Oncology Department, Dublin, Ireland Purpose or Objective Computerised Tomography (CT) remains the gold standard in radiotherapy (RT) planning, but inferior soft tissue definition remains a challenge. Intravenous contrast (IVC)

Figure 2: Usage of intravenous contrast (IVC) for radiation therapy planning scans by the UK Royal College Radiologists (RCR) Imaging for Oncology report guidelines for the disease sites where IVC should (‘recommended’) and could be used (‘suggested’)

Conclusion IVC is used extensively in Irish RT departments with some practices inconsistent between departments and with