ESTRO 2020 Abstract Book

S867 ESTRO 2020

Results The use of ABC mDIBH resulted in similar target coverage with no significant statistical differences. Median LV-V5 was 34.6 cc (range: 13.0-67.2 cc) with FB and 11.5cc (range: 0.0-35.7 cc) with ABC (p<0.001), resulting in absolute and relative reduction of 19.9 cc (range: 1.8-43.8 cc) and 67.3% (range: 8.3-100%), respectively. The use of ABC mDIBH reduced LV-V5 by 35% or greater in 90% of patients. Using ABC mDIBH, the dosimetric reduction in LV-V5 translated in an average decrease of ACE 9-year excess cumulative risk from 0.7% to 0.3% for risk0 group, from 1.2% to 0.5% for risk0.8 group, from 1.7% to 0.8% for risk1.4 group and from 2.3% to 1.0% for risk1.8 group. The impact of ABC mDIBH on ACE cumulative risk becomes increasingly remarkable with patients’ age. F or patients age >60 years, ACE 9-year excess cumulative risk decreases from 1.6% to 0.9% for risk0 group, from 2.8% to 1.5% for risk0.8 group, from 4.1% to 2.2% for risk1.4 group and from 5.4% to 2.9% for risk1.8 group. Conclusion ABC mDIBH technique resulted in a significant reduction in left ventricular V5 for left-sided breast radiotherapy. Excess risk of ACEs can be remarkably reduced, in particular for patients age >60 years. PO-1594 Impact of mean heart dose on acute coronary event excess cumulative risk in breath-hold breast IMRT F. Deodato 1 , A. Ianiro 2 , M. Boccardi 1 , G. Macchia 1 , C. Romano 2 , P. Viola 2 , V. Picardi 1 , M. Ferro 1 , M. Ferro 1 , A. Pierro 3 , E. Scirocco 4 , M. Buwenge 4 , S. Cammelli 4 , C. Sacra 5 , C.M. De Filippo 6 , V. Valentini 7 , A.G. Morganti 4 , S. Cilla 2 1 Fondazione di Ricerca e Cura Giovanni Paolo II, Radiation Oncology Unit, Campobasso, Italy ; 2 Fondazione di Ricerca e Cura Giovanni Paolo II, Medical Physics Unit, Campobasso, Italy ; 3 Fondazione di Ricerca e Cura Giovanni Paolo II, Radiology Department, Campobasso, Italy ; 4 DIMES Università di Bologna, Radiation Oncology Department, Bologna, Italy ; 5 Fondazione di Ricerca e Cura Giovanni Paolo II, Cardiology and Hemodynamics Unit, Campobasso, Italy ; 6 Fondazione di Ricerca e Cura Giovanni Paolo II, Cardiovascular Surgery Department, Campobasso, Italy ; 7 Fondazione Policlinico Universitario A. Gemelli, Radiation Oncology Department, Roma, Italy Purpose or Objective Cardiac toxicity is a major concern for left breast tangential field irradiation. A relationship between mean heart dose (MHD) and acute coronary event (ACE) rate has been reported in literature, with a cumulative incidence of ACE increased by approximately 16% per Gy. Moderate deep inspiration breath hold (mDIBH) during radiation treatment delivery helps in reducing the cardiac dose. The aim of this study was to quantify how the use of active breath control (ABC) in mDIBH during tangential IMRT breast irradiation translated in a reduction of ACE excess cumulative risk. Material and Methods Twenty consecutive patients with left-sided breast cancer who underwent adjuvant tangential IMRT with ABC mDIBH were analyzed in this study. All patients underwent CT simulation in both free breathing (FB) and mDIBH. The Elekta ABC spirometer was used for respiratory control and breath-hold length of 20–30 s. A simultaneously integrated boost (SIB) plan was created for both simulation CTs (FB and mDIBH) consisting of a prescription dose of 50 Gy to whole breast and 60 Gy to the tumor bed in 25 fractions and tangential IMRT technique. ACE cumulative risk was calculated using the prediction model developed by van den Bogaard et al., depending on MHD, patient age and pretreatment risk factors (0 for no risk and 1 for diabetes or hypertension or history of

ischemic cardiac events). ACE cumulative risk was calculated twice for each patient (once for each pretreatment risk factors, 0 and 1) simulating two cohorts grouped on the basis of the two risk factors. Results The use of ABC mDIBH resulted in similar target coverage with no significant statistical differences with respect to FB plans. Median MHD was 3.7 Gy (range:2.8-6.2 Gy) in FB and 2.5 Gy (range:1.6-4.1 Gy) in mDIBH (p<0.05), resulting in absolute and relative reduction of 1.3 Gy (range: 0.2- 2.5 Gy) and 35.0% (range: 4.7-53.6%), respectively. The use of ABC mDIBH reduced MHD by 20% or greater in 90% of patients. Using ABC mDIBH, the dosimetric reduction in MHD translated in an average decrease of ACE 9-year excess cumulative risk from 0.5% to 0.3% for risk0 group and from 2.7% to 1.8% for risk1 group. The impact of ABC mDIBH on ACE cumulative risk becomes increasingly remarkable with patients’ age. For patients age >60 years, average ACE 9- year excess cumulative risk decreased from 1.4% to 1.0% for risk0 group and from 7.0% to 4.9% for risk1 group, respectively. Conclusion ABC mDIBH technique resulted in a significant reduction of MHD for left sided breast radiotherapy. Excess risk of ACEs can be remarkably reduced, in particular for patients age >60 years. PO-1595 MR guided tumour tracking on a high field MR Linac: feasibility and first experimental results S. Nill 1 , I. Hanson 1 , F. Costa 1 , M.J. Menten 1 , A. Wetscherek 1 , U. Oelfke 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom Purpose or Objective The Elekta Unity MR Linac (MRL) consists of a high field 1.5 T MR scanner and a 7 MV linear accelerator equipped with an 80 leaf pair multi-leaf collimator, with the leaf motion being parallel to the patient caudal-cranial direction. The MRL allows 2D MR images to be continuously acquired during treatment to detect tumour motion in real-time. MLC based tumour tracking using these MR images is one promising technology that will allow further reduction of PTV margins and consequently improve healthy tissue dose sparing. Material and Methods Our in-house MLC tracking system DynaTrack developed for conventional linacs was adapted to support the MRL running non-clinical research firmware. Geometric tracking errors (root mean square of the difference between the location of a radiopaque marker and the centre of the leaf aperture opening as measured on the on-board EPID) were measured for an insert moving on a sine curve (sup-inf direction) with two breathing periods (T=4 s, 5 s) and three amplitude settings (A=5 mm, 10 mm, 15 mm) inside the Quasar MRI4D motion phantom. The motion was determined using a template matching algorithm on coronal MR images (coherent FFE sequence, update rate: 3 Hz, image latency: 200 ms) as input. A linear regression motion predictor with either a fixed look ahead time of 410 ms or for a variable time point between 243 ms and 486 ms depending on the time of the last received data point was applied within the tracking tool during the delivery. A four field treatment plan was delivered for three different arrangements using the fixed predictor setting: 1) no phantom motion – no tracking 2) phantom motion (T=5 s, A=10 mm) - no tracking 3) phantom motion – with tracking. GAFChromic EBT3 films were irradiated for each case to quantify the dosimetric impact. A comparison using dose profiles and gamma criterion pass rate between the non- tracked and tracked delivery with the results from the no phantom motion case as reference were performed.