Abstract Book

S1101

ESTRO 37

only MR guided respiration-gated radiotherapy met planning parameters and was thus clinically applicable.

The study cohort consisted of 15 prostate carcinoma patients that were treated with a hypofractionated radiotherapy protocol: 67.5 Gy in 25 fractions, 2.7 Gy per fraction, with a simultaneous boost irradiation of 69 Gy. Planning CT images were acquired in supine position and with a knee wedge and foot block for immobilization. Three fiducial gold markers were implanted into the gland under ultrasound guidance at least two weeks before the CT planning acquisition. The patients were reminded to follow an empty rectum and fill bladder preparation protocol before the TC acquisition and each fractional treatment. At each fraction a pre-treatment CBCT (CBCT pre-fiducial ) was acquired and a rigid 3D fiducial markers-registration to the planning CT was performed to an accurate positioning of the patient (Eq. 1). On alternate days, a post- treatment CBCT (CBCT post-fiducial ) was acquired with the patient still in the treatment position (Eq 2). Two more rigid 3D pelvic bony anatomy-registrations were performed off-line: one using the pre-treatment CBCT (CBCT pre-bones ) (Eq. 3) and another one using post - treatment CBCT (CBCT post-bones ) (Eq. 4). The intrafractional motion during a hypofractionated prostate radiotherapy treatment was separated in two components (Eq. 5): the patient displacement and the prostate gland motion from pelvic bony structures. The shifts based of the CBCT post-fiducial registration to the planning CT were used for total intrafractional motion determination (Eq. 6). The intrafractional component because of the patient displacement was estimated from the subtraction of the CBCT pre-bones and CBCT post-bones shifts (Eq. 7).

EP-2017 Assessing the dosimetric impact of intra- fractional prostate motion during SBRT. S. Novikov 1 , J. Melnik 1 , N. Ilin 1 , M. Girshovitch 1 , S. Kanaev 1 1 Prof. N.N. Petrov Research Institute of Oncology, Radiation oncology and nuclear medicine, St. Petersburg, Russian Federation Purpose or Objective To determine average and maximum values of prostate displacement during every fraction of stererotactic radiotherapy (SBRT) for prostate cancer. To analyze impact of this displacement on final dose distribution and to recommend PTV margins. Material and Methods Prostate intra-fractional motion was determined in 31 patients with prostate cancer that received 156 fractions of SBRT on Novalis TX system. Three reflective spheres were fixed on the skin and 3 fiducial markers were implanted in the prostate of every patient. SBRT was performed in supine position without immobilization devices under Exac-Trac infrared real time control of patient position on the table. Pre-treatment target (prostate) positioning was performed by Cone beam CT (CBCT) and subsequent verification CBCT was done after the end of each fraction with recording the information about markers shifts. On the last stage on “Eclipse” 3D planning system we performed modelling of prostate displacement during every fraction and determined final “real” dose distribution in prostate and rectum after SBRT with various PTV margins. Results: According to postfraction CBCT average marker shifts in cranio-caudal direction was 0,8+/-1,2mm, in lateral - 0.9+/-1,0mm, in anterior-posterior - 1.3+/- 1,3mm. In 5% - 8,3 % cases value of cranio-caudal and lateral shifts were above 2 mm with maximal shift -5мм. Modelling of dose distribution in prostate with considering average intrafractional shifts indicate acceptable dose distribution in clinical target volume (prostate) in patients with PTV with 1mm posterior and 3 mm – anterior, lateral and cranio-caudal margins: V100prostate – 98%, D90prostate – 101%. At the same time reduction of “standard” (3mm- posterior, 5mm – in other directions) margins to above mentioned (1mm- posterior, 3 mm – other directions) can reduce rectum volume that covered by 80% isodose. Conclusion due to determined small intrafractional displacement of prostate during SBRT sessions recommended margins for PTV can be as follows: posterior – 1mm, anterior – 3mm, lateral – 3mm and cranio-caudal – 3mm. EP-2018 Components of prostate displacement during hypofractionated radiotherapy treatment M. Roch 1 , P. Castro 1 , A. Zapatero 2 , D. Hernández 1 , L. Perez 1 1 Hospital Universitario La Princesa, Radiophysics / Radiation Oncology, Madrid, Spain 2 Hospital Universitario La Princesa, Radiation Oncology, Madrid, Spain Purpose or Objective The first objective of this study was to determine the prostate intrafraction motion using pre- and post- treatment cone-beam CT (CBCT) images with three implanted gold seed fiducial markers. The second objective was to decompose this movement in two components: the patient displacement and the prostate gland motion from pelvic bony anatomy. Material and Methods

Therefore the component because of a prostate intrafractional motion from bony anatomy was calculated from equation 5. Results The average displacement for total intrafraction motion was 3.2 mm (σ=2 mm), for patient motion was 1.7 mm (σ=1.0 mm) and for prostate gland from bone anatomy was 3.2 mm (σ=1.9 mm). In the Table 1 was shown the two components of intrafraction motion distribution in percentage of sessions.

Table 1. Intrafractional motion distribution (in percentage of sessions) during radiotherapy separated in two components: intrafractional motion of the patient and intrafractional motion of prostate gland taking pelvic bony anatomy as a reference. Conclusion The origin of geometric uncertainties during hypo- fractionated radiotherapy treatment was the prostate gland motion from pelvic bony anatomy, and not the patient displacement. EP-2019 A respiratory motion management strategy for both abdominal and thoracic VMAT radiotherapy. M. Bray-Parry 1 , J. Gesner 1 , S. Stevens 1 , A. Richmond 1 , J. Konieczek 1 , K. Finnegan 1 , I. Ho 1 1 The London Clinic, Medical Physics, London, United Kingdom