Abstract Book

S1248

ESTRO 37

perineal placement of three, 3mm gold FM into the prostate using local anaesthesia and transrectal ultrasound. Patients attended for treatment between April 2016 - May 2017. V-MAT techniques were used with total doses from 60Gy-74Gy. Fractionation was 20-37. Patients were planned and treated with a ‘comfortably full’ bladder and empty rectum; achieved by self-administering daily micro-enemas and adhering to a bladder filling protocol. All patients had daily ExacTrac imaging, which acquires two 2D kV oblique radiographs. A point based marker match was performed. Setup corrections for translational dimensions were transferred to ARIA and all shifts applied. For treatment 1-3 and weekly thereafter post correction, pre-treatment CBCT was acquired and matched prior to treatment. In accordance with our institution’s protocol if residual CBCT set-up error was ≤3mm the correction was not applied, if >3mm all shifts were applied. Translational couch shifts and residual errors were recorded. Random (σ) and systematic (∑) set-up errors were calculated. Results 114 kV image pairs and 35 CBCTs were analysed. ∑ and σ set-up errors are presented below.

imaging). In some cases however, the treatment took as long as 40 min due to large internal prostate movements. The treatment for most patients was accomplished in the standard timeslot of 15 min. Conclusion Successful implementation of intrafractional imaging for hypofractionated treatment of prostate cancer results in decreased number of fractions and reduced PTV margins. Despite slight increase in time per fraction, the treatment accuracy and dose delivery are improved. EP-2384 interfractional seminal vesicle motion for prostate cancer with/without androgen deprivation therapy T. Waki 1 , K. Katsui 2 , N. Katayama 2 , M. Takemoto 3 , S. Kanazawa 2 1 Tsuyama Chuo Hospital, Radiology, Tsuyama-shi, Japan 2 Okayama University Hospital, Radiology, Okayama, Japan 3 Japanese Red Cross Society Himeji Hospital, Radiology, Himeji, Japan Purpose or Objective We investigated differences in seminal vesicle (SV) length and interfractional SV motion relative to the prostate gland in prostate cancer patients. Material and Methods We compared 32 patients who received androgen deprivation therapy (ADT) before radiotherapy with 12 patients receiving radiotherapy alone at Okayama University Hospital in August 2008-July 2011. We examined the right and left SVs’ length and motion by computed tomography (CT) to determine the ADT’s effects and analyzed 347 CT scans in a multiple linear regression model. Results The ADT patients’ SV length was significantly shorter than the non-ADT patients’. The differences in right and left SV lengths between the ADT and non-ADT patients were 6.8 mm (95% CI 2.0-11.7 mm) and 7.2 mm (95% CI 3.1- 11.3 mm) respectively in an adjusted regression model. SV motion did not differ between the ADT and non-ADT patients in terms of interfractional motion of the SV tips and the SVs’ center relative to the prostate gland. Conclusion The ADT patients had significantly shorter SVs compared to the non-ADT patients, but no difference in SVmotion was observed. SV interfractional motion should thus be compensated using the same planning margins,regardless of whether ADT is used. EP-2385 Initial experience of ExacTrac X-Ray imaging for prostate patients with implanted fiducial markers. K. Crowther 1 , A. O'Neill 1,2 , C. Agnew 3 , D. McKay 1 , J. Smith 1 , A. McCrum 1 , P. Shiels 1 , S. Jain 1,2 1 Northern Ireland Cancer Centre, Radiotherapy Department, Belfast, United Kingdom 2 Centre for Cancer Research and Cell Biology, Queen's University, Belfast, United Kingdom 3 Northern Ireland Cancer Centre, Medical Physics Department, Belfast, United Kingdom Purpose or Objective To report the initial experience of Novalis ExacTrac Imaging system to verify the prostate position prior to treatment delivery using fiducial markers (FM). Material and Methods Five patients with localised prostate cancer were selected to participate in this pilot. All underwent trans-

Residual CBCT set-up

ExacTrac set-up error (mm) Lat Long Vert

error (mm)

Lat Long Vert

Po p ∑ err or Po p σ err or

0.6 1.2 0.6

1.5 2.1 1.7

1.6 1.5 2.5

1.4 1.9 1.4

Overall the residual setup error following ExacTrac imaging, as determined by CBCT, were small, which demonstrates high accuracy of kV localisation when FM are present. Although for 6 fractions (4/5 patients) the CBCT residual error was >3mm requiring further correction. The advantages of this method of IGRT were evident when one patient presented with a history of Ankylosing spondylitis and bi-lateral hip replacements. 2D kV image quality was not degraded by artefacts from hip joints or FM, enabling localisation of the prostate within PTV while CBCT image quality was degraded by artefact making visualisation of the prostate difficult. Conclusion Radiographers gained confidence in ExacTrac system reporting clear visualisation of FM. Limitations were: PTV coverage and preparation compliance could not be determined when CBCT was not acquired; cost and risk associated with FM placement. Patients with bi-lateral hip replacements receiving prostate radiotherapy will be offered FM, combined with daily ExacTrac imaging. However a larger pilot would be required to further assess the utilization for all prostate patients.