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exceeded 0.5cm even though 58 CBCT-scans showed X- rotations above 2 degrees. X-rotations found in the pelvic area had a moderately strong correlation (R 2 =0.36) with the residual error in the Z-direction (figure 1b). The predicted residual error was generally larger than the errors found in the clinic.

Table 1. CTV-PTV Margins ( 2.5∑ + 0.7σ) (mm) X(L/R) Y(A/P) Z(S/I) CTV-PTV (BL) 5.1 6.1 6.4 CTV-PTV (PC) 7.1 7 5.9 CTV-PTV (FM) 4.9 7.2 5.6 CTV-PTV (PG) 5.5 7.5 5.6

Conclusion Preliminary results from this study demonstrate some evidence to support the use of PCs as an alternative to FMs for prostate IGRT. Future analysis will include location of PC according to the PIRADs schema and patient feedback in relation to FM implantation. PV-0366 Stereotactic ablative RT with focal boosting in prostate cancer: feasibility on a 1.5 T MRI linac E.N. De Groot - van Breugel 1 , J. Van der Voort van Zijp 1 , B.W. Raaymakers 1 , J. Hes 1 , L.G.W. Kerkmeijer 1 , J.C.J. De Boer 1 1 UMC Utrecht, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective Recently we have treated intermediate and high-risk prostate cancer patients in a clinical multicentre phase II trial for hypofractionated prostate radiotherapy of 5 × 7 Gy, with local GTV boosts uptil 5 × 10 Gy (hypo-FLAME trial, NCT02853110). Although the targeting accuracy based on kV fiducial marker imaging was high, we expect that on a 1.5 T MRI linac we can fully account for rotations, intrafraction motion and deformations, thus further reducing PTV margins. We investigated if the current clinical MRI linac (Unity, Elekta AB) can achieve dose distributions of a quality similar to conventional state-of-the-art prostate treatments. Material and Methods The UMC Utrecht hypo-FLAME patients were treated on Agility linacs (5 mm leaf width) using 10 MV VMAT plans created in the Monaco TPS (Elekta AB). The GTVs were contoured on multi-parametric MRI (T2/DCE/DWI). The CTV-PTV margin was 4 mm. The most important constraints are summarized in Table I. The larger leaf width (7 mm) of the MRI linac, the fixed collimator angle, its IMRT delivery (beam energy 7 MV) and the strong magnetic field (in particular electron-return-effect near gas pockets) could potentially deteriorate plan quality. The clinically commissioned Monaco TPS for the MRI linac (v5.19) was used to create 7 beam IMRT plans for 10 hypo-FLAME patients using the hypo-FLAME constraint list. We compared these MRI linac dose distributions to those of the corresponding clinically delivered conven- tional plans.

Conclusion Our study showed that a restriction of 4 degree s rotation in set up of the patient would ensure a geometrical coverage in the PAO region. The rotations in the pelvic area only moderately influence the position of the PAO region. This might be explained by deformation of the lumbar and sacral spine. Reducing the threshold on rotations from 4° to 2° is not necessary in clinical practice when margins of 0.5cm are being used. PV-0365 CASPIR Trial: Calculi as an alternative to fiducial markers for IGRT in localised prostate cancer A. O'Neill 1 , R. King 2 , K. Crowther 3 , S. Osman 1 , S. Jain 1 , A.R. Hounsell 1 , J.M. O'Sullivan 1 1 Queens University Belfast, Centre for Cancer Research & Cell Biology, Belfast, United Kingdom 2 Belfast Health and Social Care Trust, Medical Physics Department, Belfast, United Kingdom 3 Belfast Health and Social Care Trust, Radiotherapy Department, Belfast, United Kingdom Purpose or Objective Image guided Radiotherapy (IGRT) for prostate cancer (PCa) frequently employs surgically implanted fiducial markers (FMs). However, it is estimated that up to 35% of prostate radiotherapy patients have prostatic calculi (PC) visible on treatment cone beam CTs (CBCT). The purpose of this clinical trial is to directly compare FMs with PCs as We report data from a single institution prospective clinical trial investigating the feasibility of using prostate calcifications as natural FMs for IGRT. Patients planned for standard prostate radical EBRT +/- brachytherapy were eligible. Prior to CT planning, 3 gold fiducial markers were inserted into the prostate by the trans- perineal route under TRUS guidance. PCs visible within the PTV were contoured. Participants were aligned to FMs for EBRT using daily CBCT image guidance on a Varian TrueBeam linac. Off-line, a single experienced user analysed CBCTs using Image Registration in Eclipse (version 13.6). Random and systematic set-up errors were determined based on FMs, PCs (where present), prostate gland (PG) and bony landmarks (BL), enabling derivation of CTV-PTV margins for each data set. Results 30 participants with PC have been recruited. Data from the first 20 patients (385 fractions) resulted in 2695 individual image registrations (8085 individual data points). The calculated PTV margins based on each reference structure are summarized in Table 1. The maximum difference between the CTV-PTV (PC) margin and CTV-PTV (FM) margin is 2.2mm in the X or L/R direction. Margins required for FM, PG and PC in the Y and Z dimensions are comparable, with a maximum difference of 0.5mm between CTV-PTV (PC) and CTV-PTV (PG) . an aid to prostate IGRT. Material and Methods