ESTRO 36 Abstract Book

S993 ESTRO 36 2017 _______________________________________________________________________________________________

generated from the Boolean sum volume of the rectum and bladder obtained from all CB-CT’s. For the rectum, the mean ± standard deviation displacement was 0.8 ± 0.3 cm. For the bladder, this was 1.5 ± 0.5 cm. In a second step, these contours will be transposed on the original plan and dose-volume histograms (DVH) will be calculated and combined, to produce single mean DVH representative of the dose actually delivered over the entire treatment course.

Conclusion Using the Yonsei QA Set, we were able to verify the error of 6DoF Couch along both the translational and rotational directions in a very simple method. This system would be useful in performing Daily IGRT QA of 6DoF Couch. EP-1841 CASPIR Trial: Interim analysis of prostatic calculi as an alternative to fiducial markers for IGRT A.G.M. O'Neill 1 , R. King 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 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). PCs represent a potential alternative to implanted fiducials. The purpose of this clinical trial is to directly compare FMs with PCs as an aid to prostate IGRT. Preliminary results are reported. Material and Methods We designed a single institution ethically approved prospective clinical trial investigating the feasibility of using prostate calcifications as natural FMs for IGRT. Patients planned for standard prostate radical EBRT +/- brachytherapy are eligible for the study. Following written informed consent, and prior to CT planning, 3 gold fiducial markers are inserted into the prostate by the trans- perineal route under TRUS guidance. PCs within the PTV are contoured. All participants are aligned for EBRT according to FM positions using daily CBCT image guidance on a Varian TrueBeam linac. Off-line, a single experienced user analyses CBCTs using Image Registration in Eclipse (version 13.6). Random and systematic treatment set-up errors are determined based on FMs, PCs (where present), prostate gland (PG) and bony landmarks (BL) and CTV-PTV margins derived for each data set. Results To date 25 participants have been recruited. 12 participants have PCs contoured, 6 of whom have completed radiotherapy. Based on 2982 individual image registrations the PTV margins required based on each reference structure are summarised in Table 1. Conclusion The maximum difference between the CTV-PTV (PC) margin and CTV-PTV (FM) margin is 1.3mm in the X or L/R dimension. This is less than the maximum difference between CTV-PTV (FM) and CTV-PTV (BL) for the same dimension (1.7mm) and comparable to the difference between CTV-PTV (FM and CTV-PTV (PG) . Preliminary results from this study demonstrate some evidence to support the use of PCs as an alternative to FMs for prostate IGRT. Recruitment is ongoing with a target of 30 participants with PCs. EP-1842 Comparison between infrared marker and surface imaging for DIBH of left-sided breast treatments A. Tini 1 , I. Pytko 1 , A. Moreira 1 , J. Sharpe 1 , C. Winter 1 , M. Guckenberger 1 , C. Linsenmeier 1 1 University Hospital Zürich, Department of Radiation Oncology, Zurich, Switzerland Table 1

Conclusion Despite a strict treatment protocol, important variation in rectal and especially bladder filling was observed. This resulted in PRV margins of 0.8 and 1.5 cm for rectum and bladder, respectively, which is not clinically possible. Therefore, truly adaptive radiotherapy is needed, depending heavily on automatization. EP-1840 Verification of accurate movement of 6DoF Couch using Yonsei QA Set. D. Jung 1 , H. Park 1 , J. Yoon 1 , S. Lee 1 , J. Kim 1 , J. Cho 1 1 Yonsei Cancer Center, Radiation Oncology, Seoul, Korea Republic of Purpose or Objective Yonsei QA Set was established to verify the movement accuracy of image-guided 6DoF(Six Degree of Freedom) Couch and to evaluate its usefulness. Material and Methods Two sets of linear accelerators equipped with 6DoF Couch and CBCT were used. Using the established QA Set, each CBCT image was obtained over 15 times through the Penta-Guide Phantom installed with off-set shift values along six translational (Translation; TX, TY, TZ) and rotational (Rotation, Pitch; RX, Roll; RY, Yaw; RZ) directions. Using this method, we compared the reference image and the registration image, and we analyzed the error calculated by measuring the positional accuracy of the modified 6DoF Couch. Results Image-guided comparison of reference image and registration image demonstrated a correlation of 0.993, revealing high calibration accuracy . Error between the modified off-set value of 6DoF Couch and the measured value along translational directions were 0.25±0.18 mm in the TX direction, 0.25±0.25 mm in the TY direction, and 0.36±0.2 mm in the TZ direction. Misalignments along the rotational axis were 0.18±0.08° in the RX direction, 0.26±0.09° in the RY direction, and 0.11±0.08° in the RZ direction. The correlation value among the rotational directions was significant at 0.958.

Purpose or Objective