ESTRO 38 Abstract book

S1212 ESTRO 38

Material and Methods Ten patients treated with IMRT for prostate cancer were included in this study. For each patient, 3 to 5 control scans (CTc) were made during the treatment period and fused with the initial scan (CTi). We manually delineated the prostate, the bladder and the rectum on the CTc. A comparative study of the different volumes was performed to determine the percentage of variation. We recorded the minimum dose (Dmin) in the prostate and the maximal dose (Dmax) in the rectum and bladder on the various CT made. Comparison of averages deviations was done using the Wilcoxon Signed Rank test for associated samples. Results We found a significant average variation of volume for the different structures. It was about 20. 67 % [-89, 198%] for the rectum (p<0.001), 16.69 % [-66, 86%] for the bladder (p<0.001) and 5.81 % [-23, 18%] for the prostate (p<0.001). The Dmin in the prostate was decreased by 4.05%, the Dmax in the rectum and the bladder by 0.16 and 0.51%. Two patients had an important decrease of Dmin in the prostate. It was about 18 and 13.75%. For those patients, the increase in prostatic volume was about 6.97% and 16.04% and the decrease in rectal volume about 27.96% and 48.91%. Conclusion The anatomical variations during IMRT in the target volume and the organs at risk were not correlated with a major dose variation. However, in two cases, the significant reduction in rectal volume was associated with a decrease in prostatic Dmin which could have an impact on the control of the disease. We insist, then, on the need to perform the dosimetric scanner with an empty rectum. EP-2197 EEBH as a method of managing respiratory movement in treating abdominal structures with SABR J. Barber 1 , B. Taylor 2 , A. Gaya 2 , A. Qureshi 2 , C. Thomas 3 , C. Hartill 1 , V. Staykova 3 , C. Sisodia 3 1 Guys and St Thomas', Radiotherapy, London, United Kingdom; 2 Guys and St Thomas', Oncology, London, United Kingdom; 3 Guys and St Thomas', Radiotherapy Physics, London, United Kingdom Purpose or Objective When treating abdominal structures with SABR, accounting for respiratory movement is mandated by the CORE and SARON trials and SABR CtE. The most common methods are to apply an ITV, meaning the target volume stays within the high dose area, or using Abdominal Compression to physically restrict the movement of abdominal structures. Utilising EEBH for delivering treatment will enable complete eradication of target volume movement and therefore the ITV. Material and Methods Following the implementation of EEBH in pre-treatment resulting in improved CT image quality, the potential impact of treating in EEBH rather than using an ITV was assessed by quantifying the reduction in target volume and the impact on OAR doses when an EEBH plan was created. This showed clear benefits to treating in EEBH, with volumes being as much as halved and OAR doses being dramatically reduced. Results Treating in EEBH has reduced target volumes by an average of 31% (values ranging from 8% to 51% in the 8 patients analysed), consequently allowing higher doses to be used in more patients and increasing the overall number of patients suitable for treatment with SABR. Conclusion There are challenges involved in treating patients in EEBH, including reproducing breath-holds and verifying the level of hold. Techniques used with DIBH techniques proved ineffective as a result of the EEBH position being within the normal range of respiratory motion, meaning AlignRT was unable to detect this as they would the significant

Purpose or Objective CBCT verification is arguably more critical to ensure set- up accuracy in hypo-fractionated SABR regimens. At our clinic all SABR lung patients receive pre-treatment 4D-CBCT and post-treatment 3D-CBCT imaging. The current default is Varian TrueBeam ‘thorax’ mode (125kV, 15mA, 46cm FoV). This is a full trajectory scan, taking 60 seconds to acquire. To improve on-set efficiency and optimise imaging dose alternative ‘spotlight’ CBCT exposures were investigated. Spotlight offers a shorter 200° trajectory and the smaller 25cm FoV is sufficient for SABR lung PTV’s. These modes were tested on the post- treatment imaging to determine if they are clinically Two thorax spotlight modes (A, B) were optimised using a CIRS lung phantom. Exposure parameters were reduced in both cases to 100kV and 10mA and 15mA for modes A and B, respectively. 5 patients were selected randomly for clinical testing (including gated and non-gated cases). For each patient, standard ‘thorax’ mode was replaced by ‘spotlight’ A or B, on subsequent fractions in no particular order. Treatment radiographers were blinded to which mode was of higher mA. Images were reviewed and scored offline by 4 RTTs of various experience and 1 physicist. Scorers were blinded to which exposures had been used. The CBCTs were scored using an adapted 5-point scoring system, where 1 is the highest score. A score ≤3 was deemed clinically suitable. For each image, the highest and lowest scores were discarded. Results CTDI w was established as 0.78 mGy (A) and 1.16 mGy (B). This represents a dose reduction of 71%-80% compared with the current standard ‘thorax’ mode (CTDI w =3.94mGy). This also corresponds to an effective dose of roughly 0.4mSv (A) and 0.6mSv (B) which is about a lifetime cancer risk of 1 in 42000 and 1 in 28000 respectively compared with the current standard ‘thorax’ mode which is 1 in 8000 (2.2mSv). Spotlight A and B achieved an average scoring of 2.4 and 2.5 across all patients, respectively. Both would therefore be suitable for clinical use. With minimal scoring difference, the lower dose Spotlight A is preferable for its reduced dose. CBCT delivery time was reduced from 60 seconds to 33 seconds. Conclusion The use of spotlight mode for verification of lung SABR has been demonstrated. Imaging dose has been optimised as low as reasonably practicable whilst fulfilling the clinical requirement for verification. Delivery time has been reduced, improving patient experience and clinic throughput. The new mode will be introduced as the clinical standard. suitable for verification. Material and Methods

Electronic Poster: RTT track: Motion management and adaptive strategies

EP-2196 Dosimetric impact of anatomical changes during IMRT for prostate cancer L. Farhat 1 , W. Mnejja 1 , H. Daoud 1 , F. Dhouib 1 , T. Sahnoun 1 , W. Siala 1 , J. Daoud 1 1 Hbib Bourguiba Hospital, Radiotherapy, Sfax, Tunisia Purpose or Objective During the treatment of prostate cancer with intensity modulated radiotherapy (IMRT), we observe geometric (volumetric and positional) variations. We proposed in this work to evaluate the dosimetric impact of these variations on prostate and organs at risk: bladder and rectum.