Abstract Book

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ESTRO 37

4mm, 5mm, APX 4mm, 3mm, 4mm, right CA ostia extensional margin for X, Y, Z three coordinates caused by heart beat were: V 11mm, 15mm, 11mm and AM 10mm, 13mm, 10mm, can include the whole range of ostia motion. Conclusion In view of the heterogeneity of different ostia, we suggest chose the individually extensional margin when considering the coronary artery as a separate organ at risk. EP-2350 Dosimetric impact of daily variations to OAR during prostate SABR treatment delivery L. Devlin 1 , S. Currie 1 , D. Dodds 1 , A. Sadozye 1 , P. McLoone 1 , A. Duffton 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom Purpose or Objective Prostate stereotactic ablative radiotherapy (SABR) is a highly conformal radiotherapy technique. Due to large fraction sizes and steep dose gradients, the effect of geometric uncertainties could be greater on organs at risk (OAR). There is little published evidence on the true dose received by OAR in conventional prostate radiotherapy and SABR. Currently the dosimetric impact of organ motion on OAR planning constraints is not well understood. With the increased conformality and dose per fraction, it is essential to evaluate the dose received by OAR. The UK SABR consortium guidelines recommend OAR doses to be as low as achievable.This study retrospectively evaluates dose delivered using CBCT images for patients who have completed treatment within a local safety, feasibility and efficacy study. Aim Is the delivered dose on treatment in agreement of the planned dose to organs at risk. Material and Methods 41 patients treated Prostate SABR linear accelerator based technique 35Gy/ 5, 10X FFF. Prostate tracked pre delivery matching to fiducial markers on CBCT. Retrospective delineation of bladder and rectum on 205 pre-treatment CBCT image sets by 1 observer. CBCT registered to planning CT at fiducial markers. Daily CBCT rectum and bladder contours overlaid on planning CT for dosimetric analysis. Recalculation of original plan to evaluate the dose volume histogram for each structure. Assessing the impact of inter motion of internal organs. The dose received by organs at each fraction measured using ratio of structure at the planning constraint. Total delivered dose received by each organ evaluated to ensure planning constraints met despite organ motion. Results In 8 patients 35% of the rectum received >18Gy. In 17 patients 10% of the rectum received >28Gy. In 17 patients 5% of the rectum received >32Gy and in 15 patients 1% of the rectum received >35Gy. In 18 patients 1% of the Bladder received >35Gy(Table 1).

constraints at the planning stage ensures dose received by OAR is kept to a minimum on treatment.

EP-2351 Reducing organ at risk dose by implementing a bladder plan-of-the-day procedure P.L. Twickler 1 , D. Scandurra-Karssens 1 , H.A.M. Vanhauten 1 , H.E. Van Herpt 1 , J.A. Langendijk 1 , H.P. Van der Laan 1 1 UMCG, Radiotherapy, Groningen, The Netherlands Purpose or Objective In order to reduce the percentage of bladder cancer patients experiencing normal tissue complications from conventional radiotherapy, we have implemented a relatively simple and resource-efficient adaptive radiotherapy (ART) procedure. The target is to reduce the irradiated volume and subsequent side effects. Material and Methods For 20 patients treated with this plan-of-the-day (POTD) technique between 2015-2017, a planning CT was performed with an empty bladder. Three PTVs were delineated (Small [S], Medium [M], Large [L]) using anisotropic margins, derived from work published by Lalondrelle et al 2011, to estimate the target volume under different bladder filling conditions. The L-PTV is slightly smaller than the conventional-PTV used prior to POTD. Image registration match structures, based on the bladder + margin, were delineated for all 3 plans. At each fraction, the most conformal plan which achieves the required target coverage is selected from a library. A cone beam CT (CBCT) was performed and the images were registered initially on bony anatomy and then using the match structures the plan of the day was chosen. Results Overall the S-plan was delivered 18%, the M-plan 62% and the L-plan 20% of the times. The S-plan was delivered three times as often in the final five fractions than at the start of treatment, which might be explained by increased patient compliance to bladder filling instructions. On a per-patient basis, there is some variation in daily filling during the treatment course: all patients required at least two plans and 50% of the patients required all three plans at some stage of treatment. However, the L-plan is only in 5% of the fractions, indicating that extremes in bladder filling are rare. In 95% a plan smaller than the L-plan was chosen. In only 1% of all fractions the chosen plan changed from one extreme to another (e.g. S to L) the next day. In none of the patients repeat CTs were deemed necessary for treatment adaptations. In a random sample of 7 bladder patients the mean doses, in the nominal plan and therefore not the accumulated dose for each fraction, with the L, M and S-plans were for the rectum 37,3 Gy, 32,7 Gy and 27,8 Gy, for the sigmoid 58 Gy, 49,5 Gy and 33,7 Gy and for the small bowel 40 Gy, 25,5 Gy and 12,5 Gy, respectively. Conclusion There is a clear benefit in using POTD with bladder carcinoma, without which in 95% of all fractions these patients would have received a higher OAR dose. In most fractions the M-plan was chosen, giving confidence that the PTV margins created at planning are neither to small nor to large. In this group we found that one planning CT is enough to adequately predict the bladder expansion. Daily variations in bladder filling show that all three plans should be available as half the patients required them at some stage of treatment.

Conclusion Due to organ motion, rectal and bladder constraints were not met for some patients. Applying strict dose