ESTRO 36 Abstract Book

S254 ESTRO 36 2017 _______________________________________________________________________________________________

binning in 10 phases, Siemens Somatom Definition AS, 2 mm slices). Results Based on r0 , 10 patients showed an intra-fractional baseline drift of more than 5 mm, in one or more directions, in 50% or more fractions. For all patients, intra- fractional differences in amplitude were not statistically significant (p > 0,05). An example of intra-fractional variability is shown in Fig 1. In terms of long-term variability, for 11 patients the difference in mean amplitude between at least 2 fractions or a fraction and 4DCT is statistically significant (p < 0,05). Fig 2 shows the mean amplitude per fraction per patient, together with the amplitude based on 4DCT. Both intra- and inter-fractional changes in hysteresis were smaller than 5 mm for all patients. No correlation could be found between long or short-term variability and tumour size or location.

Increasing the number of plans for larger motion could be beneficial for tissue sparing, but is burdensome and the dosimetric benefit is yet to be proven at an individual patient level. We investigated an easy-to-use metric to individualize the number of plans in the library, balancing gain in organs-at-risk (OAR) sparing and clinical feasibility. Material and Methods Data of 14 previously treated patients were analyzed. For each patient, plan libraries were created containing either 1 plan, based on the full-motion-range ITV as derived from the pre-treatment full and empty bladder CT-scans, or 2, 3 or 4 plans based on sub-motion-range ITVs. To create PTVs, the nodal CTV was expanded by 7-mm and the ITVs by 10-mm. For all PTVs, VMAT plans were created to deliver 46 Gy in 23 fractions. Daily CBCT scans were used for plan selection by calculating the bladder volume. For the dosimetric evaluations, the cervix-uterus and the OAR (bladder, rectum, and bowel) were contoured on daily CBCT scans. For each plan library, the OAR V 40Gy were recorded for all CBCT scans. As the gain in sparing varied over the OAR during the fractionated treatment, a composite volume was calculated by summing up the DVH parameters. Pearson correlation was estimated between DVH parameters and a maximum pretreatment extent of uterus motion defined as the Hausdorff distance (HD) (99%-ile) (MaxUtHD) . A Wilcoxon signed-rank test was used to assess statistical differences among strategies. Results Strong correlations were found between MaxUtHD and the total volume of spared normal tissue (composite gain of DVH parameters for bowel, bladder and rectum, see Fig.1). 3 patients were identified as outliers having more than 30% of the fractions an emptier or a fuller bladder than on the planning CT. They should be re-planned during treatment and were excluded from further analysis. For 2 plan (R= 0.8), 3 plan (R= 0.9) and 4 plan libra ries (R= 0.6), p ˂ 0.01. For patients with MaxUtHD >35 mm, adding a 3 rd plan would significantly reduce composite V 4 0Gy by 18 ± 6 cc on average (from that bowel 10 ± 6 cc, blad der 6 ± 2 cc and rectum 2 ± 1 cc). For patient >50 mm MaxUtHD, additional 12 cc would be spared by adding a 4 th plan to the library.

Conclusion Based on this select group of patients, it can be concluded that breathing-induced tumour motion can vary significantly over the entire course of treatment, not rarely exceeding common safety margins of 5 mm. A previous study indicates that a single 4DCT is sufficient to evaluate breathing motion, however, long- term variability was never investigated. The results in this study indicate that a single 4DCT, a snapshot in time, can not accurately predict the motion amplitude during all fractions, which is especially cumbersome when used for ITV treatment planning. OC-0485 How many plans are needed for an optimal plan library in ART for locally advanced cervical cancer? E. Novakova 1 , S.T. Heijkoop 1 , S. Quint 1 , A.G. Zolnay 1 , J.W.M. Mens 1 , J. Godart 1 , B.J.M. Heijmen 1 , M.S. Hoogeman 1 1 Erasmus MC Cancer Institute, Physics, Rotterdam, The Netherlands Purpose or Objective In our institute, locally advanced cervical cancer patients are treated with online, Plan-of-the-Day ART. For patients with a tip-of-uterus displacement >2.5 cm, as measured in pretreatment full and empty bladder CTs, the plan library contains 2 plans to cope with daily anatomy variations.

Conclusion Our results indicate that an extension of the plan library would have the most impact on sparing of bowel cavity. Patients with large MaxUtHD (>35 mm) would benefit from adding a 3 rd plan to the library and patients with the extremely large MaxUtHD (>50 mm) would benefit from adding a 4 th plan to the library. This study provides an easy-to-implement criteria to select patients who would benefit the most from additional plans in a plan library approach. OC-0486 Multi-criterial patient positioning based on dose recalculation on scatter-corrected CBCT images J. Hofmaier 1,2 , J. Haehnle 3 , C. Kurz 1,2 , G. Landry 2 , C. Maihöfer 1 , P. Süss 3 , K. Teichert 3 , N. Traulsen 4 , C. Brachmann 5 , F. Weiler 5 , C. Thieke 1 , K.H. Küfer 3 , C.