Abstract Book

S1249

ESTRO 37

EP-2386 Evaluation of image-guidance strategies of cervical and endometrial cancer A. Ryczkowski 1 , A. Jodda 1 , T. Piotrowski 1,2 1 Greater Poland Cancer Centre, Medical Physics Department, Poznan, Poland 2 Poznan University of Medical Sciences, Faculty of Health Sciences, Poznan, Poland Purpose or Objective In this study, set-up accuracy and obtained margins splitted to two regions (PTV1 - upper vagina and parametrial/paravaginal tissues; PTV2 - common, external and internal iliac lymph nodes) resulted from different image-guidance protocols used for gynecological patients were compared. Material and Methods Set-up corrections from 20 cervical and endometrial cancer patients treated on helical tomotherapy were used to simulate four types of image guidance protocols which were based on: a limited number of imaging sessions (A), reduced registration tasks during daily imaging (B), or mixed methods of imaging (C and D). Each protocol was evaluated for two referencing scenarios based on the first three fractions and first five fractions. Residual set-up error, the difference between the average set-up correction and the actual correction required, was used to evaluate the accuracy of each protocol and estimated on their basis margins.

Electronic Poster: RTT track: Motion management and adaptive strategies

EP-2387 Retrospective analysis of offline chosen plan V's online chosen plan for gynaecological patients. C. Duncanson 1 , A. Sadozye 1 , R. Harrand 1 1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom Purpose or Objective A retrospective analysis of inter-observer chosen plan-of- the-day (PoD) compared with actual treated PoD for cervical and endometrial cancer patients. Aim is to determine the level of concordance between specialist gynaecological radiographer chosen offline plan compared with real-time online plan selected by appropriately trained treatment radiographers, to evaluate if there is a need for further in-house training to improve confidence levels during plan selection. Material and Methods A total of 20 patients planned and treated with adaptive planning margins for cervix or endometrial cancers were reviewed retrospectively by the specialist gynaecological radiographer. Daily online cone beam computed tomography scans (CBCT’s) were captured for each patient. A total of 500 CBCT images were reviewed offline. The treatment plan selected for each fraction was recorded. There was a library of 2 plans available, the standard clinical protocol plan and the plan with additional margins to account for OAR movement and target motion (adaptive plan). The specialist radiographer then reviewed the CBCT’s and made an independent choice between the two PTV’s based on PTV coverage, nodal coverage, bowel dose, bladder filling and rectal volume. This was also tabled and compared with the online selected plan and the results analysed accordingly. Results Of the 20 patients within the sample, 9 patients were post-op endometrial cancer and 11 patients were intact cervical cancer patients. The range of concordance between all patients in both groups was 58.9% - 100%. There was an average discrepancy rate of 7.8% (92.2% concordance) within the post op group, however in the intact cervical group this was higher with an average 9% discrepancy rate (81% concordance). The overall concordance rate was high at 91.23%. One patient in the post-op group had a high discrepancy rate due to issues with bladder and bowel volumes which brought the average down substantially. Conclusion High overall concordance rates were observed between offline and online plan selections. Slightly lower concordance was noted within the intact cervical cancer patient group as a result of greater organ motion to uterus motion based on bladder filling and rectal emptying variability. More regular in-house CPD sessions could be offered to assist staff in reviewing CBCT in particular using any problematic patients previously treated to allow for greater education and confidence when selecting PoD for gynaecology cancer patients.

Results The first five fractions referencing scenario provides the highest reduction of the margins for each image-guidance protocol evaluated in this study. The first type of protocol is the shortest way to the effective correction of the systematic component of set-up error. For the second type of the protocol, the control of the residual errors is better and, as a result, the reduction of the PTV1 is more significant than that obtained for the first one. Although the fourth protocol allows decreasing PTV2, it is not recommended as a protocol to be used to decrease PTV1. The best control of changes connecting to the area covered by PTV1 provides the third type of protocol. Conclusion The choice of the appropriate protocol should be validated in the context of (i) institutional practice regarding patient set-up procedure and its time consumption, (ii) acceptable balance between the amount of the dose delivered to the organ at risk and the additional imaging dose and (iii) patient anatomical conditions.