ESTRO 2020 Abstract book

S634 ESTRO 2020

the proposed constraints: Dmean ≤37 Gy and V40 ≤50%. For patients with overload, we realized an optimized treatment plan to try to respect the constraints. Results For the planning plan, mean Dmean and V40 were 35 Gy [28-44] and 42% [9-96] respectively. Dose constraints were respected in 39 cases (66.1%) and in 38 cases (64.4%) for Dmean and V40 respectively. A total of 23 optimized plans were realized. After optimization, mean Dmean and V40 were 34 Gy [28-44] and 39% [9-91] respectively. Dose constraints were respected in 52 cases (88%) for Dmean and in 49 cases (83%) for V40. Conclusion Our study results show that the proposed dose constraints (Dmean ≤37 Gy and V40 ≤50%) are suitable for routine use : they can be respected in more than 80% of cases. For the remaining 20% of cases, the bladder volume should be checked and an intensity modulated radiotherapy discussed each time the overrun is important. PO-1111 Pathological lymph node staging for intermediate-risk rectal cancer patients A. Biche 1 , A. Choudhury 1 , L. Wee 1 , A. Dekker 1 , J. Van Soest 1 , M. Berbee 1 1 Department of Radiation Oncology MAASTRO- GROW School for Oncology and Developmental Biology- Maastricht University Medical Centre+- Maastricht- the Netherlands, MAASTRO clinic, Maastricht, The Netherlands Purpose or Objective Pre-operative radiotherapy and chemo-radiotherapy have been shown to reduce the risk of local recurrences after total mesorectal excision (TME) surgery. These neoadjuvant treatment strategies do not appear to improve survival and may negatively affect the quality of life after treatment. Hence, it is crucial to accurately select the patients who will or will not benefit from neoadjuvant therapy. Under-staging of the tumor and lymph nodes may lead to the omission of a beneficial pre- treatment, whereas over-staging may cause unnecessary morbidity. This study investigates the predictive value of radiomic features derived from pretreatment CT images for pathological lymph node (pN) staging for rectal cancer patients. Material and Methods A retrospective study of 64 diagnosed colorectal cancer patients treated with short-course (5x5Gy) radiotherapy followed by total mesorectal excision (TME) surgery within ten days between 2007 to 2015. The regions of interest (ROI) were delineated manually by an experience rectal radiologist. Radiomics features extraction was implemented using the Ontology-guided Radiomics Analysis Workflow (O-RAW) software. A total of 105 radiomic features extracted from each segmented ROI (Mesorectum and gross tumor volume of the primary disease (GTVp1) ) of pretreatment CT images were analyzed. Missing clinical information was imputed using the multivariate imputations by chained equations (MICE) package in R. Principal component analyses (PCA) was employed for dimensionality reduction after all features with a correlation coefficient above 0.7 have been excluded. Repeated (50) 5-fold cross-validation decision tree models are used to classify patients based on their pN status (Negative or Positive). The area under the receiver operating characteristic curve (AUC) is used to measures the performance of these models. Results Figure 1 shows the developed trees from the meserectum and tumor data, respectively. The decision tree used three radiomics mesorectum information to make a decision. However, just two variables are used for the tumor information and one for the clinical.

Conclusion In this study, older patients had a higher rate of acute urinary symptoms, in comparison with younger ones, but there was no records of grade 3 genitourinary acute toxicity (RTOG). It was found a significant higher rate of acute haematological toxicity and a trend to more frequent diarrhoea and rectal incontinence. Regarding late toxicity patterns there were no significant discrepancy noted between age-groups. PO-1110 Bladder dose constraints for conformal radiotherapy in rectal cancer: adaptation to prescribed dose N. Fourati 1 , L. Farhat 1 , Z. Fessi 1 , M. Frikha 1 , W. Siala 1 , W. Mnejja 1 , J. Daoud 1 1 Faculté de médecine Université de Sfax, Oncology Radiotherapy Department- Habib Bourguiba Hospital, Sfax, Tunisia Purpose or Objective Tolerance doses in the bladder are less than prescribed doses during conformational radiation therapy (CRT) for rectal cancer. So, the dose constraints proposed by expert are not adapted to CRT in this location. We have, in a previous study, proposed bladder dose constraints that could be applied in this indication. The purpose of our current study was to verify the applicability of these constraints in clinical routine. Material and Methods Retrospective dosimetric study of 59 patients with rectal cancer treated with preoperative CRT in our department. The prescribed dose at planning target volume (PTV) was 45 Gy (1.8 Gy / fraction, 5 fractions per week). All patients were treated with 4 orthogonal fields (anterior, posterior and 2 lateral). During dosimetric planning, the bladder was contoured as a solid organ. We retrospectively noted the doses received by this organ: mean dose (Dmean) and the volume which receives more than 40 Gy (V40). We then checked if these doses respect