ESTRO 2022 - Abstract Book

S447

Abstract book

ESTRO 2022

1 University Hospital Jena, Department of Radiotherapy and Radiation Oncology, Jena, Germany; 2 University Hospital Jena, Department of Radiotherapy and Radiation Oncology , Jena, Germany; 3 University Hospital Jena, Institute for Medical Statistics, Computer Science and Data Science (IMSID), Jena, Germany; 4 University Hospital Jena, Clinic of Nuclear Medicine, Jena, Germany; 5 University Hospital Jena, Clinic of Nuclear Medicine , Jena, Germany Purpose or Objective Several studies have evaluated the use of atlas-based auto-segmentation (ABS) in different regions, mostly for organ at risk (OAR) delineation, while the evidence for its performance in the pelvis, i.e. the clinical target volume (CTV) is poor. This study aims to evaluate ABS in patients with anal cancer for different ABS template-sizes. Materials and Methods 51 patients(pts.) with anal cancer who underwent 18 F-fluorodeeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) radiotherapy treatment planning were selected. All patients were scanned at the FDG-PET/CT scanner in treatment position and the treatment planning was performed on the CTs of the FDG-PET/CT. 27/51 pts. had FDG-PET positive lymph node metastases (LN). The positive LNs (n=27), as well as the CTV (n=51) according to the UK NATIONAL GUIDANCE FOR IMRT IN ANAL CANCER (V4.07/12/2016) were retrospectively delineated in RayStation (RaySearch Laboratories, Stockholm, Sweden). From the 51 contoured datasets one ABS-template was created. A crucial parameter for the ABS is the number datasets included in the algorithm. Five different template sizes were assessed (ABS10=10 datasets within the template, ABS20, ABS30, ABS40 and ABS50) to automatically delineate the CTVs of the 27pts with PET-positive LN, with exclusion of their corresponding dataset from the ABS template. We evaluated: 1. the dice similarity index (DSI) = 2xCV/ (mCTV+aCTV), 2. the volume falsely not covered, as well as the volume falsely covered by the aCTV, 3. the volume of the PET-positive LN covered by the aCTV(Fig.1).

Friedman Test Post-hoc analyses were performed (SPSS Inc., Chicago, IL, USA). A value of P<0.05 was considered to be statistically significant. We report median(range). Results In total, 103 LN were contoured. The LN were localized in the inguinal (n=53;51%), external iliac (n=14;14%), internal iliac(n=20;19%), mesorectal(n=10;10%) and presacral(n=6;6%) regions, respectively. There was a slightly higher accuracy of aCTVs with increasing numbers of pts within the template. For a statistically significant improvement of the DSI an ABS40 (as compared to an ABS10) was necessary (Bonferroni-adjusted p=0.017). There was no difference between ABS40 and ABS50 (DSI 0.795(range:0.705-0.847) and 0.796(0.718-0.850), respectively). With rising numbers of datasets within the template, the amount of falsely contoured and falsely not contoured volumes decreased (p<0.001). There was a higher chance for a more accurate aCTV coverage of LNs (>90% of the LN volume covered by aCTV) if the LNs were in the inguinal region as compared to other regions (odds ratio=7.23; 95%-CI [1.09;48.05]) (p=0.041). Conclusion ABS with 40 datasets in the ABS-template is a useful tool to delineate CTVs in patients with anal cancer. However, supervision and correction of ABS-based delineated volumes by an experienced radiation oncologist is still strongly recommended.

PD-0499 Bonesparing radiotherapy for anal cancer. -Initial planning quality for the DACG II trial

C. Kronborg 1 , L. Nyvang 2 , J. Hansen 2 , E. Serup-Hansen 3 , B. Havelund 4 , E. Wilken 3 , S.P. Mc Ilroy 4 , K.G. Spindler 5

1 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus N, Denmark; 2 Aarhus University Hospital, Department of Medical Physics, Aarhus N, Denmark; 3 Copenhagen University Hospital, Department of Oncology, Herlev, Denmark; 4 University Hospital of Southern Denmark, Department of Oncology, Vejle, Denmark; 5 Aarhus University Hospital, Department of Oncology and Experimental Clinical Oncology, Aarhus N, Denmark Purpose or Objective Pelvic insufficiency fracture (PIF) is a well-known late side effect of pelvic radiotherapy. We have recently found a high frequency of PIFs in patients with anal (50%) or rectal cancer (30%), 1 and 3 years after radiotherapy, and analyzed the risk